Roof Drain Calculator
Calculate the number of roof drains needed based on roof area, rainfall intensity, and drain capacity.
Results
Visualization
How It Works
Roof drains must handle the maximum expected rainfall intensity for your location. Code tables list drain capacity in square feet of roof area at a standard 4 in/hr rainfall, then adjust for local conditions.
The Formula
Drains Needed = Ceiling(Adjusted Roof Area / Drain Capacity). Adjusted Area = Roof Area x (Rainfall / 4).
Variables
- A — Horizontal roof area in square feet
- R — Rainfall intensity in inches per hour
- C — Drain capacity in sq ft at 4 in/hr
Worked Example
A 2000 sq ft roof in a 4 in/hr rainfall zone with 4" drains: 2000 / 3460 = 0.58, so 1 drain is sufficient.
Practical Tips
- Use local 100-year, 1-hour rainfall intensity from NOAA Atlas 14
- Minimum 2 drains recommended for redundancy
- Secondary (overflow) drains required by most codes
- Scupper drains are an alternative for flat roofs