Roof Downspout Sizing

Size downspouts to fit the gutter.

Roof downspout sizing.

The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. 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. Please lower the maximum spacing between downspouts to see this effect.

However too large a gutter downspout can look unattractive. Downspout sizing in sizing downspouts the following considerations apply. 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. Doubling the number of downspouts halves the required capacity for each.

It also comes in sizes 2 3 and 3 4. Gutter downspout sizes are 3 to 6 inches. 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 sizes must not exceed the bottom width of the gutter.

The sizes available in this style are 3 4 5 and 6. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Downspouts and rain gutters are sized in relation to rainfall on this basis. The size of the downspout should be constant through out its length.

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. 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. 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.

2 by 3 inches 600 square feet. 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. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Local codes may require that drainage systems only be designed for the latter.

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. As with gutters a downspout s capacity must match or exceed the expected runoff. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.

To limit the effects of thermal expansion in gutters 50 ft. The larger a gutter downspout the more water it can drain quickly. Downspouts should be constructed with conductor.