A Guide to Green Roof Functionality

by Dr. Anna Zakrisson on Tuesday, January 5, 2021 updated Friday, January 15, 2021

Guide to green roofs – what is the purpose of the vegetated roof?

Green roofs and facades help create a more decent living environment in our present, and future cities. But you need to choose the right roof for the right place.

Before you decide to install a green roof (vegetated roof), you must first ask yourself the simple question of why you are installing one. If your aim is to handle stormwater, you might go for a different type of roof than if you are interested in deep-rooted avantgarde plant aesthetics for a small private home. Are you trying to minimize irrigation needs? Perhaps, there is a way to combine all three?

This article discusses some green roof functionalities to help you get a broad overview of the current market.

Categorization based on the intensity of maintenance: extensive vs. intensive green roofs

Traditionally, green roofs, or vegetated roofs, have been divided into two broad subgroups based on the level of maintenance required and their weight loads.

Extensive green roofs have shallow, lighter weight profiles (<10cm/6 inches) and are generally self-sustaining, apart from some weeding and application of compost or slow-release fertilizer about once per year. Common species on extensive green roofs are Sedum plants due to their drought resilience. So in general extensive green roofs are functional, low weight, low cost, and easy to maintain.

In contrast, intensive green roofs are built as green spaces for people to walk onto and enjoy aesthetically. Unlike the thinner profile green roofs discussed above, they have much deeper soil profiles (20-100cm/8-40”) and can support a wide variety of plants ranging from meadows to actual trees. Of course, these profiles are much heavier than the extensive profiles. They are also often significantly more expensive to install, and maintenance costs are higher, after all, they are gardens in the sky. Intensive green roofs will need weekly irrigation in most parts of the world. However, intensive vegetated roofs may provide a vast range of benefits and, in some urban areas, the installment of a roof garden may increase the real estate value thereby generating the ROI that pays for the investment. It is all about the right roof in the right place.

If you would like to play around with interactive green roof details and learn more about weight loads and what is possible for your project, take a peek at our details page: Purple-Roof Details

Division by Functionality

Though the categorization of green roofs based on the level of maintenance and weight can be useful, it may in many cases be more suitable to think of green roofs in terms of functionality: What requirements do you have, i.e., which task should the roof perform?

Perhaps, your main task is to ensure high biodiversity, but you might also be interested in energy performance, heat island mitigation, stormwater management, or a combination thereof. There are many different products and systems on the market that can provide excellent solutions to these various challenges. Even for a single project with multiple roofs, the roofs individually may differ in the tasks they should perform, and this multi-roof system may together perform better than the sum of its parts.

Another item that needs to be evaluated clearly is the budget and potential ROI. You need to investigate which roof type might generate an ROI for your specific project. This can vary a lot between locations and might require some thought and planning to get right. Also, you should not forget to calculate your maintenance costs.

Let’s start with taking a look at the stormwater functionalities of green roofs.

General Stormwater Control/Management

I have chosen to separate general and specific stormwater management. This is because there are two ways to control stormwater using a green roof: retention and detention.
Retained water is the water that has evaporated from plants and surfaces (evapotranspiration). This is a massive engine on a green roof, and over the course of a year, 50-70% of the annual precipitation can be returned to the skies as water vapor. Of course, the efficiency of this process depends on the local climate and the vegetation used, but remember that most vegetation types that evaporate at higher levels also need additional irrigation in between rainfalls.

This reduction in annual runoff means that less water will end up in the sewers needing expensive treatment. On a city-level, this is a big win from both an ecologic and economic point of view. Also, you might lower the annual sewage costs, or stormwater fees, of your building which contributes to the return on investment calculations.

Learn more about how evapotranspiration drives vegetated roof retention and simply adding more material to the roof will not increase its retention capacity: Evapotranspiration Rates Determine Green Roof Retention

Learn more about vegetated roof retention in the video below!

Specific Stormwater Control

I chose the term “specific stormwater control” when referring to detention. The reason for this is that detention deals with the actual stormwater issues of a specific project per rain event (storm), and not with annual averages (general stormwater control).

In short: specific stormwater control is about managing the peak of the storm, lowering the intensity, and flattening the stormwater curve so that runoff is less intense and flows at a predictable and reliable rate, instead of as an uncontrollable, intense, and destructive burst during the storm.

Extensive testing of a vast variety of green roof profiles by the independent research institute Green Roof Diagnostics revealed that a green roof profile that is soaking wet after it has been pouring down for days essentially changes from acting like a sponge, to acting as a pipeline: one drop in, is one drop out. The detention delay of soaked traditional green roof profiles, irrespective of thickness, is in the range of 3-5min, which is insignificant for stormwater peak flow management.

What is stormwater detention?

As mentioned above, a retention-based green roof will remove 50-70% of the total precipitation over the course of a year. However, after a large rain event or a several-day storm, there will not be enough time for evapotranspiration to empty out the stored water, resulting in runoff rates being virtually equal to the rainfall rates. Even small subsequent rain events can now cause flooding and devastation, so engineers must install additional solutions to protect projects and cities.

To ensure solid on-site stormwater management, stormwater engineers cannot merely use a retention-based green roof. A statistical reduction of runoff over a year is excellent, but is not very useful for specific flooding prevention.

