Calculate total roof area, square feet per drain, or the number of roof drains needed from two known values in sq ft, sq m, or sq yd.
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Roof Drains Formula
The roof drains calculator uses the relationship between total roof area, the roof area served by each drain, and the number of drains.
A = P * N
P = A / N
N = ceil(A / P)
- A = total roof area
- P = roof area per drain
- N = number of roof drains
- ceil = round up to the next whole number
The calculator lets you enter exactly two values and solves for the third.
- If you enter area per drain and number of drains, it calculates the total roof area.
- If you enter total roof area and number of drains, it calculates the area assigned to each drain.
- If you enter total roof area and area per drain, it calculates the number of drains and rounds up to a whole drain.
Area units are converted to square feet internally. You can enter total area or area per drain in square feet, square meters, or square yards.
Area Unit Conversions Used
The calculator converts all area values to square feet before applying the formulas.
| Unit | Equivalent in square feet | Use in calculator |
|---|---|---|
| 1 sq ft | 1 sq ft | Base unit |
| 1 sq m | 10.7639 sq ft | Metric roof area or area per drain |
| 1 sq yd | 9 sq ft | Area measured in square yards |
Example Calculations
Example 1: Calculate the number of roof drains
You have a roof area of 12,000 sq ft. Each drain can serve 2,500 sq ft.
N = ceil(12000 / 2500)
N = ceil(4.8) = 5
You need 5 roof drains.
Example 2: Calculate square feet per drain
You have a 9,000 sq ft roof and 6 roof drains.
P = 9000 / 6
P = 1500
Each drain serves 1,500 sq ft per drain.
FAQ
Why does the calculator round the number of drains up?
You cannot install part of a drain. If the area calculation gives 4.2 drains, you need 5 drains to cover the full area based on the entered area-per-drain value.
Does this calculator size roof drains for rainfall intensity?
No. This calculator only uses roof area, area per drain, and number of drains. Actual roof drainage design can depend on rainfall intensity, drain diameter, pipe slope, roof slope, local code, overflow drainage, and drain manufacturer capacity data.
What should I enter for square feet per drain?
Enter the roof area that one drain is intended to serve. This value may come from a design standard, code table, manufacturer data, or a project-specific drainage plan.
