Untangling Stormwater Retention and Detention

Defining Retention and Detention

The latest issue of Living Architecture Magazine offers an enlightening read on retention versus detention in green roofs.

Untangling Stormwater Retention and Detention is by landscape architect, software engineer and green infrastructure researcher Brad Garner.  Over the years, Brad has worked closely with Joshua Robinson, a hydrologic engineer and together they do testing, monitoring and research at Green Roof Diagnostics.

A More Consistent Understanding

Brad found that the use of retention and detention terminology is inconsistent among researchers, hydrologists, and civil engineers around the world. In his article, Brad spells out a more hydrologically consistent understanding of retention versus detention. Understanding the difference helps assess and improve green roof performance.

At Stormwater Capture Co, we offer webinars on this subject. (Register free below.)  Here, we provide the distinction between the two.

Retention image shows water flowing out of faucet and being retained in a sponge and not reaching the faucet.

Retained water stays in the sponge and never reaches the drain.

Retention Defined

Keeping something or continuing to have something.

On a green roof, retained water will eventually be taken up by the plants or evaporated back to the atmosphere. Retention is important for plant survival and annual runoff reduction.

The image on the right shows a sponge that is retaining water. A saturated sponge needs to be wrung before it can soak up water again. Similarly, a saturated green roof needs to dry out and recharge.

Retention depends on Mother Nature to dry between rainfalls. Therefore it is not effective in managing back-to-back storms.


Water coming out of the faucet is delayed by going through a funnel and spiral which delays the water from getting to the drain.

Detained water is delayed and reaches the drain later at a slower rate.

Detention Defined

Keeping something from proceeding temporarily or delaying something.

On a green roof, detained water is held up on the roof and is allowed to slowly come out via control flow drains or friction detention layers.

As the image on the right illustrates, detention delays and reduces peak flow. During a heavy storm, detention helps prevent overloading the storm sewers and flash floods.

Detention does not depend on Mother Nature and works particularly well with back-to-back storms.

The StormCap™+Detention green roof system is a reliable and quantifiable stormwater management tool.


Advanced Green Roof Systems with Retention and Detention

Both retention and detention strategies are required to alleviate the burden of heavy stormwater on cities’ infrastructure.  Green roofs with retention and detention technology are a reliable stormwater management tool.

Green roof systems such as StormCap™ and StormCap™+Detention are powered by Urbanscape® and tailored to meet unique stormwater management needs. StormCap™+Detention pictured on the left uses innovative friction detention technology based on the Purple-Roof concept. It is a reliable and quantifiable stormwater management tool that reduces or eliminates cisterns and frees up valuable space.



Learn more about Green Roofs with Retention and Detention

Dr. Karen Liu and James Weldon present a LACES-approved webinar (1PDH)

Engineering Vegetated Roofs to Optimize Stormwater Management.

Learning objectives include:

1.The distinction between stormwater retention and detention

2. The importance of detention in urban centers

3. Existing roof top technologies

4. Using biomimicry to re-engineer vegetated roofing system

5. Collaboration: Landscape Architects, Architects, Civil Engineers






More than 2M SQFT supplied coast to coast

Next Level Stormwater Management  is Canada’s leading supplier of pre-vegetated roofs such as LiteN’Less™, Xeroflor® and StormCap™ systems. We customize to local requirements and offer warranty and maintenance support. Our team provides analytical support such as ​stormwater calculations specific to location ​and building/roof type. As a Purple-Roof partner, Next Level Stormwater Management has access to the latest green roof stormwater test data and related innovative hydrologic modeling tool. Email or call (416) 637-5772 Ext 1 for your next project.

Urbanscape Detention logo by Knauf Insulation

The image conveys this system is powered by the Purple-Roof concept.


Unbiased, scientific research for the green infrastructure industry.