Rainwater Gutter Design Calculations

Refer to bsen 12056 part 3.

Rainwater gutter design calculations.

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. Downspout sizes must not exceed the bottom width of the gutter. On a rainfall intensity of 75mm hr roof pitch not exceeding 50º gutters running full. 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.

Size downspouts to fit the gutter. Roof drainage layout and calculations. This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end. Round 3 inches 706 square feet 4 inches 1 255 square feet.

From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used. 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. For the most up to date rain intensity data visit noaa s website and use precipitation intensity as the data type. Design area wizard.

Resources aluminum series acm series column covers fastener series zinc series testing data aama pdf astm pdf gutter sizing sheet metal gages order literature northclad gutter size requirement calculator this calculator uses current data to calculate the required size of gutter as well as number and size of downspouts based on your input. 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. 15 m is a practical maximum gutter length to be served by a downspout. 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.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. We recommend resource library read more. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Working out gutter slope as a general rule gutter slope is set at 1 4 inch per 10 feet of guttering.

For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Area and flow in table 1 4 are based on 1 in 25 mm of rainfall per hour.