Roof Downspout Sizing

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

Roof downspout sizing.

Downspouts should be constructed with conductor. 2 by 3 inches 600 square feet. 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 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.

Downspout sizing in sizing downspouts the following considerations apply. 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. The larger a gutter downspout the more water it can drain quickly. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.

Please lower the maximum spacing between downspouts to see this effect. Doubling the number of downspouts halves the required capacity for each. Gutter downspout sizes are 3 to 6 inches. 3 by 4 inches 1 200 square feet.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. This type of down spout is considered the standard downspout style in households and is used with a 5 k style guttering system. However too large a gutter downspout can look unattractive. 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.

To limit the effects of thermal expansion in gutters 50 ft. Size downspouts to fit the gutter. 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. Use the chart below to figure out how many extra downspouts you need.

Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. Downspout sizes must not exceed the bottom width of the gutter. It also comes in sizes 2 3 and 3 4. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

You should have enough gutter downspouts to effectively handle the size of your entire gutter system. 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. Local codes may require that drainage systems only be designed for the latter. 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. Most downspouts are rectangular but they can also be round or square. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts.