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The Purple-Roof Concept optimizes stormwater retention and is pioneering detention in green roof- and non-vegetated roof coverings.

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The Purple-Roof Concept optimizes stormwater retention and is pioneering detention in vegetated and non-vegetated roof coverings.

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What Is A Green Roof

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<h2>TLDR: a photovoltaic stormwater tool with cooling biodiversity that pays for itself </h2>
No technology comes without side effects. This is also the case with photovoltaics systems which, in many studies, have been shown to increase urban heat island effects. However, combining photovoltaics with green roofs can alleviate this issue as the green roof evapotranspiration can cool the panels and the surrounding area. Add green roof stormwater detention, and you also have a clear ROI.

<h2>The importance of mandate wording</h2>
In many places, mandates have been put in place for either solar panels or green roofs on new commercial buildings, e.g., <a href="https://www.nature.org/en-us/about-us/where-we-work/united-states/new-york/stories-in-new-york/nyc-laws-green-roofs-solar-panels" title="Unlocking the Power of New York City&rsquo;s Roofs" rel="follow noopener" target="_blank">NYC </a>or, more recently, <a href="https://cleantechnica.com/2022/05/05/new-law-in-france-green-roofs-on-new-commercial-buildings/" title="New Law In France: Green Roofs On All New Commercial Buildings" rel="follow noopener" target="_blank">France</a>. This is something to celebrate, but we need to be aware of uncareful mandate wording that could pitch one green industry against another. In NYC, the mandate inadvertently pitched photovoltaics companies against green roofs for shares of the market, which likely was not the intended effect of the mandate.
Situations like this one can be highly damaging, as we, in this case, need a collaborative effort and not a competitive one. Especially as a combination of these two technologies might be an essential ingredient for sustainable urban areas.

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<h2>Photovoltaics &ndash; sorely needed for energy production</h2>
Europe is currently in a massive energy crisis. Furthermore, the warming climate demands fossil-free green energy such as wind power or photovoltaics. The transition to green energy is critical. Nonetheless, we need to consider the exacerbation of <a href="https://www.purple-roof.com/post/how-green-roofs-mitigate-heat-island-effects" title="How Green Roofs Mitigate Urban Heat Island Effects" rel="follow">urban heat island effects</a> through photovoltaic panels before we cover the roofs of entire metropolitan areas in PV, especially since there are green nature-based solutions that can alleviate this issue.

<h2>Solar panels and the urban heat island effect?</h2>
A heat island effect describes the localized and dangerous heating up of urbanized areas as buildings, roads, and other infrastructures absorb and re-emit heat more efficiently than vegetated areas.
Photovoltaic panels absorb around 90% of the sun&rsquo;s energy, but they are only approximately 20% efficient. This can result in significant heat loss and heating up of the environment. Furthermore, you end up with two hot surfaces: first the PV panel itself and then the roof underneath.
<blockquote>
Learn more about urban heat islands in this article: <a href="https://www.purple-roof.com/post/how-green-roofs-mitigate-heat-island-effects" title="How Green Roofs Mitigate Urban Heat Island Effects" rel="follow">How Green Roofs Mitigate Urban Heat Island Effects</a> 
</blockquote>
A <a href="https://www.sciencedirect.com/science/article/pii/S0378778821007635" title="Photovoltaics in the built environment: A critical review" rel="follow noopener" target="_blank">review of 116 papers</a> on the topic showed that photovoltaic panels heat cities during the day. Still, it should be noted that they can often cool the surrounding area during nighttime. The energy balance of these systems is complex, which is why this review was sorely needed.&nbsp; The relative effect of the PV panels on the local heat flux depends on many factors. One of them is what type of roof you put them on.

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If you put a solar panel on a black roof, you are not having a substantial adverse effect on the thermal environment. But we must ask ourselves if this is the comparison we want to make. A black roof should be a thing of the past due to the poor thermal performance concerning the urban heat island effect. Hence, I personally do not find it appropriate to use a black roof as a point of reference.
On the contrary, if you add photovoltaics panels to a white roof, you will most definitely disturb the thermal performance of that roof. A white roof, or a cool roof, is usually used to alleviate urban heat islands. It generally works well when the roof is new, but dirt can, over time, reduce the cooling efficiency, essentially turning it into a semi-black roof.
To further complicate matters, PV panel efficiency is typically tested at 25&deg;C. Due to the roof conditions, this temperature is often exceeded. Already at 60&deg;C, a 10-15% efficiency drop has been reported.
Nevertheless, many of these issues can be solved using green infrastructure in combination with photovoltaic panels.

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<h2>How green roofs can support photovoltaics</h2>
Green roofs are roofs that are covered with vegetation. Vegetation is highly effective in cooling due to the evapotranspiration process. We all know the difference between the burning sand in the sun and the cool grass next to it. Hence, the green roof vegetation can cool photovoltaics panels and reduce the loss of efficiency. Vegetated roofs are well known for significantly reducing heat island effects, and plants have been used for the cooling of buildings since the beginning of civilization. This is no new technology.
Nonetheless, there are some things to consider. A green roof only cools as long as there is access to water. This means that it is critical that you design the roof to be optimized for your climate. You can do this using our <a href="https://www.purple-roof.com/model" title="Purple-Roof Evapotranspiration / Retention modeler" rel="follow">evapotranspiration/retention modeler</a> (free of use). Also, you should consider emergency <a href="https://www.purple-roof.com/post/minimize-green-roof-irrigation" title="Minimize Green Roof Irrigation" rel="follow">drip irrigation lines</a>, preferably connected to harvested rainwater to cover extreme drought phases.
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Let us know if your city is not yet included in the modeler.
</blockquote>
It is also vital that you select an appropriate plant palette. <a href="https://www.purple-roof.com/post/how-to-select-plants-for-a-sedum-green-roof" title="Guide: Sedum Green Roof Plant Selection" rel="follow">Sedum plants</a> are hardy and drought tolerant and are generally good in combination with PV panels as they rarely grow high enough to disturb the panels. Sedum plants also do well in the shade below the panels, and data show that they transpire sufficiently for a significant cooling effect.
A green roof can also act as ballast for the photovoltaic system removing the need for anchorage and penetration of the waterproofing membrane. This leads to a reduced leakage risk. A green roof also extends the lifespan of the waterproofing membrane significantly and protects it from foot traffic, hail damage, and other mechanical damage. Contrary to what some people may think - a green roof can protect you from leaks compared with a bare roof or a PV roof.&nbsp;&nbsp;

<h2>What about the cost?</h2>
Yes, adding a green roof will add cost, and though there is an efficiency increase in the PV panels, it will take time before you see a return on investment. However, this is not all. You will also save on building energy consumption due to the cooling effect of the green roof. Also, due to the roof&rsquo;s retention capacity, your sewage fees will be reduced.
Nonetheless, a clear ROI can be added to this equation if you live in a densely populated urban area: green roof detention. This is a particular class of green roofs that can be described as stormwater tools that double as green roofs. Using detention-type green roofs, you can reduce or remove at-grade stormwater solutions, which clears up floor space that can be used for income-generating activities.
You end up with a photovoltaic stormwater tool with cooling biodiversity that pays for itself.
If you want to learn more about green roof detention and the economic possibilities, take a look at this article: <a href="https://www.purple-roof.com/post/economic-case-study-green-roof-roof-roi" title="Economic Case Study: Green Roof ROI" rel="follow">Economic Case Study: Green Roof ROI</a>

Green profits: how you build a photovoltaic stormwater tool with cooling biodiversity that pays for itself.

Green Roofs Make Solar Panels Work Better

No technology comes without side effects. This is also the case with photovoltaics systems which, in many studies, have been shown to increase urban heat island effects. However, combining photovoltaics with green roofs can alleviate this issue as the green roof evapotranspiration can cool the panels and the surrounding area. Add green roof stormwater detention, and you also have a clear ROI.

<a href="https://www.purple-roof.com/post/green-roofs-make-solar-panels-work-better" title="Green Roofs Make Solar Panels Work Better" rel="follow">https://www.purple-roof.com/post/green-roofs-make-solar-panels-work-better</a>****
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Green Roof Modeler">Go straight to Purple-Roof Green Roof Modeler</a>
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Details">Purple-Roof DETAILS</a>

<h2>Green Roofs and R and U-Values</h2>
Green roofs can be very efficient in <a href="https://www.purple-roof.com/post/how-green-roofs-mitigate-heat-island-effects" title="How Green Roofs Mitigate Urban Heat Island Effects" rel="follow">cooling a building</a> in summer due to the heat loss caused by the <a href="https://www.purple-roof.com/post/what-is-evapotranspiration" title="Evapotranspiration - the Green Roof Engine" rel="follow">evapotranspiration process</a>. In simple terms: water is converted to gas, heat is lost, and the roof is cooled. The resulting energy savings are multifold: (1) due to direct cooling of the roof, (2) but also due to the lower temperature air that is taken in by potential air conditioning modules.
Despite this effective cooling, people are often confused when they see green roofs being rated as having moderate-to-poor insulation capacity in permits. In these permits, it is common to see the green roof (and other components of the building) rated according to &ldquo;R&rdquo; or &ldquo;U&rdquo; values/factors. &nbsp;
<h2>What is an R or U value?</h2>
R-Value is a measure of thermal resistance. It is a measure of the ability of a material to resist heat flow. On the contrary, a U-Value is a measure of thermal transmittance or heat loss through a structural element. Both are used as ratings; the higher the R-value, the better the material is to resist heat flow, e.g., leading to a warmer house in winter. The opposite is the case for U-values.
The simplicity and straightforwardness of R and U-values have made them standard procedures for building insulation. They are great for static systems.

