Gutter Downspout Requirements

Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

Gutter downspout requirements.

Welcome to gutter supply your online reference for gutters gutter machines gutter guards and accessories. Use the chart below to figure out how many extra downspouts you need. The number of downspouts will depend on the cross section dimension of the downspout material. Come visit our selection of quality products.

The above recommendations assume that you have properly sized downspouts every 40 feet. From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used. Gutters and downspouts need to be sized to accommodate anticipated water loads. Allow 1 square inch of downspout cross section for every 100 square feet of roof area.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. From table 1 3 it is found that a4 in 102 mm downspout is required. As with gutters a downspout s capacity must match or exceed the expected runoff. Poorly designed or installed gutters and downspouts can cause rainwater to pool around the base of the home or back up into the gutters.

A general rule of thumb is one downspout per 20 feet of guttering but more may be necessary depending on the local weather the size of the drainage area and the design of the gutter system. Doubling the number of downspouts halves the required capacity for each.