Calculating Gutter And Downspout Needs

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.

Calculating gutter and downspout needs. Rainfall intensity is measured in a 5 minute interval translated. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Enter larger gutter widths in the yellow boxes to find out how wide your gutters need to be given the length that you want to go between downspouts. Remember two gutter lengths can share the same downspout so if you calculate the max gutter to be 40 feet you could put a downspout halfway in an 80 foot length.

Divide 1600 sq ft 149 sq m by 160 sq ft sq in 2 3 sq m 100 sq mm to determine that each downspout should have a minimum area of 10 sq in 6470 sq mm. Downspout sizes must not exceed the bottom width of the gutter. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. Now architects engineers designers and contractors can easily and accurately size downspouts and gutters according to the specifications in smacna s architectural sheet metal manual 7th edition 2012.

Measure the roof to get the watershed area the area from which water will run off. From table 1 3 it is found that a4 in 102 mm downspout is required. 15 m is a practical maximum gutter length to be served by a downspout. 3 by 4 inches 1 200 square feet.

Multiply length peak to. Max gutter per downspout ft storm requirement. To limit the effects of thermal expansion in gutters 50 ft. This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end.

To figure out what size gutters you need first you ll need to calculate the square footage of the gutter s drainage area. 2 by 3 inches 600 square feet. Please lower the maximum spacing between downspouts to see this effect. Find the rainfall intensity for your locale.

From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used. For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. How to calculate gutter and downspout capacity step 1. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering.

For a simple gable end roof you would only need to make two calculations one for each slope. This calculator uses the equations that are laid out in the smacna handbook. This calculator serves as an information source only. As with gutters a downspout s capacity must match or exceed the expected runoff.

Use the chart below to figure out how many extra downspouts you need. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Minimum top opening width max length to.