Thus, engineers are often required to add stormwater management solutions built to handle certain statistical rain events that have been pre-defined for a particular area. One example is the “100-year storm event”. This terminology is slightly unfortunate and often leads to the misunderstanding that this the event that will occur once every 100 years, making people underestimate real risks. I prefer to instead use the definition “a rain event that has a 1% chance of occurring in a given year”. The video below explains this concept in more detail:

This all means that you need to prove that your project will be able to handle that massive storm event for you to get your building permits. Basically, you will need to store 85-95% of the peak storm volume on-site, and slowly release it over time into the sewer system. Hence, if you install a retention-only green roof, you will certainly need to install additional stormwater management tools at grade to handle these types of statistical storms. Examples of such devices are below-grade stormwater tanks and cisterns, or at-grade detention ponds. Or they can opt for detention-type vegetated roofs and so control both general/annual- and event-specific stormwater on the roof.

If you are interested to figure out if a Purple-Roof compliant roof is a fit for your project, please take a look at this article: Does a Detention Roof Make Sense for Your Project?

If you are interested to learn more about blue-green & blue roofs, take a look at this article: Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water

Different types of detention roofs

A blue roof is a detention-type roof that provides only detention (specific) but no retention (general). A blue-green roof, on the other hand, is a detention + retention roof that includes vegetation. Both blue and blue-green roofs store water on the rooftop and require a completely flat deck and particular attention to membrane quality and craftsmanship. There are many excellent solutions on the market, and companies with decades of experience to help you create a highly functional, secure, and beautiful roof.

Blue-green roofs perform excellently when it comes to stormwater management due to the combination of retention and detention. Some types of blue-green roof systems store water for plant irrigation, essentially returning most (but not all) precipitation to the air as vapor. This is also where the stormwater terminology gets confusing: If most water is returned to the air as vapor, that means that most of the annual water would theoretically be retained, but it would still classify as a fully-fledged stormwater tool because it can deal with most of the design storms as well.

The exception is when the green roof sponge is full, and the storage underneath is filled up 100% as well, and yet it rains multiple days in a row which overfills the storage capacity underneath which leaves the blue-green roof as outflow. It is virtually impossible to retain 100% of the rain because of this uneven rain distribution. As you can see, understanding terminology is critical before deciding on your project's roof.

If you are looking to offer retention and detention for your project but suffer from weight constraints that make a blue-green roof an impossible solution, or there is a slope, or you cannot store water the way a blue-green roof would due to building code reasons, you might want to take a look at a Purple-Roof compliant roof. These roofs are non-proprietary and sold by various companies worldwide using their own products that have been tested thoroughly to meet the Purple-Roof Compliant label.

Stormwater – just the beginning…

What about biodiversity, heat island mitigation, use of local flora, nutrient runoff, energy savings...?
Stormwater is just one functionality-aspect of vegetated roofs.

Cooling buildings

One notable example where functionality-awareness is needed is when the aim is to use the green roof to cool a building in summer. On paper, this might be a very straightforward thing, but it does require some considerations. Firstly, you need to look at building insulation and geometry, e.g., the roof area/building volume ratio. Then, you need to look at the green roof itself.

A green roof will only significantly cool if the plants are evapotranspiring at reasonably high rates. It is the process of evaporation that aids heat transfer and subsequent loss. Hence, you cannot expect a bone-dry roof in the summer months to perform this functionality at the desirable rate. Even if an extensive sedum roof survives the summer with no irrigation, if you want the roof to actively cool your building, you might have to investigate drip irrigation during the hottest months in summer.

Perhaps, you can connect this roof with the water collected on another type of roof at another level? With some knowledge and creativity, it is possible to turn these situations into fantastic sustainable projects where different roofs can perform various tasks to create a water-resilient and well-functioning system. For this, we need product diversity, research, and market knowledge.

Urban heat island (UHI) effects and biodiversity

It should also be noted that though green roofs can cool buildings, I find it even more remarkable how they can reduce urban heat island effects (UHIs) and cool our cities. To put this in perspective: UHIs are estimated to have contributed to 2-3% of gross global warming. They also exacerbate air pollution and lead to heat-related deaths. The greening of our cities is not some “nice little project”; if I may speak frankly, it is the difference between a decent- and a troublesome future urban existence.

Finally, we should not forget the important aspect of biodiversity. The little space it was given in this article is by no means a reflection of its importance. Rather, it deserves its own article, but let me leave this with you: according to a recent UN report, 1,000,000 species are currently under threat of extinction, urban areas have more than doubled since the beginning of the 1990ies, land-based habitats have decreased by more than 20% since the 1900s. The WWF report published in 2014 reports that 52% of the world's biodiversity is gone compared with 40 years ago. This is extremely concerning not only from a moral standpoint, but also because we humans are depending on this biodiversity for ecosystem stability, food production, medicines, and frankly survival. But, let me address these matters in a different article and end this negative paragraph with a positive statement: green infrastructure has the opportunity to turn 'sterile' urban land into biodiversity oases. It is not too late.

Your project

Being aware of green roof functionalities can help you find the best fit for your project; budget, climate, local regulations, and project-specific stormwater requirements... Sometimes, the best solution for a project can be a combination of several different roof types. The market is diverse, and that is a very good thing. Let's go from gray-to-green!

We are here to help if you have any questions.
info@purple-roof.com

If this is a topic that interests you, I would like to suggest to take a look at our educational video series on green roof retention and detention: