Calculating Gutter And Downspout Needs

This calculator uses the equations that are laid out in the smacna handbook.

Calculating gutter and downspout needs.

Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts. Metal era recommends that you verify all data obtained. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. Downspout sizes must not exceed the bottom width of the gutter.

Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist. Max gutter per downspout ft storm requirement. Minimum top opening width max length to. Rainfall intensity is measured in a 5 minute interval translated.

Measure the roof to get the watershed area the area from which water will run off. To limit the effects of thermal expansion in gutters 50 ft. To figure out what size gutters you need first you ll need to calculate the square footage of the gutter s drainage area. Ratio of depth to width of gutter calculate drainage reset.

3 by 4 inches 1 200 square feet. This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end. Multiply length peak to. This calculator serves as an information source only.

Please lower the maximum spacing between downspouts to see this effect. 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. From table 1 3 it is found that a4 in 102 mm downspout is required. For a simple gable end roof you would only need to make two calculations one for each slope.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering. How to calculate gutter and downspout capacity step 1. From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used.

As with gutters a downspout s capacity must match or exceed the expected runoff. 2 by 3 inches 600 square feet. Find the rainfall intensity for your locale. For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e.

Use the chart below to figure out how many extra downspouts you need. 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. 15 m is a practical maximum gutter length to be served by a downspout.