Nrcs Gutter And Downspout Sizing

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.

Nrcs gutter and downspout sizing.

Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. The gutter and downspout size requirement depends upon the number of sections and length of the gutter sections as well as the number of downspouts. As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter. Doubling the number of downspouts halves the required capacity for each.

Downspout sizes must not exceed the bottom width of the gutter. Size downspouts to fit the gutter. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. But houses with big steep roofs or those located in climates prone to heavy downpours may need wider gutters and extra downspouts to keep rainwater from overflowing.

Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Five inch k style gutters or 6 inch half rounds the most common residential sizes are able to handle the rainfall on most houses in most parts of the country. Some trimming of the flange on the preformed outlet may be required. However 7 and 8 inches are available although the larger sizes are more suitable for larger buildings or business properties.

The sizes available in this style are 3 4 5 and 6. A 3 x 4 downspout can also fit. Adding additional sections of gutter reduces the length of each section and will reduce the gutter size. Also the gutter width in relation to height may also be adjusted.

Also please adjust the gutter width relation to height as some width height ratios will not be compatible with standard gutter sizes and the calculator will recommend a custom gutter. As with gutters a downspout s capacity must match or exceed the expected runoff. Gutter professionals have experience matching the gutters and the downspouts. In sizing downspouts the following considerations apply.

Round downspouts come in various sizes and colors and mostly used with half round gutters. Please lower the maximum spacing between downspouts to see this effect. Downspouts of less than 7 00 sq in 4515 sq mm cross section should not be used except for small areas such as porches and canopies. To limit the effects of thermal expansion in gutters 50 ft.

A 6 inch top width k style gutter can receive a 3 x 4 downspout. Types of gutter downspouts. Typically a 5 inch top width k style gutter can receive a 2 x 3 downspout. Use the chart below to figure out how many extra downspouts you need.

Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.