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<h2>Green Roof R-Values</h2>
Mineral wool (a substrate component of high-retention green roofs) has a very high R-value when dry. &nbsp;When wet, its R-value approaches zero. Similar to other components that cycle through wet and dry on the roof, but most dramatic with <a href="https://www.purple-roof.com/post/why-use-mineral-wool-on-green-roofs" title="Mineral Wool on Green Roofs - Compaction and Toxicity?" rel="follow">mineral wool</a>. &nbsp;There really isn&rsquo;t a static R-value that can be assigned to these materials, though their R-value can be assigned at fully dry, fully wet, or some point in between. But that&rsquo;s not how R-values tend to be used.

<h2>Green Roof Insulation vs. Cooling</h2>
An R- or U-value is not an optimal way to describe the ability of a green roof to manage heat energy. This is also because R and U-values are static and do not incorporate or describe the heat loss from the living part of the roof: the plants. Hence, the green roof&rsquo;s capacity to manage heat energy is not so much dependent on the static building components but on the upper part of the profile where water is evapotranspired &ndash; especially during wet and hot conditions.
The energy lost via evapotranspiration is much more difficult to calculate and describe than the static R and U-values and involves <a href="https://en.wikipedia.org/wiki/Energy_modeling" title="Energy modeling" rel="follow noopener" target="_blank">dynamic modeling</a>. Many factors affect the roof&rsquo;s ability to cool, such as climate, <a href="https://www.purple-roof.com/post/plant-selection-for-a-high-retention-green-roof" title="Plant Selection for a High-Retention Green Roof" rel="follow">vegetation</a>, water availability, slope, substrate type, and much more. A green roof with no access to water cannot evapotranspire and hence is not able to cool the building. On the contrary, a high retention green roof with access to water is an effective cooling engine. Again, biology has crashed the engineering party and messed up the straightforward equations.

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<h2>What to Do?</h2>
In many cases, R- or U-values are required to be given for the green roof. They are likely calculated for a worst-case scenario, i.e., for a soaked profile, and won&rsquo;t be spectacular looking. However, if you are interested in how much money you can save on energy using your green roof, you can contact an energy modeler or use one of the available green roof energy modelers that are available, e.g., the <a href="https://www.urbanscape-architecture.com/solutions/green-roof-system/#pet_tool" title="URBANSCAPE&reg; PET TOOL" rel="follow noopener" target="_blank">PET tool</a> from Knauf. Also, take care to design your green roof for your local climate to ensure effective evapotranspiration during the hot months. You can use our <a href="https://www.purple-roof.com/model" title="Green Roof Retention / Evapotranspiration Modeler" rel="follow">evapotranspiration modeler</a> for this (free of use).
In the end, it is about the real functionality of the roof, and no matter what the R or U-values say, a well-designed and optimized green roof will cool your building in summer and will save you energy and $.
Please, feel free to <a href="https://www.purple-roof.com/contact" title="Contact Purple-Roof!" rel="follow">contact us</a> if you have any questions!&nbsp;

R and U-Values are unsuitable for measuring the thermal performance of green roofs. This is what you should do instead!

R and U-Values are Unsuitable for Green Roofs

An R- or U-value is not an optimal way to describe the ability of a green roof to manage heat energy. This is also because R and U-values are static and do not incorporate or describe the heat loss from the living part of the roof: the plants. Hence, the green roof’s capacity to manage heat energy is not so much dependent on the static building components but on the upper part of the profile where water is evapotranspired – especially during wet and hot conditions. The energy lost via evapotranspiration is much more difficult to calculate and describe than the static R and U-values and involves dynamic modeling.

R and U-Values are Unsuitable for Measuring Thermal Performance of Green Roofs

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<h2>Purple-Roof is a Stormwater Tool that Doubles as a Green Roof</h2>
The Purple-Roof concept is far from a simple green roof upgrade. It might be clearer to describe the Purple-Roof concept as a stormwater tool that doubles as a green roof. The concept provides both stormwater <a href="https://www.purple-roof.com/post/evapotranspiration-rates-determine-green-roof-retention" title="Evapotranspiration Rates Determine Green Roof Retention" rel="follow">retention</a> and <a href="https://www.purple-roof.com/post/what-is-stormwater-detention" title="What is Green Roof Stormwater Detention?" rel="follow">detention</a> and thus fulfills the civil engineer&rsquo;s requirements for stormwater management.
Generally, the main reason for installing a Purple-Roof compliant system is that you can reduce or even remove at-grade stormwater solutions and free up floor space for income-generating development. You are managing your stormwater on an otherwise unused and neglected area: the roof. Hence, there is a <a href="https://www.purple-roof.com/post/economic-case-study-green-roof-roof-roi" title="Economic Case Study: Green Roof ROI" rel="follow">strong business case</a> in densely populated urban areas where land is expensive.
Then add all the benefits you get from a green roof, such as <a href="https://www.purple-roof.com/post/how-green-roofs-mitigate-heat-island-effects" title="How Green Roofs Mitigate Urban Heat Island Effects" rel="follow">cooling</a>, reduced energy needs, <a href="https://www.purple-roof.com/post/green-infrastructure-street-canyon-urban-air-quality" title="How Green Infrastructure Affects Urban Air Quality" rel="follow">pollution capture</a>, increased biodiversity, and LEED points, and you also get a much better-looking roof.

<blockquote>
This video explains the ROI of a Purple-Roof system in 4 min. Please, <a href="https://www.purple-roof.com/contact" title="CONTACT PURPLE-ROOF" rel="follow">contact us</a> for the whitepaper!
</blockquote>
<iframe width="560" height="314" src="https://www.youtube.com/embed/BVkDxALjLpo" allowfullscreen="allowfullscreen"></iframe>


<h2>When does the Purple-Roof Concept Make Sense for your Project?</h2>
Detention-type green roofs are incredible workhorses, excellent stormwater management tools, and can provide a clear ROI for a green roof. However, not all projects are suitable. These are some questions to consider:
Are you required to do a green roof? If so, upgrading it to Purple-Roof most likely saves you money if you live in an urban setting. However, the low real estate prices in rural settings can make at-grade stormwater solutions more favorable in the countryside. &nbsp;
Is your lot almost the same as the size of the roof? Are you occupying most of the lot with a building? 80%/20% roof/lot ratios in urban areas mean that a Purple-Roof will almost certainly be a good solution.
Can your roof handle the weight? Live load: Since we are only holding the peak amount of rain for a few hours, the practical route is to take this out of the live load budget. In most cases, the snow load budget will be enough. For instance, in most east coast cities, there is a 40-80lbs/sqft (195-391kg/m2) snow load, so utilizing 8-12lbs/sqft (39-59kg/m2) from that budget is more than acceptable.
Can we absorb the cost of the Purple-Roof? The opportunity cost: What if we can park four more cars or rent out 1200sqft (111m2) extra basement space to a fitness club? Multiply this over 50 years and reap the benefits. And there is an actual cost: Upgrading a green roof to a Purple-Roof will add $5-7/sqft ($56-78/m2) to most green roof designs. But it will eliminate the tank. Tanks cost hundreds of thousands of dollars in engineering, products, and installation.


<h2>How the Purple-Roof Concept Differs from a Blue-Green Concept</h2>
Both Purple and Blue-Green roofs are high retention green roof concepts, and both concepts can detain water. So how do they differ?
A Blue-Green roof has a void space underneath the green roof where rainwater can be stored for later use. In most cases, mineral wool wicks connect the stored water with the green roof profile, allowing upward water transport through capillary action as the profile dries out. In a way, a Blue-Green roof is a green roof on stilts over a shallow pool. The pool fills up as the outflow of water is restricted, e.g., through orifice restrictions. Blue-Green roofs can also be equipped with smart outflow systems that empty the water storage in anticipation of a large storm.
Blue-Green roofs are great at effectively using rainwater, and they can look amazing - think roof gardens! However, rainwater storage comes at a structural cost as these types of roofs can be very heavy. They also require a completely flat deck which limits where they can be built. In some instances, people have also expressed concerns over having a pool of water on their roofs. We do not consider this a genuine concern provided a premium manufacturer and installer has been used.

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<img src="https://cdn.buttercms.com/L9fQan6pRWilt2mlU2lw" alt="Different types of roof water detention" />
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<img src="https://cdn.buttercms.com/vdCKHcW9SVicmaloKdR4" alt="Four common green roof approaches" />
<blockquote>
Four common green roof approaches. From: Jeffers, Scott, et al. "<a href="https://www.chijournal.org/C484" title="Insights into green roof modeling using SWMM LID controls for detention-based designs." rel="follow">Insights into green roof modeling using SWMM LID controls for detention-based designs." Journal of Water Management Modeling (2022)</a>.
</blockquote>


The Purple-Roof is a lightweight, passive concept that does not harvest rainwater like Blue-Green roofs. Still, it delays water outflow to, e.g., meet ground percolation rates or project maximum allowable outflow rates. This is done through a fabric layer at the bottom of the green roof profile - the Detention Layer &ndash; that has thousands of woven threads that generate turbulence and slow water outflow predictably and reliably. There is no free water pooling on the roof as the water only temporarily fills up the profile. The system then recharges over hours to receive another storm.
The Purple-Roof concept can, contrary to Blue-Green systems, be installed on sloped roofs, and the Purple-Roof concept can also be placed under pavers and gravel to achieve full detention functionality, but without the plants.


