What is Green Roof Retention and Why is it Important?

by Dr. Anna Zakrisson on Thursday, March 12, 2020

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Green roofs: the difference between retention and detention

There are so many reasons to put a green roof on your building, but the most common by far is stormwater management. There are two aspects to this stormwater management: retention and detention.

In the video above, we focus on retention as it is important to understand this concept before we move onto detention.

n short, we can describe retention and detention as follows:

  • Retention: water captured by the green roof that only LEAVES AS VAPOR.
  • Detention: water that is TEMPORARILY held by the green roof that flows out at A LATER TIME.

Notice that the main difference is how the water leaves. Retained water leaves up as vapor and detained water FLOWS OUT or down.

How to visualize green roof retention

We can think of retention capacity like water in a cup or a sponge. In both cases, these items can hold water, but both have a LIMITED CAPACITY.

When you pour water into the cup, the cup retains water. Similarly, when you pour water onto a sponge, the sponge retains water. The water retained and doesn’t leave in any direction other than what evaporates and leaves as water vapor. No runoff is generated.

This is kind of like rain in miniature. When it rains, we fill up the green roof retention capacity and this LOWERS RUNOFF. And that’s great!

But if we add more water into the already full cup or sponge, we immediately generate runoff as the cup overflows and the sponge can not hold any additional water. Once retention capacity is full, runoff occurs at the same rate as rainfall.

This overflow/runoff is uncontrolled, and this is the situation that can result in flooding and damage. This is why a civil engineer needs additional stormwater management tools and cannot solely rely on a traditional green roof for stormwater management.

A detention green roof provides such stormwater management.

You can read more about the different detention roofs that are available on the market in the following article:
Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water.

This video above is a short animation showing the difference between a traditional green roof, a high-retention green roof (a "sponge roof"), and a Purple-Roof detention roof.

Evapotranspiration is the driver of retention

What drives green roof retention? Plants have pores called stomata on the undersides of leaves. When the plant breathes or transpires, this releases water vapor. Hence, plant roots draw water from the soil, and release water to the sky.

Evapotranspiration: the process of water escaping as vapor through evaporation + plant transpiration.

With this in mind, we are going to refine our original definition of retention to this:

Retention: water captured by the green roof that only leaves as vapor THROUGH EVAPOTRANSPIRATION

Thus, retention and evapotranspiration always represent THE SAME VOLUME. If you want to increase retention by 5L, you need to increase evapotranspiration by 5L.

If you want to INCREASE WATER RETENTION, you need to increase evapotranspiration.

Hence, it’s not possible to simply add more substrate or media to increase retention. You need to ensure that the plants have the capacity to evapotranspire the additionally stored water. This capacity varies a lot between different climates and seasons and is one of the reasons you should invest some time to ensure that your green roof is adapted to your local climate.

How to model green roof retention?

Green Roof Retention Modeler to investigate the most optimal green roof setup for your project based on local climate and structural constraints.

Let us know if your city has not yet been added to the modeler by sending us a quick message over the Purple-Roof CONTACT FORM.