How Does the Purple-Roof Concept's Detention Layer Work?

by Anna Zakrisson on Tuesday, April 19, 2022 updated Thursday, August 4, 2022

A detention-based green/vegetated roof slows the outflow of water also when the green roof is soaked with water from a previous rainfall. This slowing down of water, or stormwater detention, is analogous to the functionality of a bioretention cell where water is collected and the peak outflow is reduced and delayed in order to prevent flooding damages. For the Purple-Roof concept, the detention layer is the "magic ingredient".

Do All Green Roofs Provide Detention?

Far from all green roofs have detention capacity, though this is often claimed due to a misconception and confusion around two concepts: retention and detention.
As it starts to rain and the green roof is dry or relatively dry and there is a lot of pore space still available in the roof profile. At this point there is no runoff – this is retention filling up and not a detention-based outflow delay.

The profile fills up and capillary forces hold the water in the profile micropores. The problem is the substrate macropores where the capillary forces are not strong enough to hold the water against the gravitational pull. Then, runoff begins due to these gravitational forces pulling on the water through substrate preferential flow paths.

Please note that it is essential to separate real retention from the results of the ASTM E2399, which is the standard used to assess the maximum load of a saturated profile. Using ASTM E2399 as a measure of retention is severely flawed and will lead to an overestimation of the retention capacity of the profile.

But back to detention! The million-dollar question is if the profile can delay the outflow of water when it is filled up like a soaked bath sponge, or does it become a pipeline with essentially no peak outflow reduction & delay? Also, even if there is a peak outflow reduction & delay, is the magnitude of these parameters enough to satisfy a stormwater engineer? A 5min peak outflow delay and 5% peak outflow reduction during a significant storm event such as a 100-year storm is insignificant as a flooding protection measure.

Many years of data collection and hydrologic modeling by Green Roof Diagnostics have revealed that traditional green roofs do not provide significant detention to act as a replacement for, e.g., a bioretention cell or a stormwater tank. Traditional green roofs are great at providing retention and many other tremendous secondary benefits, but they do not provide true detention, no matter what you might hear.

What is Detention, and Why is Friction so Important?

There are several detention-type green roof solutions on the market with various solutions to delay the outflow of water. Some use orifice restrictions, meaning that water is collected on the roof, and the drainage outflow points are set to a specific diameter resulting in a predictable outflow rate. There are also smart roofs on the market that will open or restrict the outflow depending on predicted storm intensities. Most of these solutions are Blue-Green roofs and need a completely flat deck to function, making these solutions a good fit for beautiful roof gardens.

The Purple-Roof concept is suitable for flat and sloped roofs and uses a friction-based layer to delay & reduce peak outflow. This friction-based layer is slightly analogous to the natural friction encountered in a watershed. Water follows the path along the sloped forest floor; some water is absorbed (retention), and some is delayed through the crisscrossing over mosses, sticks, and stones. Traditional green roofs have little to no friction as they are built to expedite water away as fast as possible, making it impossible to saturate the profile thoroughly using the macropore spaces.

How Does the Purple-Roof Concept Work?

As it begins to rain, the Purple-Roof concept profile slowly fills up. First, the water seeps into the soil substrate and then into the mineral wool. Due to the (toxin-free!) needled mineral wool, extra micropore space is available that ensures that more water can be held in the profile compared with substrate-only profiles. The mineral wool also helps reducing the preferential flow paths, thus increasing the retention capacity of the vegetated roof.

Underneath the mineral wool is a honeycomb-shaped void space that can be adjusted in height depending on project-specific needs. There is no horizontal flow of water between cells. Below the honeycomb detention reservoir, the water reaches the detention layer. This is where the detention magic happens.

Dr. Anna Zakrisson from Green Roof Diagnostics shows how the detention layer works

As the water seeps into the detention layer, it is met with many hundreds of thin threads per cm in the 5mm thick detention layer fabric. These threads break up any laminar flow; the resulting turbulence increases with increasing head pressure. The more water stored in the honeycomb detention reservoir, the more turbulence there is in the detention layer. In short: thread-induced friction results in turbulence, and this causes water to back up into the profile. The outflow is predictable and reliable and has been measured and modeled thoroughly by Green Roof Diagnostics.

Interestingly, the detention layer can also be used underneath pavers and gravel with full detention capacity. This means that the entire roof space often can be utilized for detention though perhaps only, 80% can be utilized for retention (the vegetated areas). 100% of a roof can rarely be vegetated often due to fire regulations and HVAC and other equipment.

Return on Investment

The concept can be sized for virtually any design storm and project and is already in use in the US and internationally as a replacement for at-grade stormwater solutions. This is particularly valuable in urban areas where land is expensive. The move from an at-grade stormwater solution to green roof-based detention can mean many $ saved and many $ made as the ground can now be used for income-generating activities instead of merely hosting a stormwater tank. Plus you get all the secondary benefits of a green roof.


Please, don't hesitate to contact us if you have any questions. Also, Green Roof Diagnostics can offer support in the stormwater permitting process in many cases. Don't hesitate to contact GRD HERE for European and North American requests.