<h2>How does the Purple-Roof Concept Detention Layer Work?</h2>
A detention-based green/vegetated roof slows water outflow when the green roof is soaked with water from previous rainfall. In the Purple-Roof concept, this is achieved using the Detention Layer, a 0.2&rdquo; (5mm) thick polyester fabric with densely woven threads that break up laminar flow, hence generating turbulence. The higher the head pressure, the higher the turbulence and the slower the water outflow.
As the head pressure increases, the turbulence increases, and water is backed up into a void space &ndash; the HoneyComb Reservoir. During extreme storms, water backs up into the mineral wool and finally into the substrate and vegetation layer. The concept can be sized according to project stormwater requirements.

<blockquote>
This short video shows the hydrology behind the Purple-Roof concept
</blockquote>
<iframe width="560" height="314" src="https://www.youtube.com/embed/u7VygyBbSxI?feature=youtu" allowfullscreen="allowfullscreen"></iframe>

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<img src="https://cdn.buttercms.com/NXCukvZ0TF6eTRyqL4Zs" alt="Extensive Purple-Roof concept detention profiles" />
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<blockquote>
Purple-Roof concept example. The concept can be sized according to project needs.
</blockquote>
 &nbsp;
 In addition, the capillary forces of the detention layer suck the root protection layer dry as the system empties, avoiding standing water.&nbsp;
The hydrologic behavior is predictable, reliable, and has been carefully modeled. The hydrology behind the detention layer has been studied in many thousands of tests by the independent green roof research institute Green Roof Diagnostics and confirmed in <a href="https://www.chijournal.org/C484" title=" Insights into green roof modeling using SWMM LID controls for detention-based designs" rel="follow">academic publications</a>.

<blockquote>
In this short video Dr. Anna Zakrisson from <a href="https://www.greenroofdiagnostics.com/" title="Green Roof Diagnostics Home" rel="follow noopener" target="_blank">Green Roof Diagnostics</a> (GRD) shows the detention layer in detail. The <a href="https://www.purple-roof.com/" title="Purple-Roof HOME" rel="follow">Purple-Roof concept</a> has been researched and tested by GRD.
</blockquote>
<iframe width="560" height="314" src="https://www.youtube.com/embed/CnuJkdRlk3c" allowfullscreen="allowfullscreen"></iframe>
Please, contact us for whitepapers and more information!

The Purple-Roof concept is a stormwater tool that doubles as a green roof! The concept provides both stormwater retention and detention.

A Stormwater Tool that Doubles as a Green Roof!

The Purple-Roof concept is far from a simple green roof upgrade. It might be clearer to describe the Purple-Roof concept as a stormwater tool that doubles as a green roof. The concept provides both stormwater retention and detention and thus fulfills the civil engineer’s requirements for stormwater management.

Generally, the main reason for installing a Purple-Roof compliant system is that you can reduce or even remove at-grade stormwater solutions and free up floor space for income-generating development. You are managing your stormwater on an otherwise unused and neglected area: the roof. Hence, there is a strong business case in densely populated urban areas where land is expensive.

Then add all the benefits you get from a green roof, such as cooling, reduced energy needs, pollution capture, increased biodiversity, and LEED points, and you also get a much better-looking roof.

Green roof detention can help reduce pressure on at-grade green stormwater solutions preventing blow-outs and increasing performance.

Roof-to-Ground: Green Roof Detention Treatment Train

A treatment train is a sequence of stormwater treatments designed to maximize results in a specific environment. This means that we need to take a step back to get a good overview and see how we can connect different stormwater solutions to reestablish the natural hydrologic cycle. This is especially important for the city/municipality as it will further assist the city in reducing stormwater treated by the sewage treatment plants and reducing combined sewer overflows.
One such example could be to combine detention-type green roofs with bioretention ponds or bioswales to create a green stormwater infrastructure treatment train.

How Green Roof Detention can Improve the Functionality of At-Grade Green Stormwater Infrastructures

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<figure class="image"><img src="https://cdn.buttercms.com/2N3CIF99RDqIBrrCWMpY" alt="Healthy vegetated roof free of pests and bugs" />
<figcaption>A healthy green roof free of pests and bugs</figcaption>
</figure>
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<h2>Pest infestations on Green Roofs?</h2>
In my 15 years, I have been on over 500 green roof visits. I have been asked many times to diagnose a failing green roof. Apart from the South, more or less, all these visits took place from LA to Vancouver, from Minneapolis to Chicago to Toronto, and from Boston to Washington DC. I have never seen a bug infestation on our roofs, but sometimes they do pop up, and if they do happen, it is because the green roof has been mistreated: too much/little water and/or too little nutrients, and the infestation could easily have been avoided. Weirdly enough I have never seen any ants nests on green roofs.&nbsp;
In the US and Canada, we also grow 200 acres of <a href="https://www.purple-roof.com/post/how-to-select-plants-for-a-sedum-green-roof" title="Guide: Sedum Green Roof Plant Selection" rel="follow">Sedum crop</a> in three locations, and we have never had to react to any pest-related bugs in any way, shape, or form. Nonetheless, if we see a plant covered in aphids, for instance, which sometimes happens, we do not panic. We let the aphids be, as they only attack plants that are already weakened. Once done, the aphids simply disappear.
****

<figure class="image"><img src="https://cdn.buttercms.com/pdXD00NTZHe69jz5IIPA" alt="Ladybug on a plant-eating aphids" />
<figcaption>Ladybugs are a natural pest control when it comes to aphids</figcaption>
</figure>

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<h2>The Biodiversity of a Green Roof</h2>
In the fall, during the early morning hours at the farms, we see the dew collect on what appears to be hundreds of thousands of funnel-shaped spider webs. As these spiders can only survive if there is something for them to eat, I can only assume they catch something airborne because, on the plants, we simply do not see a lot of bugs.&nbsp;
We see these web funnels on green roofs as well, and of course, we see many birds landing looking for food. On my vegetated roof at my house, I see birds on the roof throughout the day, picking at something that I can only assume to be food in the form of insects or spiders.
At the nursery, we also have deer, wild pigs, rabbits, raccoons, and whatever else scrounges over the green roof plant fields at night, and somehow our <a href="https://www.purple-roof.com/post/green-roof-plants-how-do-they-survive" title="Green Roof Plants - Masters of Survival" rel="follow">Sedum crop</a> must not taste well, so we don&rsquo;t even have to worry about larger animals. Insects and animals tend to eat crops that grow in abundance. Plants that grow in adverse, highly stressed locations like deserts (cacti) or rock crevices on mountains (Sedums) tend to develop defense mechanisms that protect their slowly accumulated resources. Cacti do this with, for example, prickly needles, whereas <a href="https://www.purple-roof.com/post/can-you-install-sedum-green-roofs-in-winter" title="Can You Install Sedum Green Roofs in Winter?" rel="follow">Sedums </a>do so by chemical defenses (alkaloids and polyphenols) and compensatory growth strategies as a response to herbivory1. Hence, we tend to see bugs but no pest explosions that would require insecticides.&nbsp;

<h2>The Link Between Water and Bugs</h2>
In forest-like moist environments, insects tend to be present in large numbers, where lots of organic material is degrading, supporting a rich chain of life. You see way more bugs in the jungle than in the desert and it&rsquo;s a simple supply problem. If there is little to feed, there is little to harvest.
Similarly, a green roof is also a very harsh environment, as it tends to be on the drier side, especially in summer. There is very little organic material on a green roof, perhaps 2-3cm (1&rdquo;) of dead leaf debris, but the moisture swings from wet to bone dry, making it hard for bugs to thrive. Furthermore, the pH of the roof soils tends to be on the high side (7.5-8.5), and this might be something that does not foster insect explosions.

