Sizing Of Roof Downspouts

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.

Sizing of roof 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. Then use the information below to see what size gutter you need. The size of the downspout should be constant through out its length. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage.

5 inch 5 520 square feet. Doubling the number of downspouts halves the required capacity for each. If a roof s various drainage areas call for different size gutters go for the biggest one k style. 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 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. Please lower the maximum spacing between downspouts to see this effect. Downspout sizing in sizing downspouts the following considerations apply.

Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.