Roof Downspout Sizing

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.

Roof downspout sizing.

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. Size downspouts to fit the gutter. This type of down spout is considered the standard downspout style in households and is used with a 5 k style guttering system.

You should have enough gutter downspouts to effectively handle the size of your entire gutter system. 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. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. 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. Most downspouts are rectangular but they can also be round or square. Use the chart below to figure out how many extra downspouts you need. As with gutters a downspout s capacity must match or exceed the expected runoff.

15 m is a practical maximum gutter length to be served by a downspout. Gutter downspout sizes are 3 to 6 inches. It also comes in sizes 2 3 and 3 4. Doubling the number of downspouts halves the required capacity for each.

Please lower the maximum spacing between downspouts to see this effect. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. The larger a gutter downspout the more water it can drain quickly. 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.

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. Downspouts and rain gutters are sized in relation to rainfall on this basis. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. The size of the downspout should be constant through out its length.

Downspout sizes must not exceed the bottom width of the gutter. 2 by 3 inches 600 square feet. 3 by 4 inches 1 200 square feet. 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 sizes available in this style are 3 4 5 and 6. Downspout sizing in sizing downspouts the following considerations apply.