<h2>Green Roof Plant Drought Tolerance</h2>
Sedums and many other green roof plants that can tolerate drought are survivors that tend to adapt or cope with adversity better than most. It is essential to understand that it has little to do with preference and everything to do with tolerance. When you observe plants in large drifts, these plants do not prefer to be there more than other plants. They grow in drifts because they can tolerate that particular condition better than others.
If you disturb soil (either due to a storm or artificial excavation), a wide selection of plants scrambles to settle themselves. As it turns out, that open soil wound spot might be flooded regularly, or perhaps it is bone-dry for six months, or it gets eaten by goats or rabbits, whatever extreme adversity you can think of. Over time there will be a handful of plants that can handle that regular abuse. Those plants will remain in large drifts because of their tolerance to accept the harsh conditions. It certainly is not a preference.
The same is true with the vegetated roof; the plants that do well on a green roof are the plants that have the most tolerance for heat, drought, coldness, wind scours, etc. Their leaves are tough, often leathery, and simply difficult to chew through, and they taste bad, so you can assume few bugs are going after your typical green roof plant foliage. You can grow other plants on a green roof too, but then you need to plan for frequent irrigation unless you live in a cool temperate climate with few drought periods and frequent small rain showers. You can check the optimal green roof in the online <a href="https://www.purple-roof.com/model" title="ET modeler" rel="follow">evapotranspiration/retention modeler</a>.&nbsp;

<h2>There is One Insect of Concern &ndash; But Only if Your Green Roof is Already Sick</h2>
This is a real-life story of a grub-infestation-on-a-green-roof story in the <a href="https://maps.apple.com/place?q=North%20Chevy%20Chase%20Elem%20School&amp;ll=38.9989247%2C-77.0729433&amp;auid=7170391528499505936&amp;lsp=9902&amp;address=3700%20Jones%20Bridge%20Rd%2C%20Chevy%20Chase%2C%20MD%20%2020815%2C%20United%20States" title="Montgomery County School District" rel="follow noopener" target="_blank">Montgomery County School District</a>. A story about a larva, a whitish immature form of a Japanese beetle called the grub. Imagine mama Beetle wants to lay her eggs somewhere, and it is flying through this Montgomery neighborhood, and it can lay these eggs everywhere. It can do so on a lawn, a ditch, or a vegetated roof. In this case, the school had two vegetated roof areas (Green roofs A and B) roughly the same size, approximately the same conditions, same soil depth, and same plant genus.
As any mother would know, you want to place your offspring where their survival chances are in their favor, so mama Beetle decides to drop her eggs on Green roof B. &nbsp;Why B and not A? &nbsp;Was B closer, and Mama Beetle was simply lazy and didn&rsquo;t want to fly the extra 50 yards? &nbsp; No. She decided that B was showing all the signs of a perfect place for her offspring. Somehow these bugs can identify plants or drifts of plants that are sick, stressed beyond normal, or dying. &nbsp;They perhaps use color and UV signatures; maybe they can sense pheromones, who knows, but somehow mama Beetle knows precisely where to lay her eggs, and B was the perfect place.
A few months later, in August, the green roof had been ravaged. Plants are unrooted and even tossed over the roof edge laying on the ground a few stories down. We have a look and it truly looked as if someone had made a deliberate effort to uproot 80% of roof B. &nbsp;
Grubs had eaten the roots of the Sedums because the plants had shown signs of stress. Next, the birds (usually crows), and sometimes raccoons, notice the tasty grubs and use their feet and/or beaks to uproot the plants, which is now easy because they are like a toupee layer without roots. There were 5-20 grubs per 0.1m2 (1 sqft), which basically is an all-you-can-eat buffet. More birds came, and within 24-48 hours, 370m2 (4000 sqft) of Sedum roof was uprooted and destroyed.&nbsp;

<iframe width="560" height="315" src="https://www.youtube.com/embed/Cq9cYWIMAQQ" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>
<blockquote>
The video shows the grubs in closer detail

</blockquote>
<h2>What Was the Cause of the Grub Infestation?</h2>
The green roof system provider for Green roof B was called in, and they said it was evident that the grubs destroyed the green roof and they were the cause. Later, we clearly stated that the issue was not the grubs. The grubs simply responded to a wide-open door. Let me clarify: Green roof B was made up of vulnerable small plants in distress, and the grubs simply took advantage of it. The vegetation was sick because there was a drought combined with nutritional deficits. Soil samples taken by us revealed the soil was missing a few critical nutrients and this &lsquo;toppled&rsquo; the Sedum crop into a very desirable beetle egg-laying location, as ten-thousand or more grubs were thriving on Green roof B. &nbsp;
In a contrast, at the exact same time, just 50 feet away, Green roof A was perfectly fine. How can that be? Green roof A was a different system that held onto water much better. As it held onto water much better, it, therefore, leached out fewer nutrients, and the plants had just a tad more water to bridge the drought and a tad extra nutrients which made for more robust bigger plants with much bigger reserves.&nbsp; Mama Beetle flew over that roof too, and she decided that her offspring would have a hard time battling that healthy crop on Green roof A and preferred B instead.
Grubs are a symptom, not a cause. If you design a green roof to bridge regular periods of drought properly, you will avoid most grubs. If you make sure the vegetation is getting at least the bare minimum of nutrients, they will be able to grow into large plants that can build nutritional reserves allowing them to be OK with a few roots chewed off by grubs, and they have the resources to fight off pests naturally. &nbsp;&nbsp; Please use the <a href="https://www.purple-roof.com/model" title="Evapotranspiration modeler" rel="follow">evapotranspiration/retention modeler</a> to verify that your roof configuration is optimal for your climate.
****

<figure class="image"><img src="https://cdn.buttercms.com/zbY0dZEQQrS7fctRdJzT" alt="Grub infested green roof" />
<figcaption>The grub-infested green roof</figcaption>
</figure>
<figure class="image"><img src="https://cdn.buttercms.com/5o53yEsRvyhGmGTWoUMm" alt="Healthy green roof free of pests" />
<figcaption>A healthy green roof free of pests just a few meters away from the infested roof</figcaption>
</figure>

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<h2>How Did We Get Rid of the Grubs?</h2>
Green roofs have thin profiles and highly coarse soils that quickly dry out. Winter precipitation usually washes out much of the nutrients by spring, and even though Sedums need very little food, they do need some, especially in the 1st four to five years when they are getting established. After that, they require less because of the accumulation of organic material over time. We take soil samples in spring and fall, and we apply only the nutrients that are in short supply. We never add fertilizers that are not needed. Water can be stored in water-absorbing layers like needled mineral wool or cups in the drainage layers.
Within three days, we visited the remainder of the Montgomery County School District green roofs in a 48km (30-mile) radius, and of the 26 schools at the time, 19 had severe green roof problems due to grubs. These failed roofs were from two different green roof system suppliers, and they had one thing in common: excessively porous system design without much water holding capacity. &nbsp;
The other seven roofs were also of various brands (three suppliers), and as all schools were getting the exact same maintenance protocol, we could conclude the grub problem was not due to a lack of applied nutrients. The seven that looked perfectly fine all had one thing in common: they were trays as well but had solid walls, and larger water reservoirs to bridge the drought, and due to the systems, they lost fewer nutrients.
But the story gets a bit more complex. You might ask if only the needled mineral wool or water cup reservoir is the entire secret solution? No. That same week we inspected many more roofs in the area. None of them had grub problems, except for two others, located in Fairfax County, just 20 miles away. At both these roofs, we saw extensive grub problems. You could uproot a few thousand square feet of green roof vegetation without roots, frail-looking and stressed because they couldn&rsquo;t tap into the water reserves. The foliage looks gray-ish or red-isch and plants look shrunken. These two green roof systems had the capacity to store enough water, so in this case, it wasn&rsquo;t merely water-related, and yet there was indeed a massive grub infestation. &nbsp;
We took soil samples, and it was clear that this roof was utterly devoid of any nutrients. These plants had only water to feed themselves with, and they got weak. People were weeding the roof, but they were not feeding the roof. Plants, like us, need water and food. You give them both and the community of plants will live forever.&nbsp;
****

<figure class="image"><img src="https://cdn.buttercms.com/CZBBrOGtQVWKrzSJPjO7" alt="Pests on a green roof" />
<figcaption>An already weakened vegetation was an easy target for the grubs</figcaption>
</figure>

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<h2>The Final Grub Solution</h2>
How to deal with grubs? There are ways to kill the grubs, but these are nasty chemicals that we prefer for you to avoid. The safest way is to let the damage continue for a while (by the time you notice it, it is too late already, and let these larvae develop into beetles, and they will fly away. Then treat the roof with Milky Spore, a natural product that affects the egg stage of the beetle, and this will be a natural defense system that will prevent 95% of the problem from reoccurring next year. &nbsp;
Milky Spore contains spores of the bacteria Paenibacillus&nbsp;popilliae, which work specifically against the grub stage of the Japanese Beetle (Popillia japonica). It poses no threat to people, animals, plants, or beneficial insects. When Milky Spore is introduced into the soil, it will lie dormant until grubs begin feeding on roots where the bacteria are present. Once ingested, the spore multiplies inside the grub and kills Japanese Beetle grubs about a week after infection. As the grub decomposes, the billions of spores it contains are released back into the soil to start the whole process again. Over time, Milky Spore fills out the soil creating a soil environment that Japanese beetles simply cannot survive in. Like other bacteria, it is highly survivable in drought conditions but suffers in temperatures of Zone five and colder.
You might say: &nbsp;I will treat my roof with Milky Spore before it&rsquo;s a problem. I disagree. I think it is better to take care of your green roof. If you just pay a little more attention to your green roof, test your soil, and keep them fed, you will not have to worry about grubs or treat them for grubs, and your maintenance crew will thank you as well since healthier plants mean less open soil and less weeding.&nbsp;

Feel free to <a href="https://www.purple-roof.com/contact" title="CONTACT PURPLE-ROOF" rel="follow">contact us </a>if you have any feedback or questions!



