The solution spells DETENTION.

Green roofs are generally an excellent stormwater run-off volume reduction tool. Reducing the volume of stormwater run-off has huge benefits on a city scale as less water must be processed by sewage or stormwater treatment plants. This results in substantial savings for the taxpayers.

RETENTION is defined as water that stays on the roof and never becomes runoff. Retained water only leaves as evapotranspired vapour and never as runoff. Retention is great as it re-establishes the natural water cycle which has been interrupted by intensive urbanization. In the urban areas the rainwater does not infiltrate into the ground, but drains into the sewage systems. This is why many progressive cities have regulations in place that promote green roofs.

Nonetheless, stormwater professionals still do not choose to put a green roof on top of every roof they encounter. The reason for this is that the average green roof design does not detain water sufficiently. Retention does not work if the roof is already saturated which can happen during extreme storms, storms that carry on for several days or if the weather conditions do not allow for effective evapotranspiration. In these conditions, the green roof works like a cup which is full… and the water runs off.

So what to do? The solution spells DETENTION.

The trick is to slow down the flow of water when “the cup is full” which usually happens during heavy and intense storms. Such storms cause flooding, pollution and river stream erosion (and headaches for the stormwater professionals).

Detention delays the outflow of the water from the roof and thus reduces the peak volume of water entering the sewers around the building. The water is kept on the roof for a bit longer before it is allowed to run off. Detention buys time which in this case is the critical factor.

At Urbanscape®, we offer a new line of Urbanscape® Detention Green Roof Systems featuring special detention layers functioning as a barrier to slow down run-off. There are several alternatives currently available on the market depending on the types of “super storms” specific to individual countries, regions or even municipalities.

Urbanscape® Detention Green Roof System
This set-up of the Urbanscape® Green Roof System is an optimal solution for intense yet short storms with high rainwater volumes. The detention layer with increased friction will significantly slow down the water run-off.

During the storm, the water table in a green roof will rise and fill all the air space in a full green roof assembly. After the storm ends, the excess water in the system will slowly run off and enter the sewage or stormwater system, leaving the green roof assembly fully saturated to the maximum retention capacity. The system is upgraded by using a special Urbanscape® Honeycomb water storage put on top of the Urbanscape® Detention Layer to become a superior detention and retention tool for longer-lasting and intense storms.

Evaluation of performance
To assist stormwater professionals, we use sophisticated green roof modelling software that allows us to simulate the real retention and detention performances of green roofs for specific areas and specific buildings.

Detention calculation done in Storm-water lab

Retention calculation done with P.E.T. Software

Urbanscape Detention Green Roof System @ Knauf Insulation Experience Center, Škofja Loka, Slovenia