Flat Roof Drainage Calculator

Neither -- Design Flow Rate is calculated by multiplying Roof Area directly by Design Rainfall Rate, so a given percentage increase in either input raises the flow rate by that same percentage. The two inputs have exactly equal proportional influence on the result.

No -- Design Flow Rate depends only on Roof Area and Design Rainfall Rate, both multiplied together and converted to gallons per minute. Drain Size only affects how many drains are needed to carry that already-calculated flow, not the flow rate itself.

Roof Slope is checked independently against the recommended 1/4 inch per foot minimum to report whether your pitch supports positive drainage, but the drain count and scupper count formulas never reference the slope value -- they respond only to flow rate and drain size.

It divides the calculated Design Flow Rate by the rated capacity of your selected Drain Size (roughly 30 GPM for a 2-inch drain up to 275 GPM for a 6-inch drain), then rounds up to the next whole drain, since a fraction of a drain isn't installable.

It's the method the plumbing code itself uses. The International Plumbing Code sizes roof drains, leaders, and vertical conductors from projected roof area and design rainfall rate in § 1106.2 and its accompanying Table 1106.2 -- the same two inputs this calculator asks for. Always check your local jurisdiction's adopted plumbing code and rainfall-rate map, since some municipalities amend the base IPC tables or rainfall design values.