Detention Roof Rotterdam.

Rotterdam, Nederland

This is an example case of a Detention Roof project in Rotterdam, the Netherlands. Below you can read with which type of rain shower has been calculated and what the results are of the application of the Detention Roof. Within this calculation, all water absorbing components are fully water saturated.

Roof slope

Roof surface

3,412 m2

Maximum permissible roof load

110 kg/m2

Surface Detention Roof

2,953 m2

Maximum roof load Detention Roof

80 kg/m2

Other roof area (vegetation-free zones)

459 m2

Type of roof drains

Pluvia-system ø 100 mm

Number of roof drains

45 pcs


Type of rainstorm.

For this water calculation, rainstorm R10 has been used. Due to the current climate change, calculating with R8 showers is no longer sufficient for the current sewer system. During a rain shower R10, 35.7 mm of rain water falls in 45 minutes. The maximum permitted run-off speed on the sewer system is 2.78 litres per second per hectare. When a Detention Roof is applied the calculated peak run-off falls within this standard.



Calculated peak run-off of Detention Roof

2.69 litres per second per hectare

Calculated peak run-off of vegetation-free roof

260.62 litre per second per hectare

Peak run-off reduction

In this case the Detention Roof ensures a peak run-off reduction of 99%

Average delay time

1,225 minutes - this means that after 1,225 minutes 50% of the total amount of rainwater has been drained

Time frame between the end of the rain storm and the detention of rainwater

After 66 hours, the roof stops draining the rainwater

How many mm has been detained?

61 mm is temporarily stored and slowed down

How many mm is retained?

35 mm is absorbed in the Detention Roof


Detention is (temporarily) storing water, which is ultimately (delayed) drained. Retention is when the water is absorbed in the green roof layers and eventually is returned to the atmosphere in the form of vapor by the plants (transpiration) and the soil (evaporation). The Urbanscape Green Roll and the Sempergreen Sedum-mix blanket are water-absorbing components. The Detention layer and the Storage layer are water-detaining components.

Water calculation for your project

Would you like to know how a Detention Roof can contribute to your project? Request a water calculation.

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