1. Adamski, P., Margielewska, K. &amp; Witkowski, Z. <a href="http://maxbot.botany.pl/cgi-bin/pubs/data/article_pdf?id=350" title="Compensation and induced defense in response to herbivory in Sedum maximum" rel="follow noopener" target="_blank">Compensation and induced defense in response to herbivory in Sedum maximum</a> (Crassulaceae). Fragm. Florist. Geobot. 45, 193&ndash;202 (2000).

&nbsp;

Green roofs very rarely get pests and if they do, it is mostly because the roof was in poor shape: a case study.

How to prevent green roof bug infestations

Green roofs rarely get pest and bug infestations and if they do, it is because the vegetation was in bad shape e.g. wrong watering regimes or too little nutrients. Pests on green roofs are avoidable by keeping your vegetation strong and healthy.

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<figure class="image"><img src="https://cdn.buttercms.com/0MTbJNQfQlO8dgPjuCwA" alt="Purple-Roof concept detention type green roof installation" />
<figcaption>Purple-Roof concept detention model output</figcaption>
</figure>

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A detention-based green/vegetated roof slows the outflow of water also when the green roof is soaked with water from a previous rainfall. This slowing down of water, or stormwater detention, is analogous to the functionality of a <a href="https://en.wikipedia.org/wiki/Bioretention" title="Bioretention" rel="follow">bioretention cell</a> where water is collected and the peak outflow is reduced and delayed in order to prevent flooding damages. For the <a href="https://www.purple-roof.com/" title="Purple-Roof HOME" rel="follow noopener" target="_blank">Purple-Roof concept</a>, the detention layer is the "magic ingredient".
<h2><iframe width="560" height="315" src="https://www.youtube.com/embed/u7VygyBbSxI" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe></h2>
<h2>Do All Green Roofs Provide Detention?</h2>
Far from all green roofs have detention capacity, though this is often claimed due to a misconception and confusion around two concepts: retention and detention.&nbsp; As it starts to rain and the green roof is dry or relatively dry and there is a lot of pore space still available in the roof profile. At this point there is no runoff &ndash; this is retention filling up and not a <a href="https://www.purple-roof.com/post/what-is-the-difference-between-green-roof-retention-and-detention" title="What is the Difference Between Green Roof Retention and Detention?" rel="follow">detention</a>-based outflow delay.
The profile fills up and capillary forces hold the water in the profile micropores. The problem is the substrate macropores where the capillary forces are not strong enough to hold the water against the gravitational pull. Then, runoff begins due to these gravitational forces pulling on the water through substrate <a href="https://www.purple-roof.com/post/how-water-flow-in-soil" title="How Water Flows Through Soil" rel="follow">preferential flow paths</a>.
Please note that it is essential to separate real retention from the results of the <a href="https://www.technormen.de/norm/astm-e2399-e2399m-19-1.2.2019.html" title="ASTM E2399/E2399M-19" rel="follow noopener" target="_blank">ASTM E2399</a>, which is the standard used to assess the maximum load of a saturated profile. Using ASTM E2399 as a measure of retention is severely flawed and will lead to an overestimation of the retention capacity of the profile.
<iframe width="560" height="315" src="https://www.youtube.com/embed/q9Dd1PZiFs0" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>

But back to detention! The million-dollar question is if the profile can delay the outflow of water when it is filled up like a soaked bath sponge, or does it become a pipeline with essentially no peak outflow reduction &amp; delay? Also, even if there is a peak outflow reduction &amp; delay, is the magnitude of these parameters enough to satisfy a stormwater engineer? A 5min peak outflow delay and 5% peak outflow reduction during a significant storm event such as a 100-year storm is insignificant as a flooding protection measure.
Many years of data collection and hydrologic modeling by Green Roof Diagnostics have revealed that traditional green roofs do not provide significant detention to act as a replacement for, e.g., a bioretention cell or a stormwater tank. Traditional green roofs are great at providing retention and many other tremendous secondary benefits, but they do not provide true detention, no matter what you might hear.
<h2></h2>
<h2>What is Detention, and Why is Friction so Important?</h2>
There are <a href="https://www.purple-roof.com/post/detaining-water-on-a-roof-blue-green-and-purple" title="Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water" rel="follow">several detention-type green roof solutions</a> on the market with various solutions to delay the outflow of water. Some use orifice restrictions, meaning that water is collected on the roof, and the drainage outflow points are set to a specific diameter resulting in a predictable outflow rate. There are also smart roofs on the market that will open or restrict the outflow depending on predicted storm intensities. Most of these solutions are Blue-Green roofs and need a completely flat deck to function, making these solutions a good fit for beautiful roof gardens.
The <a href="https://www.purple-roof.com/post/a-stormwater-tool-that-doubles-as-a-green-roof" title="A Stormwater Tool that Doubles as a Green Roof!" rel="follow">Purple-Roof concept</a> is suitable for flat and sloped roofs and uses a friction-based layer to delay &amp; reduce peak outflow. This friction-based layer is slightly analogous to the natural friction encountered in a watershed. Water follows the path along the sloped forest floor; some water is absorbed (retention), and some is delayed through the crisscrossing over mosses, sticks, and stones. Traditional green roofs have little to no friction as they are built to expedite water away as fast as possible, making it impossible to saturate the profile thoroughly using the macropore spaces.
****
<img src="https://cdn.buttercms.com/oSg31iSou7Nu93s3JDQk" alt="natural-water-cycle-detention-retention-post.jpg" />

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<h2>How Does the Purple-Roof Concept Work?</h2>
As it begins to rain, the Purple-Roof concept profile slowly fills up. First, the water seeps into the soil substrate and then into the mineral wool. Due to the (toxin-free!) needled mineral wool, extra micropore space is available that ensures that more water can be held in the profile compared with substrate-only profiles. The mineral wool also helps reducing the preferential flow paths, thus increasing the retention capacity of the vegetated roof.
Underneath the mineral wool is a honeycomb-shaped void space that can be adjusted in height depending on project-specific needs. There is no horizontal flow of water between cells. Below the honeycomb detention reservoir, the water reaches the detention layer. This is where the detention magic happens.
****
<figure class="image"><img src="https://cdn.buttercms.com/f8zwwXo6RGpQ8k8uBzNL" alt="Purple-Roof green roof detention and retention profiles" />
<figcaption>Purple-Roof concept profiles in SI and US units</figcaption>
</figure>
****
<iframe width="560" height="315" src="https://www.youtube.com/embed/CnuJkdRlk3c" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>
<blockquote>
Dr. Anna Zakrisson from Green Roof Diagnostics shows how the detention layer works

</blockquote>
As the water seeps into the detention layer, it is met with many hundreds of thin threads per cm in the 5mm thick detention layer fabric. These threads break up any laminar flow; the resulting turbulence increases with increasing head pressure. The more water stored in the honeycomb detention reservoir, the more turbulence there is in the detention layer. In short: thread-induced friction results in turbulence, and this causes water to back up into the profile. The outflow is predictable and reliable and has been measured and modeled thoroughly by <a href="https://www.greenroofdiagnostics.com/" title="Green Roof Diagnostics HOME" rel="follow noopener" target="_blank">Green Roof Diagnostics</a>.
Interestingly, the detention layer can also be used underneath pavers and gravel with full detention capacity. This means that the entire roof space often can be utilized for detention though perhaps only, 80% can be utilized for retention (the vegetated areas). 100% of a roof can rarely be vegetated often due to fire regulations and HVAC and other equipment. &nbsp;&nbsp;
<h2><iframe width="560" height="315" src="https://www.youtube.com/embed/YJZEYP_hi3A" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe></h2>
<h2>Return on Investment</h2>
The concept can be sized for virtually any design storm and project and is already in use in the US and internationally as a replacement for at-grade stormwater solutions. This is particularly valuable in urban areas where land is expensive. The move from an at-grade stormwater solution to green roof-based detention can mean many $ saved and many $ made as the ground can now be used for income-generating activities instead of merely hosting a stormwater tank. Plus you get all the <a href="https://www.purple-roof.com/post/top-10-benefits-of-a-green-roof" title="These are the Top Benefits of a Green Roof" rel="follow">secondary benefits</a> of a green roof.
LEARN MORE ABOUT DETENTION GREEN ROOF ROI: Economic Case Study: <a href="https://www.purple-roof.com/post/economic-case-study-green-roof-roof-roi" title="Economic Case Study: Green Roof ROI" rel="follow">Green Roof ROI</a>
Please, don't hesitate to contact us if you have any questions. Also, Green Roof Diagnostics can offer support in the stormwater permitting process in many cases. Don't hesitate to contact GRD <a href="https://www.greenroofdiagnostics.com/contact" title="CONTACT GREEN ROOF DIAGNOSTICS" rel="follow">HERE </a>for European and North American requests.&nbsp;

A detention-based green roof system can kick out your at-grade stormwater tank and free up space. Learn how!

How Does the Purple-Roof Concept's Detention Layer Work?

A detention-based green/vegetated roof slows the outflow of water also when the green roof is soaked with water from a previous rainfall. This slowing down of water, or stormwater detention, is analogous to the functionality of a bioretention cell where water is collected and the peak outflow is reduced and delayed in order to prevent flooding damages.

