Roof Gutter Drainage Calculator

Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed.

Roof gutter drainage calculator.

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. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering.

Calculator notes and assumptions. For example in the u k. 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. Take the roof s total square footage and divide by the total square footage handled by one drain.

Simplified suitable for flattish roofs with little or no vertical faces or steep roofs draining to one side of box gutter only or any roof if you are in a hurry and make the necessary allowances for all catchments. 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. Gutter calculator this calculator uses the equations that are laid out in the smacna handbook. Box gutter with rain water head at end of box gutter.

Metal era recommends that you verify all data obtained. The result is the number of drains needed. Size downspouts to fit the gutter. Doubling the number of downspouts halves the required capacity for each.

Or 12 drains 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. Calculator is based on chapter 1 of smacna s architectural sheet metal manual 7th edition asmm. 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.

This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end. We recommend using this calculator as a basic tool to begin determining the requirements for your drainage system however all information should be verified by hand prior to ordering or installing your gutter system. For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. Designers specify compliance with bs en 12056 3 2000 that includes a.