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