****
<img src="https://cdn.buttercms.com/1jmp3uxqQuqQsj1zfGkv" alt="Saratoga Springs vegetated roof for stormwater management" />
****
<a class="purple-button" href="https://www.purple-roof.com/model" title="Purple-Roof
Green Roof Modeler">Go straight to Purple-Roof Green Roof Modeler</a>
<a class="purple-button" href="https://www.purple-roof.com/details" title="Purple-Roof
Details">Purple-Roof DETAILS</a>

<h2>How Detention Green Roof ROI saved a Project</h2>
<a href="https://www.bonacio.com/portfolio/the-moderne-condominiums/" title="The Moderne Condominiums" rel="follow noopener" target="_blank">The Moderne</a> is a new 5-story condominium located in Saratoga Springs, New York. Constructed in 2021, it contains a mix of 24 high-end residential units with exterior and underground parking. It also includes a 1020 m2 (11,000 sf) rooftop amenity space with a green roof.
During the early stages of construction, issues with excessive groundwater and soil stability required additional foundation work to be completed. That work required the relocation of the underground stormwater tank. Due to the tight space constraints of the site, that new tank location would eliminate two parking spots. Local zoning requirements stipulated that losing the two parking spots would reduce the allowable number of units. That reduction would have translated into financial ruin for the project.

<blockquote>
This video shows you the summary of this article.
</blockquote>
<iframe width="560" height="315" src="https://www.youtube.com/embed/BVkDxALjLpo" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>
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<img src="https://cdn.buttercms.com/oTv0BYAjSoOgqj0lec30" alt="Saratoga Springs detention type green roof project" />
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<h2>Where to Put the Stormwater?&nbsp;</h2>
The design team was immediately tasked with finding a new method of <a href="https://www.purple-roof.com/post/novel-flooding-prevention-using-green-roofs" title="Novel Flooding Prevention using Green Roofs" rel="follow">managing stormwater</a>, and eliminating any parking was not an option. Construction had already begun, so a decision was needed quickly. The first option discussed was to install a traditional storage system inside the building. That would absorb approximately 112 m2 (1,200 sf) of interior space and cost roughly $75,000 to install. It would require annual inspections and maintenance, but most importantly, it would cost the developer a great deal of sales revenue with the loss of interior space. This option was not well received.
Fortunately, the project team found a new method of stormwater management. By upgrading the traditional green roof to a <a href="https://www.purple-roof.com/" title="Purple-Roof HOME" rel="follow">Purple-Roof concept</a> green roof, which utilizes <a href="https://www.purple-roof.com/post/detention-roof-a-fit-for-your-project" title="Does a Detention Roof Make Sense for Your Project?" rel="follow">detention</a>, there may be no need to lose parking or interior space.&nbsp;
****
<img src="https://cdn.buttercms.com/EK5mCznRgeMJKKQae5so" alt="saratoga-springs-post.jpg" />
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<blockquote>Read more about detention type green roofs in these articles:
<a href="https://www.purple-roof.com/post/what-is-stormwater-detention" title="What is Green Roof Stormwater Detention?" rel="follow">What is Green Roof Stormwater Detention?</a>
<a href="https://www.purple-roof.com/post/detention-roof-a-fit-for-your-project" title="Does a Detention Roof Make Sense for Your Project?" rel="follow">Does a Detention Roof Make Sense for Your Project?</a>
<a href="https://www.purple-roof.com/post/detaining-water-on-a-roof-blue-green-and-purple" title="Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water" rel="follow">Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water</a>

</blockquote>
<h2>The Stormwater Plan and Green Roofs</h2>
Through careful hydrologic modeling by <a href="https://www.greenroofdiagnostics.com/" title="Green Roof Diagnostics HOME" rel="follow noopener" target="_blank">Green Roof Diagnostics (GRD)</a>, a Purple-Roof system was engineered and modeled to provide the specific storage and outflow reduction required, without any need for additional systems or loss of space. After confirming with the structural team and local regulators, the decision was made to implement the Purple-Roof concept and eliminate the tank system.

<blockquote>
Learn more about green roof detention in this video:
<iframe width="560" height="315" src="https://www.youtube.com/embed/MORdaL6A2xg" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>
</blockquote>

For permitting purposes, the design storm used was a historical Type II, 100-year, 150 mm (5.9&rdquo;), 24-hour storm. The required storage volume for the building was 170,000 L &nbsp;(6,000 cf), and the maximum allowable outflow rate was 28 L/s/ha (0.4 cf/s/a). To achieve these metrics, the green roof was upgraded by removing the traditional drainage panel and then adding 50 mm (2&rdquo;) of needled mineral wool, a 150 mm (3&rdquo;) honeycomb reservoir, and a 5 mm (3/16&rdquo;) thick detention layer.&nbsp;
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<figure class="image"><img src="https://cdn.buttercms.com/3hteW144TfuRj9lBPPkW" alt="Purple-Roof detention concept profile" />
<figcaption></figcaption>
</figure>
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<blockquote>
Overview of the 735m2 (7905 sf) vegetated Purple-Roof compliant profile. VE=vegetation, 1.5cm (0.5&rdquo;), GRS=green roof soil, 11cm (4.5&rdquo;), NMW=needled mineral wool, 5cm (2&rdquo;), HC=honeycomb, 7.6cm (3&rdquo;), DL=detention layer, 5mm (0.2&rdquo;).
</blockquote>
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<figure class="image"><img src="https://cdn.buttercms.com/U8a3qEaTR2ryuY4bt8Xn" alt="Purple-Roof detention paver concept profile" />
<figcaption></figcaption>
</figure>
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<blockquote>
Overview of the 280m2 (3011sf) Purple-Roof compliant paver profile. NMW=needled mineral wool, 5cm (2&rdquo;), HC=honeycomb, 7.6cm (3&rdquo;), DL=detention layer, 5mm (0.2&rdquo;).&nbsp;
</blockquote>
<a class="purple-button" href="https://www.purple-roof.com/details" title="Purple-Roof
Details">Purple-Roof DETAILS</a>

The needled mineral wool provides excellent retention and filtration properties while minimizing weight. The honeycomb detention reservoir provides a temporary storage that is always functioning, regardless of the soil saturation. The detention layer provides controlled and consistent outflow at an engineered rate over the entire green roof area.&nbsp;

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<figure class="image"><img src="https://cdn.buttercms.com/aHxONLT5GnSR9KXy6Iwx" alt="2peak-flow-reduction-post.jpg" />
<figcaption></figcaption>
</figure>
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<figure class="image"><img src="https://cdn.buttercms.com/dCZwI0MTgIXT9KSDWgUA" alt="peak-flow-reduction-post.jpg" />
<figcaption>Peak flow reduction and delay of the Purple-Roof concept system compared with unmitigated runoff.</figcaption>
</figure>
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<a class="purple-button" href="https://www.purple-roof.com/model" title="Purple-Roof
Green Roof Modeler">Go straight to Purple-Roof Green Roof Modeler</a>

<h2>Green Roof Stormwater Management using Detention-Type Green Roofs</h2>
By managing the stormwater on the roof, the developer provided 180m3 (6,336 cubic feet) of stormwater storage, equal to over two shipping containers. The Purple-Roof also reduced the peak stormwater runoff by 95% and provided a 42-hour drain-down delay. The decision also kept 15 truckloads of debris from being excavated, hauled, and dumped in a landfill. Finally, the Purple-Roof concept allowed the Developer to sell all 24 units, including the 112 m2 (1,200 sf) of salvaged interior space at the market rate of $550 per sf, adding $540,000 to the developer&rsquo;s sales revenue.

<h2>Financial Summary</h2>
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<img src="https://cdn.buttercms.com/Ncvh0URbQZWCQOD0omdf" alt="Example of return on investment for a vegetated roof" />
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<img src="https://cdn.buttercms.com/CBmHwQPnSOKhfRZFjVgp" alt="Detention green roof ROI calculations" />
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The additional cost of the Purple Roof was $196,488 The credit for no longer requiring the underground storage tank was approximately $75,000 Therefore, the actual added cost for the Purple-Roof compliant system was $121,500.
<blockquote>
By investing $121,500 to manage the stormwater on the roof, the developer was able to sell two additional units for a total of $1,800,000.
</blockquote>

<a href="https://www.purple-roof.com/contact" title="CONTACT US" rel="follow">Contact us</a> to learn more and to get the whitepaper!

By investing $121,500 on green roof stormwater management, the developer was able to sell two additional units for a total of $1,800,000.

Economic Case Study: Vegetated Roof ROI

ECONOMIC CASE STUDY: A project always needs a stormwater plan to protect the property and the city from the devastating effects of flooding. These solutions must work every single time and must be completely independent of the health of the greenery, or the saturation status of the green roof.  By investing $121,500 to manage the stormwater on the roof, the developer was able to sell two additional units for a total of $1,800,000.

