Roof Area Per Downspout

Max area of roof draining to one down.

Roof area per downspout.

Roof area square feet per linear ft of gutter underneath. 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. Secondary cause of gutters not working properly is improperly maintained or failing. This wizard makes it easy to calculate the roof design area by providing various shape calculations and the calculation for vertical walls area per international plumbing code ipc.

A large roof is going to move a lot of water which means the downspout locations may need to be adjust to compensate. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Large gutters can hold more water. Up to 6 12 7 12 8 12.

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. Rainfall intensity should always be checked with the local authorities. Gutters come in many sizes. The table is based on rainfall intensity 100 mm per hour 4 inches per hour.

Doubling the number of downspouts halves the required capacity for each. The more roof surface above the gutters the more rainwater will be caught and funneled down into those gutters.