A Stormwater Tool that Doubles as a Green Roof!

by Anna Zakrisson on Thursday, August 4, 2022

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Purple-Roof is 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.

This video explains the ROI of a Purple-Roof system in 4 min. Please, contact us for the whitepaper!

When does the Purple-Roof Concept Make Sense for your Project?

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.

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.

How the Purple-Roof Concept Differs from a Blue-Green Concept

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.

Four common green roof approaches

Four common green roof approaches. From: Jeffers, Scott, et al. "Insights into green roof modeling using SWMM LID controls for detention-based designs." Journal of Water Management Modeling (2022).

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 – 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.

How does the Purple-Roof Concept Detention Layer Work?

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” (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 – 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.

This short video shows the hydrology behind the Purple-Roof concept

Purple-Roof concept example. The concept can be sized according to project needs.

In addition, the capillary forces of the detention layer suck the root protection layer dry as the system empties, avoiding standing water.

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 academic publications.

In this short video Dr. Anna Zakrisson from Green Roof Diagnostics (GRD) shows the detention layer in detail. The Purple-Roof concept has been researched and tested by GRD.

Please, contact us for whitepapers and more information!