A blog by IKO Polymeric... The number and location of rainwater drainage outlets for a flat roof system is calculated using BS EN 12056-3.

The calculation takes into account rainfall intensity, the roof form, and the use of the building. Outlets can be external downpipes served by gutters, or internal downpipes. National building regulations also set out requirements for roof drainage, with accompanying guidance.

The position and number of rainwater outlets should discharge water from the roof as quickly and efficiently as possible in order to avoid putting undue stress on the flat roof structure or the flat roof system. Depending on the designed roof falls, ponding water may be avoided by positioning outlets at points of maximum deflection.

Do green and blue roofs affect rainwater outlet positioning?

Where the flow of rainwater on the flat roof is designed to support a planted green roof, or assist the water attenuation of a blue roof, the rate of discharge should be controlled accordingly. However, it does not inherently change the position of outlets. Outlet manufacturers supply data for their products concerning capacity.

IKO Polymeric can provide a roof drainage calculation service for your project, and also offer rainwater outlets as part of our product range.