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Drain Calculator

Factors which affect proper roof drainage are deck slope, drain size, and placement of drains. The full flow capacity of a drain is not realized until the water reaches approximately 2.5" above the inlet in a four-inch drain and 3.5" in a six-inch drain. The sizing of roof drains must be large enough to quickly carry away all water entering the roof drains. Any blockage or restrictions in the system may result in water backing onto or under the roof membrane. Horizontal pipe capacity must be checked as part of an adequate design.

When placing roof drains, it is good design practice to locate the drains no more than 50 feet from the roof perimeter, nor more than 100feet apart. It is important to remember that no roof area should ever have less than two roof drains. Size and placement of rooftop equipment may necessitate additional roof drains. Consideration should also be given to local codes whish may require separate overflow drain systems. The primary considerations in designing a roof drainage system are:

  • The amount of rainfall landing on the roof membrane
  • How long it rains
  • The speed at which the rain collects at the drain.

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. Remember when making your selections that Roof Drains are available in 3", 4", and 6" diameters and Scupper Drains are available in 2", 3", and 4" diameters.

Roof Size (sq ft)


Please enter the square footage of your roof