Roof Area Per Downspout

The roof plan area that is drained by each downspout is plan area 40 x 40 1600 sf given the area factor b in table 10c the design area is design area plan area x b 1600 x 1 05 1680 sf from table 10a column b2 the area drained per square inch of downspout is 130 sf.

Roof area per downspout.

A large roof is going to move a lot of water which means the downspout locations may need to be adjust to compensate. A high slope moves water more quickly and if gutters have trouble dealing with a lot of water in a short period of time an extra downspout can alleviate that issue. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Gutters come in many sizes.

Gutters and downspouts are vulnerable for obstructions caused branches and leafs from surrounding trees and similar. The table is based on rainfall intensity 100 mm per hour 4 inches per hour. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Large gutters can hold more water.

Smacna s new downspout and gutter sizing calculator enables the user to manually enter the design area of the roof or use the design area wizard for the calculation. Rainfall intensity should always be checked with the local authorities. Secondary cause of gutters not working properly is improperly maintained or failing. Roof area square feet per linear ft of gutter underneath.

The more roof surface above the gutters the more rainwater will be caught and funneled down into those gutters. Up to 6 12 7 12 8 12.