Roof Downspout Sizing

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.

Roof downspout sizing. 3 by 4 inches 1 200 square feet. To limit the effects of thermal expansion in gutters 50 ft. 15 m is a practical maximum gutter length to be served by a downspout. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.

However too large a gutter downspout can look unattractive. It also comes in sizes 2 3 and 3 4. The size of the downspout should be constant through out its length. You should have enough gutter downspouts to effectively handle the size of your entire gutter system.

This type of down spout is considered the standard downspout style in households and is used with a 5 k style guttering system. But houses with big steep roofs or those located in climates prone to heavy downpours may need wider gutters and extra downspouts to keep rainwater from overflowing. Smacna s new downspout and gutter sizing calculator enables the user to manually enter the design area of the roof or use the design area wizard for the calculation. 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. Most downspouts are rectangular but they can also be round or square. 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. Five inch k style gutters or 6 inch half rounds the most common residential sizes are able to handle the rainfall on most houses in most parts of the country.

Please lower the maximum spacing between downspouts to see this effect. The sizes available in this style are 3 4 5 and 6. 2 by 3 inches 600 square feet. Gutter downspout sizes are 3 to 6 inches.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Size downspouts to fit the gutter. It takes 96 15 square feet 8 93 square meters of surface with 1 inch per hour 25 mm hr of water to correspond with 1 gpm 0 063 l s flow rate. Downspouts should be constructed with conductor.

The larger a gutter downspout the more water it can drain quickly. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. Downspout sizes must not exceed the bottom width of the gutter. As with gutters a downspout s capacity must match or exceed the expected runoff.

Downspouts and rain gutters are sized in relation to rainfall on this basis. Use the chart below to figure out how many extra downspouts you need. This wizard makes it easy to calculate the roof design area by providing various shape calculations and the calculation for vertical walls area per international plumbing code ipc.