Economic Case Study: Green Roof ROI

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<img src="https://cdn.buttercms.com/wRRKSQXVQ62Dg911McNz" alt="The dysfunctional water cycle exemplified with flooding on a street" />
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<a class="purple-button" href="https://www.purple-roof.com/model" title="Purple-Roof
Green Roof Modeler">Go straight to Purple-Roof Green Roof Modeler</a>
<a class="purple-button" href="https://www.purple-roof.com/details" title="Purple-Roof
Details">Purple-Roof DETAILS</a>

<h2>Nature-Based Solutions and Water</h2>
Water is a precious resource and should be treated as such. In many urban areas, water is merely viewed as a problem, and any precipitation is rapidly expedited out of the city in a gray infrastructure network of pipes and sewers. We can do better! We can also do it cheaper.
<a href="https://ec.europa.eu/info/research-and-innovation/research-area/environment/nature-based-solutions_en" title="Nature-based solutions - definition" rel="follow noopener" target="_blank">Nature-based solutions</a> are low-tech, cost-effective, and nature-inspired solutions that help build urban resilience by working with nature instead of against it. Examples of <a href="https://www.purple-roof.com/post/are-green-roofs-nature-based-solutions" title="Green Roofs as Nature-Based Solutions" rel="follow">nature-based solutions are green roofs</a>, rain gardens, bioswales, and living walls.
<blockquote>
Read more about gray vs. green stormwater solutions in this article: <a href="https://www.purple-roof.com/post/what-stormwater-management-options-are-there" title="Green &amp; Grey Stormwater Management Options" rel="follow">Green &amp; Grey Stormwater Management Options</a>
</blockquote>
An essential task of many urban nature-based solutions is restoring the natural water cycle. This is crucial as for the green to happen, we need to deal with the blue. As my hero, marine biologist Prof. Sylvia Earle once said: "<a href="https://oceantoday.noaa.gov/sylviaearle/" title="Sylvia Earle TED Conference" rel="follow noopener" target="_blank">No water, No life. No blue, No Green</a>".
If we don&rsquo;t take care of the urban water cycle, our chance of highly biodiverse urban areas is also diminished. It is all interlinked and begins with access to water in the right amounts at the right time.&nbsp;

<h2>The Urban vs. Natural Water Cycle</h2>
The natural water cycle is disturbed in urban areas mainly due to the sealing of the ground. This sealing of the ground is a crucial problem as water can no longer percolate into the ground and replenish the groundwater. This non-percolated water accumulates on the surface and can cause flooding during high-intensity rain events.

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<img src="https://cdn.buttercms.com/0mEQBs6zSlyZOfBmuusa" alt="The natural water cycle drawn on a forest floor" />
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<h2> How do we solve this?</h2>
There are many interesting solutions, ranging from parks to rain gardens and bioswales. However, due to the lack of free space, or costly land, in densely populated urban areas, we frequently run into issues with these at-grade solutions. Often stormwater is led into the mixed canalization systems and expedited out from the city to an expensive sewage treatment plant.
However, there is a simple and easy solution for such areas: we can use roof areas to manage stormwater. One of the purposes of green roofs is to re-establish the natural water cycle in urban areas.
The aims are to reduce the volumes of water that needs to be treated by sewage treatment plants, use the evaporative cooling of the plants to cool buildings and surrounding areas (reducing <a href="https://www.purple-roof.com/post/how-green-roofs-mitigate-heat-island-effects" title="How Green Roofs Mitigate Urban Heat Island Effects" rel="follow">urban heat islands effects</a>), reduce horrendous combined sewer overflows, and reduce flooding.
<blockquote>
Read more about combined sewer overflows here: <a href="https://www.purple-roof.com/post/what-is-a-combined-sewer" title="What is a Combined Sewer?" rel="follow">What is a Combined Sewer Overflow?</a>
</blockquote>
Green roofs can provide these ecosystem services, but not every green roof can provide all of them at every location. Some of these services can only be provided by a <a href="https://www.purple-roof.com/post/guide-to-green-roof-functionality" title="Green Roof Guide: How to Select your Vegetated Roof" rel="follow">subgroup of green roofs</a>.

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<figure class="image"><img src="https://cdn.buttercms.com/EqlPMOQ4S0m31FIMLqjW" alt="Unused bare roofs in an urban area" />
<figcaption>Many unused roof areas could be used for greening.</figcaption>
</figure>
&nbsp;
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<h2>The Natural Water Cycle</h2>
In a forest, rain falls onto the ground and percolates into it. Some of the water is directly taken up by plants, some is absorbed into the ground, and the part that cannot be absorbed continues flowing along its path.&nbsp; The part of the water absorbed by the ground and taken up by the plants will slowly be returned to the sky as water vapor through transpiration and evaporation.
<blockquote>
Learn more: <a href="https://www.purple-roof.com/post/how-water-flow-in-soil" title="How Water Flows Through Soil" rel="follow">Preferential Flow Paths in Soil Substrates</a>
</blockquote>
Transpiration notwithstanding, evaporation is also substantially more effective in a forest compared with asphalt or paved surfaces. The forests&rsquo; evaporative surface area is vast compared with an asphalt road; think of all the leaves, crevices, sticks, and mosses &ndash; all increasing and adding to the evaporative surface area.
The water that is evapotranspired is the volume of water that the system can retain. This water will not become runoff.
The water that is not absorbed or taken up by roots continues its journey above and through the ground. Some of it will percolate down into the groundwater, and some will directly replenish streams and lakes. The flow of this water is slowed down by the friction provided by sticks and stones along its way. This is stormwater detention (or slowing down of water).
We can look at a system&rsquo;s detention capacity by looking at peak inflow vs. the peak outflow rates, e.g., there is a rainfall coming down at 500L/s/ha, and the saturated system slows this to reach an outflow rate of 10L/s/ha. Also, the system provides a shift in the peak flow of, e.g., 30min. This means that the system provides detention.
It is worth noting that the filling up of the dry system is not detention &ndash; this is retention. This delay is not peak outflow delay and is the most common misunderstanding of this concept. If the water remains in the system and is evapotranspired, the volume represents retention.

<h2>Delayed Outflow and Peak Flow Reduction</h2>
We are trying to re-establish the natural water cycle on a green roof to recreate the path of water through a forest but on a roof.&nbsp; A large portion is soaked up by the green roof substrate and is evapotranspired by the plants. This water represents the retained water. The annual retention capacity of a green roof is somewhere between 40 to 70% depending on type and location. Once these profile types become saturated, they essentially become a pipeline, a bit like a soaked bath sponge. This water is the remaining 30 to 60% that becomes runoff annually.
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<img src="https://cdn.buttercms.com/Jx8mXz2ThyxZ8clP1Moz" alt="A traditional green roof acts as a soaked bath sponge (graphic)" />
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The runoff is generated by extreme storm events or repeat storm events, i.e., it rains many days in a row, which is a common weather phenomenon. To prevent flooding, erosion, and devastation, each construction project needs to provide the municipality/stormwater agency with a stormwater plan that proves that the maximum allowable outflow rates are met during a specific set statistical storm, e.g., a 100-year storm event. This means that if this 100-year storm event happens, the project maximum allowable outflow rate will not be exceeded, e.g., 10L/s/ha. This is essential to provide security for health, ecology, and the economy as flooding events can be disastrous.
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<img src="https://cdn.buttercms.com/0tKLr6PJRZuaUhzCpKka" alt="Different types of storms" /><img src="https://cdn.buttercms.com/3N5TfwFhRYSwlXMSElTg" alt="Storm distribution" />
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Though excellent at retaining water, a traditional green roof is not capable of detaining stormwater to a sufficient degree to be relevant for a stormwater engineer. However, some green roof types and concepts provide detention, e.g., <a href="https://metropolder.com/en/" title="Metropolder Home" rel="follow noopener" target="_blank">Blue-Green roofs</a>, Blue Roofs, and <a href="https://www.purple-roof.com/" title="Purple-Roof Home" rel="follow">Purple-Roofs</a>.

<blockquote>
Learn more about the detention capacity of the <a href="https://www.purple-roof.com/post/how-does-the-detention-layer-work" title="How Does the Detention Layer Work?" rel="follow">Purple-Roof concept</a> in this video:
</blockquote>
<iframe width="560" height="314" src="https://www.youtube.com/embed/u7VygyBbSxI?list=PLGA_cLAszevMTAg4vkW4o_5uTSp8D6weY&amp;index=2" allowfullscreen="allowfullscreen"></iframe>

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<img src="https://cdn.buttercms.com/k8gsuASfRFa83h4dCkUU" alt="Different detention type roof concepts" />&nbsp;
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<h2>The Green Solution for Urban Areas</h2>
This article is not saying that detention-type green roofs are superior to other types of roofs. What it is saying is the right roof in the right place.
Suppose you live in a rural area or a small town where land is inexpensive. In that case, your best green nature-based stormwater solution is most likely a rain garden and a traditional green roof, or even a rain garden and no green roof (but perhaps you want the green roof for other reasons than stormwater management &ndash; there are so many reasons to get a green roof!).
Nonetheless, if you are building in an urban area where land is expensive, and a rain garden would be a financial disaster, a detention-type green roof is your best friend. We have seen many projects where massive sums have been saved by managing stormwater on the roof instead of the ground.
Detention-type green roofs can provide a <a href="https://www.purple-roof.com/post/economic-case-study-green-roof-roof-roi" title="Economic Case Study: Green Roof ROI" rel="follow">significant ROI</a> in urban and industrial settings.
<blockquote>
Learn more: <a href="https://www.purple-roof.com/post/green-roof-roi" title="Green Roof ROI Explained!" rel="follow">Green Roof ROI Explained!</a>
</blockquote>
Don&rsquo;t hesitate to <a href="https://www.purple-roof.com/contact" title="Contact Purple-Roof" rel="follow">contact</a> us if you have any questions!
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<figure class="image"><img src="https://cdn.buttercms.com/BlEckaLjRduCNivumnqE" alt="Showing green roof ROI as increased parking space underneath building" />
<figcaption>Increased parking generates a Return on Investment (ROI)</figcaption>
</figure>
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Detention-type green roofs can replace stormwater tanks and cisterns and ensure that max allowable outflow rates are met. Learn more...

