Flat Roof Drainage Design

Size vertical storm drainage piping.

Flat roof drainage design.

Flat roof drains design and installation are very critical. The one challenge with flat roofing is ensuring proper drainage. A rooftop garden or additional outside living space are the top reasons people choose flat roofs. Select number of roof drains and calculate the roof area sloped to each drain.

Most draining systems on flat roofs have a poor design or are not correctly installed. Poor drainage can lead to serious issues with flat roofing from leaks to structural damage. It is a requirement of building regulation part h that adequate provision is made for rainwater to be carried from the roof of the building. Underneath the basics flat roofs are usually constructed with a ceiling nailed to joists which support a deck usually wood in residential construction a vapor barrier insulation and a roof board.

Unlike other roofing types that are slanted or sloped to allow water to drain automatically flat roofs need a built in drainage system to help remove moisture and debris. This applies for rain falling vertically or if it falls at an angle when blown by the wind. The 3 most common drain systems of a flat roof flat roofs have unique architectural characteristics that make them desirable in many situations. Appendix b in the ipc provides rates of rainfall for various u s.

According to bs 6229 bs 8217 flat roofs should be designed with minimum falls of 1 40 to ensure a finished fall of 1 80 can be achieved allowing for any inaccuracies in the. From there the flat roof system you choose will determine the materials you apply. Cities based on inches per hour that may fall during a storm of one hour duration and a 100 year return period. Size horizontal storm drainage piping.

The ultimate draining system that will prevent a roof from flooding. So often we find that inner drains are high on the roof causing water to pool around them. In the case of flat roofs the catchment area is the same as the footprint area of the roof being drained. Carson dunlop associates explain in their sketch above left that some flat roof designs relying on scupper drains and downspouts provide a set of secondary roof drain scuppers located 2 above the roof membrane.