How To Size Number Of Downspouts Needed For Roof

Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

How to size number of downspouts needed for roof. Size downspouts to fit the gutter. To limit the effects of thermal expansion in gutters 50 ft. The size of the downspout should be constant through out its length. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour.

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. Downspout sizes must not exceed the bottom width of the gutter. Adding additional sections of gutter reduces the length of each section and will reduce the gutter size. Also the gutter width in relation to height may also be adjusted.

Downspouts should be constructed with conductor. Multiply the drainage area by the roof pitch factor and rainfall intensity to find out the adjusted square footage. Downspout sizing in sizing downspouts the following considerations apply. The capacity of a sloped rectangular gutter may be approximated by using a gutter cross section area not less than that of a semicircular gutter and a depth to width ratio of at least 0 75.

6 inch 7 960 square feet. The gutter and downspout size requirement depends upon the number of sections and length of the gutter sections as well as the number of downspouts. Rectangular downspouts move slightly less water at 600 square feet for 2 by 3 inch or 1 200 square feet for a 3 by 4 inch downspout. 5 inch 5 520 square feet.

Then use the information below to see what size gutter you need. Multiply the number of downspouts by square footage of each.