Flat Roof Drainage Design

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.

Flat roof drainage design.

Size horizontal storm drainage piping. Roofs of this design will have one or more roof drains or scuppers installed. Most draining systems on flat roofs have a poor design or are not correctly installed. The downside to flat roofing however is that it s vulnerable to water damage in a way that other roofing solutions aren t.

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. There are three types of drain systems commonly used on flat roofs. Cities based on inches per hour that may fall during a storm of one hour duration and a 100 year return period. Flat roofs have a long history in the united states especially for commercial construction.

There are roof coating products available if there isn t enough slant or slope to accomplish this. Size vertical storm drainage piping. There are three types of drain systems on a flat roof. 50 000 4 400 11 36.

However flat roofs depend greatly on proper drainage of rain and snow melt to preserve their integrity. Poor drainage can lead to serious issues with flat roofing from leaks to structural damage. The one challenge with flat roofing is ensuring proper drainage. So often we find that inner drains are high on the roof causing water to pool around them.

From there the flat roof system you choose will determine the materials you apply. Select number of roof drains and calculate the roof area sloped to each drain. Or 12 drains required. Unlike sloped roofs flat roofs can t rely on gravity alone to prevent water from pooling on their surfaces.

Of course the best designed flat roof has the correct amount of pitch to drain the water off naturally. Appendix b in the ipc provides rates of rainfall for various u s. 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. The drainage requirements for flat roofs on buildings present some special problems that we describe and illustrate here.

Without it pooling water can occur and cause leaks deterioration in the roof membrane and ice formation during winter. It is very critical to position drains for maximum effect. At page top and at left our photos show a low slope roof surrounded by a parapet. Watch for clogs blockage at the roof drain or scupper.

Each of these systems has its own pros and cons. Take the roof s total square footage and divide by the total square footage handled by one drain.