Roof rainwater flow calculator
Calculate the primary drainage flow for a roof catchment. Enter rainfall and effective area, then select the roof type to apply its runoff coefficient.
Example inputs: 600 m², 180 mm/h and C = 1 give 30 L/s. Replace the example rainfall with your project data.
1. Enter roof and rainfall data
2. Primary drainage flow
- Runoff coefficient C
- Unit rainfall
- L/s/m²
- Equivalent flow
- m³/h
Enable JavaScript to calculate. The formula and worked example are below.
For multiple catchments, downpipe sizing and outlet capacities, use the Excel workbook.
How is roof rainwater flow calculated?
Q = r × A × C
Q is flow in litres per second, A is effective area in square metres and C is the runoff coefficient. When rainfall i is in mm/h, r = i / 3600. If rainfall is already in L/s/m², no further conversion is needed.
600 m² worked example
At an illustrative 180 mm/h, r = 180 / 3600 = 0.05 L/s/m². For 600 m² and C = 1, Q = 0.05 × 600 × 1 = 30 L/s, equivalent to 108 m³/h. This is a flow rate, not a storage tank volume.
Which roof coefficient applies?
The presets follow the roof build-up, slope and layer-thickness conditions shown in the selector. A mixed roof needs separate catchments. For an unlisted roof or a manufacturer-specific green roof, select the manual option and enter documented data. Coefficient source: LORO.
What should be checked next?
This tool calculates the primary flow only. Check actual catchment boundaries, gutters, outlet performance at the permitted water head, pipes and downstream drainage. Reduced runoff coefficients do not size the emergency overflow. Select rainfall duration and return period from the applicable project basis.