How much drop should a flat roof have to shed water?

Written by sarah sweetman
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How much drop should a flat roof have to shed water?
Flat roofs are increasingly common, but it is important to plan how water will drain. (Getty Images)

The slope of your flat roof is essential to making sure water drains quickly and smoothly. Design considerations for the overall aesthetic of the building must be balanced with this functional need. A drop of 1/4 inch every 12 inches (or 1/4:12) should be fine for most structures. This means that for every 12 inches of roof, there should be a drop of 1/4 inch.

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Adequate slope for drainage

The slope of your flat roof is essential to making sure water drains quickly and smoothly. Design considerations for the overall aesthetic of the building must be balanced with this functional need. A drop of 1 cm every 48 cm (or 1 inch every 48 inches), equal to a ratio of 1 in 48, should be fine for most structures. This means that for every 48 cm of roof, there should be a drop of 1 cm.

Extra insurance

Some builders use a ratio of 1 in 40, meaning that for every 40 cm of roof span there is a drop of 1 cm in the pitch of the roof (or for 40 inches of roof span, a 1 inch drop). This is usually done as a means of adding extra insurance to roof drainage. It is more than is necessary to give adequate drainage, however.

Bottom line

The general standard for appropriate flat roof slope is 1 cm every 48 cm (1 inch every 48 inches). This slope will allow for adequate drainage of water from the roof. Do be certain to plan how your roof will shed water, for example into gutters or built drains, as slope alone is not enough.

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