When Are Gutters Needed on a Flat Roof?

Flat Roof Pitch

• A pitch, or slope, of 1 inch rise to 4 feet of length is the practical minimum to drain a flat roof, but depressions will likely develop on large surfaces and cause water to pond. Australian researchers Martin and Tilley noted that increases in pitch up to 1 inch in 5 feet will drain 50 percent more water. Water that drains from flat roofs exits through internal pipes or through scuppers that empty into gutters or downspouts.

Internal Drains

• Flat roofs typically have drains on the surface that are connected to internal downpipes that lead to the ground or to storm sewers. The width and number of these drains depend on the size of the roof and local rainfall. Internal drains are usually located between structural roof supports that produce slight slopes necessary to guide water to low points. When two low points meet, they are called valleys. Areas called crickets and saddles help guide water to drains located in valleys. Internal drains are generally located several feet from walls and roof edges. A good internal drainage system should drain a small flat roof in 15 minutes or less.

Scuppers

• When a flat roof is flanked by a rail or low wall, the slight slope of the roof guides water into scuppers, holes at the base of the rail. Scuppers empty into downspouts or gutters that lead to downspouts.

Flat Roof Gutters

• If a gutter helps drain a roof that rises to 2 inches in 1 foot -- the definition of a flat roof -- it should be located 1 inch below the edge of the roof. A gutter should have a slope of 1/16-inch to 1 foot or greater to flush out leaves and debris that wash off the roof. The bottom of the gutter should be wider than the downspout.