How Roof Runoff is Calculated
The rational method provides the foundation for estimating stormwater runoff from impervious surfaces such as rooftops. The core relationship is straightforward: one millimetre of rainfall falling uniformly over one square metre of impervious surface generates one litre of runoff — assuming no infiltration losses. The runoff coefficient (C) accounts for minor losses due to surface absorption, initial wetting, and membrane surface texture. For standard flat roofing membranes (TPO, PVC, EPDM, bituminous systems), runoff coefficients typically range from 0.85 to 0.95. This means that between 85% and 95% of total rainfall becomes runoff. For design purposes, engineers apply this formula across a storm duration matching the time of concentration — the time for water to travel from the most remote point on the roof to the outlet. For most flat or low-slope roofs, this is a matter of minutes, meaning the entire rainfall volume translates almost immediately into outlet flow.