Calculating Gutter And Downspout Needs

How to calculate gutter and downspout capacity step 1.

Calculating gutter and downspout needs.

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. Ratio of depth to width of gutter calculate drainage reset. Rainfall intensity is measured in a 5 minute interval translated. Max gutter per downspout ft storm requirement.

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. 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. To limit the effects of thermal expansion in gutters 50 ft.

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. Find the rainfall intensity for your locale. 2 by 3 inches 600 square feet.

For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used. Use the chart below to figure out how many extra downspouts you need. The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts.

For a simple gable end roof you would only need to make two calculations one for each slope. Metal era recommends that you verify all data obtained. As with gutters a downspout s capacity must match or exceed the expected runoff. 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.

As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering. From table 1 3 it is found that a4 in 102 mm downspout is required. 3 by 4 inches 1 200 square feet. 15 m is a practical maximum gutter length to be served by a downspout.

The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Measure the roof to get the watershed area the area from which water will run off. Multiply length peak to.

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. This calculator uses the equations that are laid out in the smacna handbook. Please lower the maximum spacing between downspouts to see this effect.