Rooftop stormwater management technologies: Climate change adaptation and resilience

Friction-detention green roofs

A friction-detention green roof consists of an enhanced retention green roof on top of a friction detention mat (Figure 3c). The retention of the upper green roof is enhanced by a mineral wool and an optional reservoir cell. The detention mat consists of thousands of fine vertical fibers sandwiched between two geotextiles. These fibers create friction to slow down runoff traveling to the drain to achieve detention. Water is backed up, filling up the reservoir layer and saturating the retention layer and growing medium.

The friction-detention mat allows small amounts of runoff to flow through unimpeded but slows down large volumes of runoff. As a result, runoff comes out at a slower rate over a longer time, which prevents overloading the storm sewers. The detention mat enables the system to manage back-to-back rainfall events and large intense storms regardless of antecedent weather conditions (i.e. even when the green roof is fully saturated). The water is released slowly within 24 hours and the system is ‘recharged’ for the next storm.

It is important to note because detention happens at the drainage level across the entire green roof, this avoids clogging issues associated with single-flow restrictors such as control flow drains. It also enables the system to be implemented on low-sloped roofs without losing efficiency and sloped roofs effectively. Lastly, the friction detention system is particularly economical on irregular shaped roofs where roof drains would require flow restrictors.

A successful friction-detention green roof design requires collaboration between several disciplines—architects, landscape architects, civil, and mechanical engineers—to provide project-specific details such as size, location, design storm, maximum allowable outflow rate, etc. Using this input data, a proprietary detention modeling program accurately predicts performance and calculates the appropriate green roof profile to meet detention and retention requirements of the project.

The research behind vegetated systems with friction-detention technology

Figure 4 compares the water retention and detention performance of three distinct green roof systems. Water fills and exists differently given a “traditional” system using simple drainage cups to expedite fast drainage, an “enhanced retention” system with a high performing water retention layer, and a “friction-detention” green roof with both retention and friction-detention layers. A short explanatory video can be viewed at:

Performance comparison of three rooftop stormwater management solutions – hydrographs.

Figure 5 illustrates the effectiveness of the friction-detention green roof by comparing hydrographs from the same three systems in an actual rain test.

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