What is the Difference Between Green Roof Retention and Detention?

by Dr. Anna Zakrisson on Wednesday, March 11, 2020

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What is the difference between green roof retention and detention?

Green roof retention and detention are both hydrological terms, i.e., they deal with water. However, these terms are often confused which leads to unnecessary misunderstandings. This is a short article explaining the difference between green roof retention and detention.

What is green roof retention?

Green roof retention is defined as the water that never becomes runoff. This water is passively removed from the vegetated roof as the plants transpire and as water evaporates from the roof. Retained water is returned to the air as water vapor.

The combined process of evaporation and transpiration is called evapotranspiration. Evapotranspiration is the process by which green roofs can cool buildings and how vegetated roofs can reduce urban heat island effects.

Retention also reduces the volume of water that hits the streets. On average and in a temperate climate, a vegetated roof can reduce the annual volume of roof runoff by about 50%. That is a substantial volume that will not have to be treated by sewage treatment plants. This leads to significant taxpayer savings for a city.

Also, in many European regions, releasing runoff from a project into a sewer system might be costly, so reducing this volume by 50% might lead to significant savings for the individual project owner.

Why retention is not enough for stormwater management

However, this 50% is a statistical volume over the course of a year and does not mean that traditional green roofs are sufficient as stormwater management tools.

This 50% of the volume represents 80% of the storms, meaning that the remaining 50% of the volume comes down as high-intensity XL storms (the remaining 20% of the storms).

For this remaining volume, a traditional green roof cannot do much. Once the roof is wet, it becomes a pipeline. We can compare the wet green roof with a soaked bath sponge; it simply cannot absorb more water. Green Roof Diagnostics have tested soaked green roof profiles and can provide data on the behavior of soaked green roof profiles.

To deal with this remaining water, we need the process of detention.

What is green roof detention?

Green roof detention deals with the runoff, the remaining 50% of the volume that cannot be taken care of by using retention alone.

There are many different ways to create detention, but all of these methods create a delay in the outflow of the runoff.

This delay results in a delayed peak outflow and a reduced peak outflow volume. In lay terms, this means that instead of the storm dumping all of the water at the same time into the sewers and on the land, the water gets held back and slowly released so that the sewers have time to empty and the land infiltration capacity has time to recharge.

This is great for the environment as combined sewer overflows are prevented.

Detention green roofs are also beneficial for your wallet, especially if you live in an urban area where land is expensive as you can ditch old and outdated grey infrastructure such as tanks and cisterns in favor of modern, resilient green infrastructure that comes with a long list of secondary benefits.

Purple-Roof is a friction-based detention green roof. CONTACT us if you have any questions!

Find out if a detention roof is a good fit for your project in this article:
Does a Detention Roof Make Sense for Your Project?