Sizing Of Roof Downspouts

Downspouts should be constructed with conductor.

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. 5 inch 5 520 square feet. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

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. Size downspouts to fit the gutter. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Doubling the number of downspouts halves the required capacity for each.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Then use the information below to see what size gutter you need. If a roof s various drainage areas call for different size gutters go for the biggest one k style. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage.

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