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Benefits of a traditional green roof
Traditional vegetated roofs are excellent at capturing rainwater to reduce annual runoff volumes through their retention capacity. This reduction leads to reduced sewage volumes that have to be treated by sewage treatment plants and cools our cities.
Limitations of a traditional green roof
Nonetheless, a traditional vegetated is not a fully functional stormwater management tool as water cannot be absorbed when the roof is already wet such after XL- or repeat storms.
Thus, a traditional vegetated roof cannot be relied upon to meet the maximum allowable flowrates of a single project leading to a need for additional expensive stormwater solutions such as tanks or cisterns.
What can a detention green roof do?
Our goal was to make this grey infrastructure obsolete and provide all stormwater management on the roof.
Green Roof Diagnostics is an independent research organization and the home of the largest green roof laboratory in the world. The researchers at GRD have spent the past years researching two main topics: stormwater retention and detention.
Retention is the water that leaves the roof as vapor and Detention as dealing with the water that becomes runoff.
Firstly, they quantified vegetated roof retention through lysimeter measurements of several platforms over two years.
What they found was somewhat deviating from the standard industry claims: Only about 50% of the annual precipitation volume, representing 80% of the storm events is retained in an average temperate climate.
The remaining 20% of the storm events are not retained but left the roof as runoff.
The second main challenge was to quantify green roof Detention by testing profiles wet-to-wet. This means testing runoff from a vegetated roof that is already soaked such as after several days of rain or a big rainfall.
GRD found that the detention capacity of a wet green roof is negligible.
Together with German textile producer Browatech, GRD continued to research materials to find a way to turn a green roof into a stormwater management tool that would also function on sloped roofs.
The detention layer
A detention layer, or friction layer, replaces a traditional drainage layer providing a precisely engineered flow rate. This slows water and allows water to backfill layers above. This stored water drains out at a slow, predictable, and engineered pace.
Suddenly, the vegetated roof act as a proper stormwater management tool equivalent to a tank or a cistern. The best thing is that it is Detention on top of Retention. Best of both worlds so to speak.
With a detention roof, you can remove or reduce stormwater tanks and cisterns and use that space for development instead.
Contact us to learn more about detention roofs and how they can help you generate a return on investment (ROI) for your project.
The Retention Modeler
Also, take a look at our RETENTION MODELER, a free tool that allows you to model the most optimal live roof for your local climate. We should not build green roofs as one-size-fits-all. Instead, let’s build roofs that are resilient and low maintenance.
Our goal has been to develop tools that are easy to use and intuitive. It is important to be able to identify complexities and challenges early in the planning phase as this will save both $$ and headache down the line.
For a greener future!
Learn more about different types of detention roofs:
Blue Roofs, Blue-Green Roofs, and Purple Roofs all DETAIN water
Purple-Roof is a non-proprietary concept for an optimal green roof.
Learn more about green roofs as nature-based solutions and economic incentives:
Are Green Roofs Nature-Based Solutions?