For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e.
Calculating gutter and downspout needs.
How to calculate gutter and downspout capacity step 1.
To figure out what size gutters you need first you ll need to calculate the square footage of the gutter s drainage area.
Measure the roof to get the watershed area the area from which water will run off.
The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website.
Use the chart below to figure out how many extra downspouts you need.
Please lower the maximum spacing between downspouts to see this effect.
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.
To limit the effects of thermal expansion in gutters 50 ft.
Adding a 2 by 3 rectangular downspout for instance boosts your gutter s capacity by 600 square feet of drainage area.
15 m is a practical maximum gutter length to be served by a downspout.
From chart 1 2 it is determined that a 9 5 in 241 mm half round gutter should be used.
Metal era recommends that you verify all data obtained.
Ratio of depth to width of gutter calculate drainage reset.
Adding additional downspouts will require less gutter and downspout volume resulting in smaller gutters downspouts.
Minimum top opening width max length to.
This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end.
As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering.
3 by 4 inches 1 200 square feet.
Find the rainfall intensity for your locale.
2 by 3 inches 600 square feet.
Multiply length peak to.
As with gutters a downspout s capacity must match or exceed the expected runoff.
From table 1 3 it is found that a4 in 102 mm downspout is required.
Downspout sizes must not exceed the bottom width of the gutter.
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.
This calculator uses the equations that are laid out in the smacna handbook.
Calculations are based on level gutter capacity as experimentally determined by the national institute of standards and technology nist.
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.
Rainfall intensity is measured in a 5 minute interval translated.
Max gutter per downspout ft storm requirement.
