Rainwater Gutter Design Calculations

Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

Rainwater gutter design calculations.

Working out gutter slope as a general rule gutter slope is set at 1 4 inch per 10 feet of guttering. Calculate effective maximum roof area b c 2 x length of roof area in m installation tips for gutter fixings onto wooden fascias use 25mm x no 10 round head rust proof screws. Round 3 inches 706 square feet 4 inches 1 255 square feet. Downspout sizes must not exceed the bottom width of the gutter.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. 15 m is a practical maximum gutter length to be served by a downspout. Doubling the number of downspouts halves the required capacity for each. From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used.

For the most up to date rain intensity data visit noaa s website and use precipitation intensity as the data type. This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Size downspouts to fit the gutter.

Overflow provisions rain shadow roof slopes how to find rainfall intensity how to calculate the area of any catchment how to find the hydraulic grade of charged pipes how to design box gutters using the code how to use the extra features of the dp calculator box gutters allowable width slope length other plumbing codes how to exceed the. Subtracting 3 4 for under the roof edge and 1 8 outside edge allowance this is the width that is available for the rainwater to enter the gutter total width of a flat piece of building material maximum total gutter drop before any downspout this assumes 4 7 per hour maximum rain intensity. To limit the effects of thermal expansion in gutters 50 ft. Design area wizard.

Divide these areas by the local rain fall rate in inches per hour to determine the actual roof area to be served by the gutter diameter. 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. For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. 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.

On a rainfall intensity of 75mm hr roof pitch not exceeding 50º gutters running full. Area and flow in table 1 4 are based on 1 in 25 mm of rainfall per hour. We recommend resource library read more.