Purple-Roof Detention Explained

by Oscar Warmerdam on Thursday, April 4, 2019 updated Wednesday, June 23, 2021

Purple-Roof is a detention-based green roof recipe that also excels in retention.

Detention + Retention = True Stormwater Management.

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This is how Purple-Roof works:

Retention explained

Retention is water absorbed by a green roof through micropore capillary action. In lay-terms: retention is like water held by a sponge. This water is held in place, and the only way it can escape is through UPWARD via evapotranspiration.

Evapotranspiration is the combined effect of evaporation and plant transpiration meaning that the water becomes water vapor, i.e., the sponge dries out.

Water is either retained or not. Either it stays in the sponge, or it leaves as runoff. Any water leaving as runoff is not retained but DETAINED.

What is green roof detention?

Detention is stormwater that is in excess of retention. Detention deals with the runoff.

The goal of a detention-oriented green roof is to delay peak runoff so that sewage systems have time to drain. It is this delay that what saves us from floods. Thus, detention has a time component added.

Almost all of the rain that falls onto a fully saturated profile will quickly drain through and flow underneath a traditional green roof profile. The water will run to the drain in such a short time frame that a traditional green roof cannot claim to have much more than 10-30 seconds of DETENTION TIME.

Runoff is DOWNWARD water movement in comparison to the UPWARD movement of retained water.

It is accurate and fair to say that in the purest form of the word green roofs, once fully saturated, do not offer much Detention (time).

How does green roof detention work?

There are micropores and macropores in a green roof profile.

When a roof is fully saturated, the micropores are full due to capillary action, and when they are full, it may take 7-14 days to empty out once again. This is what makes a green roof a good volume reducer, but it is also the reason that a green roof rarely Retains more than 50% on an annual basis.

It is also the reason that civil engineers do not think that green roofs are a primary stormwater management tool. Green roofs are not a reliable stormwater peak flow delay and reduction tools that will function EVERY time, simply because the 2nd-day storm or the XL storm overwhelms the system almost instantly.

To clarify: a saturated soil is not 100% filled with water. Instead, it has a maximum water table within the soil profile; that is what makes it ‘saturated.’ For instance, in a 10cm (4 inches) soil column on a green roof, the bottom 2.5cm (1 inch) is soaking wet, and the top 7.5cm (3 inches) is very moist. This is a huge difference! In this scenario, the water table is at 2.5cm (1 inch). Only this section is truly saturated.

In a normal green roof profile, the MACROPORES and the drainage layer, which is also a macropore-filled space, are never filled up to full capacity. Water simply falls out of these pores as they are too big to hold onto the water. Also, the drainage layer is an optimized water facilitator, not a restrictor.

How did Purple-Roof solve this?

With the Purple-Roof concept, we replace the traditional drainage layer with a Detention Layer to restrict the outflow speed in the drainage zone. This allows water to back up into the green roof profile base. First, we completely fill up the drainage layer, and then as pressure resistance builds up, we also raise the water table from 2.5cm (1 inch) all the way up to 10+cm (4 inches).

The water amount that is temporarily added into the profile is easily 4-5cm (+/- 2 inches) extra water content. If you want to store even more you can do so by changing the profile.

Then, as the storm rages over the property, this water finds a storage space in the macropores where it can reside for 12-360 minutes, depending on slope and distance calculations. As the Purple-Roof Detention layer slowly empties, the water level in the soil starts to lower from the peak of 10cm (4 inches) back down to 2.5cm (1 inch), and the Detention layer itself will drain empty. This creates an additional long-tail effect.

As a result, our Purple-Roof System can absorb the peak of the storm, temporarily store it, release it slowly, and repeat it 2-3 hours later, over and over and over again.

Please, contact us if you have any questions or would like to discuss specs or projects or perhaps look at some monitoring data.
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Also, check out our article on green roof ROI: Green Roof ROI - What's the Deal?

Reading tip - green roof hydrological performance:

Stovin, Virginia, et al. "The influence of substrate and vegetation configuration on green roof hydrological performance." Ecological Engineering85 (2015): 159-172.

Read more about stormwater retention and detention in Living Architecture Monitor: Untangling Stormwater Retention and Detention by Brad Garner.