What Is the Minimum Slope for Flat Roofs?

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A flat roof has no wasted space as this peaked roof does.
A flat roof has no wasted space as this peaked roof does. (Image: Roof image by Gonçalo Carreira from Fotolia.com)

Flat roofs are a common architecture feature in homes of all styles. From a simple lean-to to a multi-million dollar mansion a flat roof is attractive and functional. Flat roofs are easier to install, cost less, and have fewer maintenance problems.

No Flat Roofs

Technically, very few roofs are actually flat. The greatest problem with a completely flat roof is drainage. Water puddles on the roof, which increases the risk of a leak developing. Snow stays on a flat roof longer because it has no way to slide off.

Low-Slope Roof

The flat roof look is achieved with what is called a low-slope roof. According to the American Society for Testing and Materials Standard E 1918-97, a low-slope roof has a maximum slope of 2 inches "rise" for 12 inches "run."

Example of Rise

A low-slope roof that is 30 feet long would be 60 inches taller at the peak than the bottom edge of the roof. This allows proper drainage.

Hiring a Contractor

Flat roofs are more complex for waterproofing. Carefully check a builder's credentials, understanding of flat roof design and experience installing them.

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