Green Roof Detention

The colorful blob above represents a rainstorm on a weather radar map. As the storm passes over a location, rainfall increases toward the center of the storm. The hydrograph at left illustrates this.

Retention effectively reduces runoff

... if starting fairly dry.

Retention is somewhat effective if not too wet.

Retention is ineffective once saturated.

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Detention is a delay of water.

Water still flows off, just later.

Detention is very reliable!

This example of detention

  • delays runoff

But reduction of intensity is also critical.

This example of detention

  • delays runoff, and
  • reduces peak intensity

This example of detention

  • delays runoff,
  • reduces peak intensity, and
  • reduces overall volume.

Purple-Roof+ combines detention and retention to achieve these cumulative benefits.

And it works every time!




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.

Purple-Roof is a retention-oriented green roof, providing the highest retention available.

Purple-Roof+ is a detention-and-retention green roof, and is currently the only true green roof (vs. blue-green hybrid) 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

Purple-Roof+ is not your only option for detention in combination with a green roof. Purple-Roof+ is currently the only green roof (i.e. not hybrid) that offers detention. Hybrid options are generally characterized as "blue-green", which is usually 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. Purple-Roof+ is also generally less expensive than a blue-green roof.

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 Purple-Roof+, sometimes the original Purple-Roof might be best. 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.