Roof Gutter Drainage Calculator

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.

Roof gutter drainage calculator.

This calculator serves as an information source only. Rainfall wizard is based on table 1 2 rainfall data and drainage factors page 1 2 1 3 of the architectural sheet metal manual 7th edition. Designers specify compliance with bs en 12056 3 2000 that includes a. The result is the number of drains needed.

The roof pitch or slope factors for roof pitch are used to multiply or increase the rainfall rate if we are draining a sloped roof the roof drain or gutter or drain piping or downspout leader size diameter other size constraints e g. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. Size downspouts to fit the gutter. Doubling the number of downspouts halves the required capacity for each.

Or 12 drains required. Calculator is based on chapter 1 of smacna s architectural sheet metal manual 7th edition asmm. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering. 50 000 4 400 11 36.

A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. The roof drain calculator below is based on the maximum amount of rainfall in inches per hour of the hypothetical worst storm in the past 100 years and which has a two percent chance of occurring in any given year. The number of roof drains shown should be verified with local code requirements. Take the roof s total square footage and divide by the total square footage handled by one drain.

To use this calculator simply enter the information in the form on the left and your data will be output on the right side. Calculator notes and assumptions. The downspout and gutter sizing calculator is located on the tools cad and apps page of the smacna website. Metal era recommends that you verify all data obtained.

For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. For example in the u k. Box gutter with rain water head at end of box gutter. Gutter calculator this calculator uses the equations that are laid out in the smacna handbook.