Sizing Of Roof Downspouts

The size of the downspout should be constant through out its length.

Sizing of roof downspouts.

Downspouts of less than 7 00 sq in 4515 sq mm cross section should not be used except for small areas such as porches and canopies. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. 5 inch 5 520 square feet. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Downspouts should be constructed with conductor. If a roof s various drainage areas call for different size gutters go for the biggest one k style.

Then use the information below to see what size gutter you need. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. Now architects engineers designers and contractors can easily and accurately size downspouts and gutters according to the specifications in smacna s architectural sheet metal manual 7th edition 2012. Doubling the number of downspouts halves the required capacity for each.

Please lower the maximum spacing between downspouts to see this effect.