Sizing Of Roof Downspouts

Downspouts should be constructed with conductor.

Sizing of roof downspouts.

Doubling the number of downspouts halves the required capacity for each. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. The size of the downspout should be constant through out its length. 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. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. 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. Please lower the maximum spacing between downspouts to see this effect. 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.

Downspout sizing in sizing downspouts the following considerations apply.