Calculating Gutter And Downspout Needs

Use the chart below to figure out how many extra downspouts you need.

Calculating gutter and downspout needs.

To figure out what size gutters you need first you ll need to calculate the square footage of the gutter s drainage area. 15 m is a practical maximum gutter length to be served by a downspout. Minimum top opening width max length to. Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.

Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area. Max gutter per downspout ft storm requirement. Measure the roof to get the watershed area the area from which water will run off. As with gutters a downspout s capacity must match or exceed the expected runoff.

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

Rainfall intensity is measured in a 5 minute interval translated. Metal era recommends that you verify all data obtained. How to calculate gutter and downspout capacity step 1. Find the rainfall intensity for your locale.

Please lower the maximum spacing between downspouts to see this effect. From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used. 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.

2 by 3 inches 600 square feet. Ratio of depth to width of gutter calculate drainage reset. For a simple gable end roof you would only need to make two calculations one for each slope. Downspout sizes must not exceed the bottom width of the gutter.

From table 1 3 it is found that a4 in 102 mm downspout is required. 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. For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.

The gutter and downspout size required is highly dependent on the amount of gutter and the spacing of the downspouts. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering.