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.

Sizing of roof downspouts. Downspout sizing in sizing downspouts the following considerations apply. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. 5 inch 5 520 square feet. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.

If a roof s various drainage areas call for different size gutters go for the biggest one k style. Please lower the maximum spacing between downspouts to see this effect. 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.

Downspouts should be constructed with conductor. Then use the information below to see what size gutter you need. 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.

The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts.