Use the chart below to figure out how many extra downspouts you need.
Gutter downspout requirements.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
Allow 1 square inch of downspout cross section for every 100 square feet of roof area.
Size downspouts to fit the gutter.
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Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
As with gutters a downspout s capacity must match or exceed the expected runoff.
From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used.
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Poorly designed or installed gutters and downspouts can cause rainwater to pool around the base of the home or back up into the gutters.
Place downspouts at least 20 feet apart but no more than 50 feet apart.
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.
The number of downspouts will depend on the cross section dimension of the downspout material.
Divide 1600 sq ft 149 sq m by 160 sq ft sq in 2 3 sq m 100 sq mm to determine that each downspout should have a minimum area of 10 sq in 6470 sq mm.
Doubling the number of downspouts halves the required capacity for each.