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Calcimator

Flat Roof Drainage Calculator

Size drains and scuppers for flat roofs based on roof area, rainfall intensity, and slope.

Inputs

sq ft
in/hr
in/ft

Results

Drains Needed

2

Scuppers Needed

5

Design Flow Rate207.84 GPM
Slope AdequacyAdequate
How to Use This Calculator
  1. Enter the roof area (sqft) and design rainfall rate (in/hr) for your climate zone.
  2. Set the roof slope (in/ft) and select the interior drain size (2" to 6").
  3. Review the required drain count, scupper count, and design flow rate in GPM.
  4. Check the slope adequacy result to confirm your roof slope meets the recommended 1/4 in/ft minimum.
  5. Use the drain and scupper counts to plan drain layout and overflow scupper placement on your roof.

What each input means

Roof Area
Total flat roof area in square feet
Design Rainfall Rate
Maximum rainfall intensity for your area (check local building code)
Roof Slope
Roof slope in inches per foot (minimum 1/4 in/ft recommended)
Drain Size
Interior drain diameter in inches

How this is calculated

Worked example, using the default values

  1. Identify Input Parameters
    4 parameters
    Roof Area = 5000, Design Rainfall Rate = 4, Roof Slope = 0.25, Drain Size = 4 = 4 input(s) provided
  2. Calculate Drains Needed
    Drains Needed
    2 = 2
  3. Calculate Scuppers Needed
    Scuppers Needed
    5 = 5
  4. Calculate Design Flow Rate
    Design Flow Rate
    207.84 = 207.84
  5. Calculate Slope Adequacy
    Slope Adequacy
    Adequate = Adequate

Engine last updated . Checked against 2 independently-derived tests — how we verify calculators.

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