HOW A VEGETATED GREEN ROOF CAN ACHIEVE MORE DETENTION: “A new technology involves replacing the traditional fast-draining drainage layer with a ‘friction’ or ‘detention’ layer to ensure that during large storms, runoff rates are lower than rainfall,” stated Sasha Aguilera (November 2019)

Green roofs are social places (photo credit: Wilkinson, Brown & Ghosh)

Reinventing the Traditional Vegetated Roof for Detention

In November 2019, Construction Canada published a featured article by Sasha Aguilera and Brad Garner which is titled Reinventing the traditional vegetated roof for detention. The essence of their co-authored article is that:

“Traditional vegetated roofs achieve retention by holding a certain amount of rainfall onsite. After they are fully saturated and retention is maximized, additional rainfall drains through as quickly as it falls. Now, a detention layer in vegetated roof systems helps manage the excess stormwater runoff onsite by mimicking friction found in watersheds and aquifers.”

Friction-detention technology offers uniformity and redundancy, explain the authors. “Detention is activated by the friction-detention layer’s own large surface rather than relying on a single-flow restrictor at a roof drain (which can fail or clog),” they write.

About the Authors

Based in Toronto, Sasha Aguilera, B.Arch, GRP, is design consultant for Next Level Stormwater Management. More than a decade as a designer and technical consultant on various vegetated roofing projects across the country has established Aguilera as one of Canada’s foremost green roof professionals. She was honoured with the F. Ross Browne Award in 2017, named for the late Construction Canada columnist. It is awarded annually to the author of an exemplary article that appears in their magazine.

Brad Garner is a stormwater researcher at Green Roof Diagnostics, where he blends his experience as a landscape architect with his software engineering skills. Garner’s custom software aggregates and manages data to evaluate solutions for managing urban stormwater. “I like solving problems, organizing things, and the challenge of making the complex appear simple. I like getting stuff done! I’m drawn to the web’s infinite scalability and interconnectedness that fits in your pocket,” says Brad Garner.

To Learn More:

To read the complete article by Sasha Aguilera and Brad Garner, download a copy of Reinventing the traditional vegetated roof for detention. The following image is reproduced from their article to encourage the interested reader to take the time to read the article and understand how detention-friction technology works.

The macropores of the detention layer profile continue to fill up contrary to the other two profiles. Some runoff is generated, but at much lower rates than the rainfall. The reason for this is that the detention layer is restricting the maximum flow rate, causing some backup in the profile. Runoff will have the same rates and rainfall until full saturation is achieved on the green roof. The detention profile then slowly drains out, thereby delaying the peak runoff volume of the rainfall.

Watch the simulation video on YouTube:

And visit Sasha’s blog at