Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
Gutter downspout requirements.
Place downspouts at least 20 feet apart but no more than 50 feet apart.
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Gutters and downspouts need to be sized to accommodate anticipated water loads.
The above recommendations assume that you have properly sized downspouts every 40 feet.
A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.
Poorly designed or installed gutters and downspouts can cause rainwater to pool around the base of the home or back up into the gutters.
Allow 1 square inch of downspout cross section for every 100 square feet of roof area.
The number of downspouts will depend on the cross section dimension of the downspout material.
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Size downspouts to fit the gutter.
From table 1 3 it is found that a4 in 102 mm downspout is required.
Doubling the number of downspouts halves the required capacity for each.
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.