Flat Roof Drainage Design

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Flat roof drainage design.

There are three types of drain systems on a flat roof. 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. 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.

The result is the number of drains needed. There are roof coating products available if there isn t enough slant or slope to accomplish this. This means they require a purpose built flat roof drainage system for the task. Flat roofs have a long history in the united states especially for commercial construction.

Cities based on inches per hour that may fall during a storm of one hour duration and a 100 year return period. The one challenge with flat roofing is ensuring proper drainage. Of course the best designed flat roof has the correct amount of pitch to drain the water off naturally. There are three types of drain systems commonly used on flat roofs.

Size horizontal storm drainage piping. Or 12 drains required. 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 construction. Without it pooling water can occur and cause leaks deterioration in the roof membrane and ice formation during winter.

The drainage requirements for flat roofs on buildings present some special problems that we describe and illustrate here. The downside to flat roofing however is that it s vulnerable to water damage in a way that other roofing solutions aren t. From there the flat roof system you choose will determine the materials you apply. 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. However flat roofs depend greatly on proper drainage of rain and snow melt to preserve their integrity. Appendix b in the ipc provides rates of rainfall for various u s. Take the roof s total square footage and divide by the total square footage handled by one drain.

Watch for clogs blockage at the roof drain or scupper. 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. Unlike sloped roofs flat roofs can t rely on gravity alone to prevent water from pooling on their surfaces.

Select number of roof drains and calculate the roof area sloped to each drain. Size vertical storm drainage piping.