Are Green Roofs True Stormwater Management Tools?

by Oscar Warmerdam on Tuesday, January 15, 2019 updated Tuesday, February 22, 2022

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Why green roof retention is great for stormwater management on a city scale!

Green roofs are often sold as stormwater management tools. And that is true: green roofs do manage stormwater but mostly as a volume reduction tool. This volume reduction has huge benefits on a city scale. The benefactors are the taxpayers, citizens of what is now a city with cleaner water, and cleaner air.

This is why many progressive cities have regulations in place that promote green roofs.

But, if that’s the case… If green roofs are an excellent volume reduction tool on a city scale, why don’t civil engineers put one on top of every roof they can? The answer is simple but will be a surprise to most: most green roofs do not detain water well enough. Let us explain...

Academic research of most cities in Europe and North America states that a green roof offers a water volume reduction of around 50% in most climates. The roofs are very efficient in reducing the mean annual volume of mostly smaller routine storms. They do this through retention.

Retention is great! Retention cools the city, waters the roof vegetation, minimizes the amount of water that the city needs to clean up, and thereby saves substantial sums of money for the taxpayers. Also, any water that is retained is less likely to cause sewer overflows in case of a storm. Routine rain events are most often completely retained by a green roof.

But, wait a second! What about that other 50%? Why can the green roof retain only 50% of the water? What happens to the rest and how can we improve it?

Why retention is unreliable for stormwater management at the building scale.

Retention can be unreliable because it doesn’t work in repeat storms, large storms, or during cool and cloudy weather. Why? Water that is retained, by definition, leaves the green roof only via evapotranspiration. If there is no time for evaporation, such as during big storms or repeat storms, or if weather conditions do not permit much evaporation, then the retention-oriented green roof sits there like a wet sponge.

2^nd-day storms may not be substantial enough on their own to cause trouble but may cause huge problems when combined with the precipitation from the first-day storm. Extra-large storms also often exceed the capacity of the green roof as they outflow with little or no delay. These types of extreme storms happen only a few times a year (5-7), but it is precisely these storms that cause flooding, pollution, and river stream erosion.

You cannot practically retain this second 50% with a green roof using retention only. You can only detain and temporarily store the second 50% and release it over time. And I am sorry to say… standard green roofs do not really detain water.

Any study that says so mislabels what is retention and what is true detention.

Now, detention-oriented solutions are appearing on the market, and we think this is promising. We believe that this is the first step in a substantial transformation of the entire green roof industry. Globally.

What is the difference between retention and detention?

Retention is like filling up a bucket with rain, and once it is full, the water can only escape out of the bucket upward in the form of evaporation (water released from the soil) and transpiration (water released from the plant). The combined effect of evaporation and transpiration is called evapotranspiration (ET).

Once evapotranspiration has taken place, more rain fits into the bucket. Detention, however, is any water that spills over the bucket, and which goes down to the drain.

Thus, the key to our problem is the element of time. How long does it take once spilled to travel to the drain?

The confusion in the green roof world, and in academia, is when “filling up the bucket” during a rainstorm is labeled as “detention.” This is simply not correct: while it is raining, the time that it takes for a relatively dry roof to become saturated is often considered detention. But it is not, the time it takes to fill up the bucket is merely the rain filling up the bucket to maximize retention. Once the bucket is full and overspills, that is the moment detention starts on a green roof.

Unfortunately, roofs are designed to facilitate water to flow off the roof in an expedited fashion. Time is minimized instead of maximized. Traditional green roofs do not detain stormwater to a substantial degree.

This is the reason that a civil engineer at a building scale still requires a second detention device in the form of a tank, a sand filter, or bioretention. These work by holding the water for a limited amount of time, but every drop of detained water is indeed released, just a little later. Unlike a green roof, these solutions work every time, and this is why they are the preferred choice for a civil engineer at the building scale.

Green roof detention explained

Detention is the process of absorbing or holding water during a storm, then slowly releasing it. Unlike retention, detention works regardless of the weather. When a detention-oriented system drains, it recharges. So, a detention-oriented green roof can recharge quickly, and be ready for the next storm, or the tail end of a large storm. Even in dark and cloudy weather! Retention cannot do this, retention takes weeks to re-charge, and it is good otherwise the plants would not have enough water.

Detention slows the flow of water, so that the hydrologic characteristics more closely resemble that of a forest, with peak runoff being delayed from peak rainfall.

This is how natural detention works:
In a watershed, once the soil is saturated, water starts to flow downhill. The time it takes to flow to the river, or lake, is determined by the following three factors:

The distance from the high to the low point, zig-zags preferred.
The angle of the slope
The lateral friction through vegetation and land-based bumps, rocks, and other imperfections.

In a traditional green roof, the slope and distance to the drain are fixed. Also, because the water flows through a green roof soil profile onto the smooth, sloped deck almost effortlessly, unlike in nature the water doesn’t get to experience any lateral friction in the drainage layer.

Adding detention to a green roof

Several companies have been researching and developing detention-oriented green roofs over the past few years. Some store water in a reservoir at the bottom of a green roof module and use smart controls to slowly release the water through valves.

Other products, similar to the above, have a built-in-place assembly; these systems use plastic supports to elevate the green roof, and then store water in a reservoir below the green roof, above the membrane, and then slowly release this water via roof drain flow restrictors.

Our detention-oriented concept is called Purple-Roof. It is a non-proprietary specification and not a product.

The way we added detention is by creating resistance to horizontal flow inside the drainage layer at the bottom of the profile. Not like mineral wool does since mineral wool basically has no horizontal flow. This is all below the mineral wool and basically works similar to an aquifer: water enters and flows slowly, but as it builds up the resistance increases. Eventually, the water temporarily rises into the soil profile, which offers real-time detention and, as a bonus, detention offers increased retention. It never clogs and will continue to flow after the storm and thus gets ready for the next event.

This is quite similar to what happens in a rain garden or during bioretention.

Purple-Roof does all this while addressing everything else that is important about a green roof, such as plant health and good soil. Also, remember we are adding detention without sacrificing retention.

What about green roof ROI?

By adding detention on the roof as an addition to retention, you can avoid building expensive bioretention tanks and ponds. You free up valuable development space!

What if, instead of an overflow pond, you can build an additional Café or be able to park two more cars!

Well, you can see how this quickly will pay for your roof, and even create a profit for you! Once the green roof has a justifiable expense as it replaces alternative detention tools, the secondary benefits of a green roof come into play: doubling the life of the membrane, energy savings, increased property values, increased rents, hail damage protection, and of course the PR that bees and butterflies bring to the roof.

Contact us for more details!

Most of you reading this are experts in stormwater management, so we’re hoping this is different from what you’ve heard before.

We would be happy to have a chat about the components, specs, and details, look at some modeling, look at some platform monitoring, or whatever else you want to cover.

Just let us know! You can contact us using the contact form below.

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Reading tip: stormwater management

We recommend the research review: Akther, Musa, et al. "A review of green roof applications for managing urban stormwater in different climatic zones." Sustainability 10.8 (2018): 2864. for further reading on current research into stormwater management in different regions.

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