What Is the Minimum Slope for Flat Roofs?

Jack Burton

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.

A flat roof has no wasted space as this peaked roof does.

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.