Roof Drains Calculator

The area per drain depends on the roof type, slope, local rainfall intensity, and building codes. For flat commercial roofs, a common guideline is one drain per 10,000 to 15,000 square feet. Steep roofs in heavy-rain areas may need more drainage capacity.

You cannot install a fraction of a drain. If the calculation yields 4.3 drains, you need 5 drains to ensure the entire roof has adequate drainage. Under-draining a roof risks water ponding and structural damage.

Yes. Flat roofs or low-slope roofs need more drains because water does not naturally flow to the edges. Steeply sloped roofs shed water more quickly and may need fewer drains or can rely on gutters instead.

Yes. Enter both values in the same unit (for example, square meters). The formula divides area by area, so the result is a unitless count of drains regardless of the measurement system.