Detention Roof Manchester.

Manchester, UK

This is an example case of a Detention Roof project in Manchester, UK. On the right you will find the details of the roof. Below you can read with which type of rainstorm 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

2 degree

Roof surface

956 sqm

Surface Detention Roof

856 sqm

Other roof area (vegetation-free zones)

100 sqm

Type of roof drains


Number of roof drains

7 pcs


Type of rainstorm.

For this water calculation, a rainstorm with intensity T=100 is used. In this rainstorm, 47.5 mm of rain falls in 60 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

2.73 litre per second per hectare

Calculated peak run-off of vegetation-free roof

238.01 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

1,393 minutes - this means that after 1,393 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 and 42 minutes, the roof stops draining the rainwaterand the detention

How many mm has been detained?

59 mm is temporarily stored and slowed down

How many mm is retained?

34 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.

Request here