Novel Flooding Prevention using Green Roofs

Water is a precious resource and should be treated as such. In many urban areas, water is merely viewed as a problem, and any precipitation is rapidly expedited out of the city in a gray infrastructure network of pipes and sewers. We can do better! We can also do it cheaper. Green roofs are nature-based solution that can provide this much needed support for urban areas.

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<img src="https://cdn.buttercms.com/pLp6FlgXTHOulT4W3prQ" alt="Vegetated roof plant color pattern at Skyview Park" />
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Green roofs can have many purposes, such as <a href="https://www.purple-roof.com/post/guide-to-green-roof-functionality" title="A Guide to Green Roof Functionality" rel="follow">stormwater management</a>, heat island mitigation, or simply as a recreational space in an urban asphalt desert. Another fully valid reason is beauty, especially if the roof is clearly visible and relays a message.&nbsp; A well-designed green roof looks good and can be a fantastic marketing investment on top of the potential ecological and financial benefits it brings.
One often requested item for visible roofs and walls is green roof plant color patterns. These patterns are created using different plant palettes and can look spectacular. In this short article, we hope to summarize some pros and cons of such a design decision and discuss the financial and maintenance burden.

<h2>Green Roof Color Patterns &ndash; A Fight Against Nature</h2>
Green roof color patterns can look amazing, but we need to manage expectations; these are living systems, and as the plants grow, develop, and die back, the designs are bound to change unless we put up a big fight against this natural change.
 Take, for example, some of these highly structured planting designs from Disney World:

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<img src="https://cdn.buttercms.com/HfpUdeQjRTawAZEnDCc5" alt="Disneyland flower pattern" />
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<img src="https://cdn.buttercms.com/EsIXisrFRLmSQ6KeqY4x" alt="Flower patterns at Disneyland" />
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Immediately upon viewing these, you might think, &ldquo;those are great, but they require a lot of maintenance!&rdquo;. That is correct. These are striking horticultural displays used in an application where such &ldquo;magic&rdquo; is an integral part of the Magic Disney Kingdom. Designers sometimes try to apply some of this magic to green roofs, and we think that is great! &nbsp;But is the intent to maintain the patterns permanently? Or perhaps for just a few years? &nbsp;Either is possible, but the level of effort and corresponding cost must be considered.

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<img src="https://cdn.buttercms.com/uSw3ErtlRvuPoaW40bBg" alt="Sedum green roof color pattern" />
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The planting design above is an example of the successful use of various colored pre-grown Sedum mats or tiles to create a bit of &ldquo;magic,&rdquo; &ldquo;drama,&rdquo; and overall visual appeal on a green roof. A standard level of maintenance (i.e., an ecologically focused, cost-efficient maintenance approach) might allow these patterns to evolve into more of a mixed palette over two to five years, such as below:

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<img src="https://cdn.buttercms.com/7PFrAeRjSwuKXpkcRDmR" alt="Sedum green roof with mixed plant palette" />
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In most cases, we believe the most appropriate approach is to allow for the natural succession of the green roof vegetation instead of requiring that maintenance crews maintain a rigid geometry. It is a battle with nature. Either is possible, and the decision is primarily a matter of cost (and water accessibility).

<h2>Succession: Green Roof Stripes and Patterns Over Time</h2>
When a <a href="https://www.purple-roof.com/post/how-to-select-plants-for-a-sedum-green-roof" title="Guide: Sedum Green Roof Plant Selection" rel="follow">mixed color standard pallet sedum green roof</a> is installed, it generally contains a blend of 15-20 different species of different forms and shapes and growth patterns. One of the reasons for this is to ensure that over time, a stable community will form that naturally and effortlessly (from a human perspective) has been optimized for that specific location and roof. Diversity means resilience, and a resilient green roof means less maintenance and a low risk of costly vegetation crashes.
Over time, the plants that are the best adapted for the specific location (including distinct microclimates on the roof) will outcompete those less adapted. Sometimes this also means fluctuations in community composition within a year. Fast-growing plants with shallow roots, such as ruderal creepers (e.g., Sedum acre, sexangulare, and album), are often the first to &ldquo;wake up&rdquo; in early spring before they are taken over by slower-growing perennial plants with deeper root systems (e.g., Sedum spurium John Creech, Immergr&uuml;nchen, kamtschaticum, floriferum, and Phedimus takesimensis) as the year progresses into the hot summer season.

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<img src="https://cdn.buttercms.com/wRQSYzjSQIm6gcwKK5qp" alt="Sedum spurium Immergrunchen" />
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Learn more about ecological succession in this 6min video!

<iframe width="560" height="315" src="https://www.youtube.com/embed/jZKIHe2LDP8" title="YouTube video player" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen="allowfullscreen"></iframe>

<h2>Do The Patterns Look Good All Through the Year?</h2>
Ruderal creepers can double in size within a week at optimal conditions, but they tend to be water sensitive and quickly die back or go dormant when heat and <a href="https://www.purple-roof.com/post/minimize-green-roof-irrigation" title="Minimize Green Roof Irrigation" rel="follow">water shortages</a> strike. Still, they are essential for the roof to protect the soil during winter when other plants are completely dormant.
Here we arrive at another design problem: will your color designs look good throughout the year, or will you end up with patches of dead or dormant plants? Some design combinations can look splendid but may require irrigation and a lot of tender loving care. This, of course, comes at a financial and ecological cost that should be transparent already at the design stage.
Even on the same building, it is sometimes possible to see roofs with different dominant species depending on profile depth and sun/shade exposure. Thus, these roofs often have different colors. It is also worth noting that stressed and unstressed plants of the same species sometimes have different colors, as stress can lead to an accumulation of red and orange pigments such as anthocyanins. Hence, stress can also mix up your color palette. There is a lot of work and funds involved in keeping a clean and stable green roof pattern.
I would like to state that I personally think such color patterns can look amazing, and I think they can be an excellent marketing idea, but there must be awareness and honesty of the maintenance required. Another option could be to design patterns and, as natural succession proceeds, view it as natural art. Over the following years, nature slowly paints its own pattern, establishing a stable, beautiful, and resilient roof.


<h2>Are interested in learning more about green roof plant selection? Check out these articles on the <a href="https://www.purple-roof.com/" title="Purple-Roof home" rel="follow">Purple-Roof</a> blog!</h2>
<a href="https://www.purple-roof.com/post/can-you-install-sedum-green-roofs-in-winter" title="Can You Install Sedum Green Roofs in Winter?" rel="follow">Can You Install Sedum Green Roofs in Winter?</a>
<a href="https://www.purple-roof.com/post/how-to-select-plants-for-a-sedum-green-roof" title="Guide: Sedum Green Roof Plant Selection" rel="follow">Guide: Sedum Green Roof Plant Selection</a>
<a href="https://www.purple-roof.com/post/plant-selection-for-a-high-retention-green-roof" title="Plant Selection for a High-Retention Green Roof" rel="follow">Plant Selection for a High-Retention Green Roof</a>
<a href="https://www.purple-roof.com/post/green-roof-spring-maintenance" title="Green Roof Spring Maintenence" rel="follow">Green Roof Spring Maintenence</a>

Green roof color patterns can look wonderful, but do they last? Learn more about the associated maintenance requirements and costs.

Green Roof Color Patterns: Maintenance & Cost

Green roof color patterns can look amazing, but we need to manage expectations; these are living systems, and as the plants grow, develop, and die back, the designs are bound to change unless we put up a big fight against this natural change.

Urban canyons trap air pollutions at street level causing bad health and exacerbate heat island effects. Green infrastructure is a solution.

How Green Infrastructure Reduce Urban Canyon Effects

Green infrastructure provides a high surface area for pollution deposition, acts as a permeable barrier, and can alter local air dispensation patterns. However, the real impact of green infrastructure on air quality is complex and is depending on a range of parameters such as local meteorological conditions, urban canyon geometry, and vegetation characteristics such as geometry and transpiration rates.

Publications

An Experimental Study to Evaluate the Effectiveness of Green, Purple (Detention) and Traditional Roofs for Urban Stormwater Management

Mohammed A. Alim, Sayka Jahan, Ataur Rahman, Mohammad A Rahman, Mark Liebman, Robert Griffith, Merran Griffith and Brad Garner

Published on : 2023-09-18

Insights into green roof modeling using SWMM LID controls for detention-based designs

Scott Jeffers, Brad Garner, Derek Hidalgo, Dionisi Daoularis, and Oscar Warmerdam

Published on : 2022-03-15

GRD-SWMM-Modeling - Insights into Green Roof modeling using SWMM LID Controls for detention-based designs

Scott Jeffers, Brad Garner, Derek Hidalgo, Dionisi Daoularis, and Oscar Warmerdam

Published on : 2022-03-15