Roof Gutter Drainage Calculator

Gutter calculator this calculator uses the equations that are laid out in the smacna handbook.

Roof gutter drainage calculator.

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. Metal era recommends that you verify all data obtained. Designers specify compliance with bs en 12056 3 2000 that includes a. The number of roof drains shown should be verified with local code requirements.

Calculator is based on chapter 1 of smacna s architectural sheet metal manual 7th edition asmm. Roof area the locale s rainfall intensity and gutter capacity determine how many square inches of downspout are needed. This means the downspout end of the gutter should be set 3 4 of an inch lower than the other end. Calculator notes and assumptions.

Size downspouts to fit the gutter. As a general rule gutter slope is set at 1 4 inch per 10 feet of guttering. The result is the number of drains needed. 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.

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. This calculator serves as an information source only. 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.

Box gutter with rain water head at end of box gutter. 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. Or 12 drains required. To use this calculator simply enter the information in the form on the left and your data will be output on the right side.

For example if you have 30 feet of guttering the calculation is 1 4 x 3 0 75 i e. A square inch of downspout can drain 1 200 square feet of roof during a rain with an intensity of one inch per hour. 50 000 4 400 11 36. For example in the u k.