Green Roof Detention

Detention is "flattening the curve" of stormwater management. A detention roof, such as Purple-Roof or a Blue-Green-Roof provides rooftop detention. Notice how this is different from stormwater retention, which only works when antecedent conditions are relatively dry.

We get a lot of questions about green roof detention. The graphic above attempts to explain the concept.

Green roofs typically provide stormwater retention, which is capturing water (preventing water from draining) and then allowing the water to leave only via evaporative processes. Retention is absolutely essential to green roofs, as retention provides water that sustains plants. But retention is not always effective. Why? In short, retention fills up and takes time to dry out.

The Sponge-Roof concept is a retention-oriented green roof, providing the highest retention available.

The Purple-Roof concept is a detention-and-retention green roof, and is currently the only true green roof concept (vs. the Blue-Green hybrid option) that offers detention.

Detention is slowing water down, but still allowing it to drain. Slowing drainage solves most stormwater problems. And by draining, Purple-Roof is ready for the next storm within hours.

Detention is extremely reliable! This is because detention is not weather-dependent. Detention-oriented approaches can handle significantly larger volumes than retention-oriented approaches.

The graph above is obviously a simplistic example. See the graph below for some actual test results. Testing is ongoing to examine a wide range of scenarios.

Green Roof Detention Test Results

Testing performed by Green Roof Diagnostics. Image courtesy of Green Roof Diagnostics.

The graph above follows the same general format as the graphic at the top of the page. Volume is the vertical axis. Time is the horizontal axis. Rain is blue. Runoff is the two other shapes. This graphic adds two dashed lines representating the volumetric water content of the test specimens.

The "yellow test" started off fairly dry, representating a condition after no rain for about a week. Notice that it produced runoff from about minute 48 to minute 60 of a 60-minute storm. A lot of this reduction in runoff is due to retention.

The "green test" started off fairly wet, representing a condition after no rain for about a day. Notice that it still delays runoff by about 30 minutes, reduces runoff peak intensity by almost 50%, and reduces overall runoff volume by about half.

Other Options

The Purple-Roof concept is not your only option for detention in combination with a green roof. The Blue-Green Roof concept is currently also offers detention, as Blue-Green Roofs consist of a green roof directly above a blue roof. These hybrid options may provide even greater detention!

Advantages of Purple-Roof are that it can be installed on a wide variety of roof slopes, sizes, and configurations, as site-specific constraints of a Blue-Green approach do not apply. E.g. a Blue-Green roof is generally only installed on a dead-flat roof, and may only make sense to install over a minimum roof area. The Purple-Roof concept is also generally less expensive than Blue-Green options.

We think it is important to have options so that each project can achieve the maximum benefits possible. Sometimes Blue-Green might be best, sometimes the Purple-Roof concept, sometimes a high-retention Sponge-Roof might be the best concept. Several Purple-Roof partners also offer Blue-Green solutions.

Incorporating Green Roof Detention

How can green roof stormwater detention help your project? And what other information would you need from us?

Most projects already have to comply with stormwater regulations that require a detention approach; green roofs just haven't been part of the equation. So incorporation of a detention-oriented green roof may be quite easy.

Contact us for assistance for your project.