The number of downspouts will depend on the cross section dimension of the downspout material.
Gutter downspout requirements.
From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used.
Size downspouts to fit the gutter.
Use the chart below to figure out how many extra downspouts you need.
Allow 1 square inch of downspout cross section for every 100 square feet of roof area.
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.
Poorly designed or installed gutters and downspouts can cause rainwater to pool around the base of the home or back up into the gutters.
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Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
The above recommendations assume that you have properly sized downspouts every 40 feet.
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.
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Gutters and downspouts need to be sized to accommodate anticipated water loads.
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.