As with gutters a downspout s capacity must match or exceed the expected runoff.
Nrcs gutter and downspout sizing.
Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.
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.
Types of gutter downspouts.
Doubling the number of downspouts halves the required capacity for each.
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.
Typically a 5 inch top width k style gutter can receive a 2 x 3 downspout.
Also the gutter width in relation to height may also be adjusted.
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.
Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.
Use the chart below to figure out how many extra downspouts you need.
Gutter professionals have experience matching the gutters and the downspouts.
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.
Size downspouts to fit the gutter.
Some trimming of the flange on the preformed outlet may be required.
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.
However 7 and 8 inches are available although the larger sizes are more suitable for larger buildings or business properties.
A 3 x 4 downspout can also fit.
To limit the effects of thermal expansion in gutters 50 ft.
Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.
Also please adjust the gutter width relation to height as some width height ratios will not be compatible with standard gutter sizes and the calculator will recommend a custom gutter.
In sizing downspouts the following considerations apply.
As far as downspouts go they run at 2 x3 and 3 x4 in size or 3 or 4 inches in diameter.
There is a range of downspout styles which are available in the market as listed below.
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 a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
15 m is a practical maximum gutter length to be served by a downspout.
The size of the downspout should be constant through out its length.
A 6 inch top width k style gutter can receive a 3 x 4 downspout.
