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 written 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

May 20, 2021 1:00 PM

Sasha Aguilera - Green Roof Consultant

Sasha Aguilera and Rodney Chaney 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

Rodney Chaney

3. Existing roof top technologies

4. Using biomimicry to re-engineer vegetated roofing system

5. Collaboration: Landscape Architects, Architects, Civil Engineers



Webinar Reviews

“Amazing presentation. Highly recommended you catching the next one.”  Peter McQuaid, President and Founder, MCQ Sales LLC 

“Great opportunity and a pleasure. “Thanks to the speakers for their invaluable knowledge.” Francisca Tapia Jr Sustainability Consultant, ABUD

“Excellent seminar” Dan DiMucci, Pennoni Sr. Consultant, RLA

“Exceptional resources for integrating green roof design with storm water management strategies.  Interactive….a lot of valuable design information.” Meg Needle Lord Aeck Sargent, AIA Atlanta

“The presentation was highly informative, and I personally learned a lot. I found the design tools particularly helpful.” Ganesh Nayak, President, Metier Consulting

Stormwater Capture Co Your Green Roof Specialist logo over an image of a green roof on a condo in an urban city.

Ready to Start Your Next Green Roof Design? Contact us!

With proper planning any rooftop – whether industrial, commercial or residential – can be transformed into a useful, green space. Stormwater Capture Co is a one-stop-shop for green roof systems and technical support.  We supply a range of  systems from lightweight and biodiverse, to detention green roofs.  We have custom green roofs that meet specific stormwater management and biodiversity goals. Contact us to transform your roof. Email or call (855) 786-7626 for stormwater calculations specific to your project.

Urbanscape Detention logo by Knauf Insulation

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

As a Purple-Roof partner, Stormwater Capture Co has access to the latest green roof stormwater test data and related innovative hydrologic modeling tool. Email or call (855) 786-7626 for stormwater calculations specific to your project.

Testing, Monitoring and Research by Green Roof Diagnostics

Unbiased, scientific research for the green infrastructure industry.