DIY & materials
Roof drainage flow Calculator
Calculate rainwater design flow and outlets from rated capacity. This free roof drainage flow calculator shows the calculation and its assumptions so you can compare your own figures.
Your results
Calculate rainwater design flow and outlets from rated capacity.
Calculated outcomes
- Hourly Runoff Litres
- 11,250 L
Assumptions and estimates
- Method: Flow in litres/second = catchment m² × rainfall mm/hour × runoff fraction ÷ 3,600. Outlet count rounds up.
- Enter the design catchment and intensity required for the building and location. Gutter gradients, outlet head, blockage allowance and hydraulic verification are excluded.
Useful next steps
- Use Compare options to test an alternative side by side.
- Copy, print or download a record of this calculation when you need one.
Reports open your email app addressed to s@s1.ie with the public page address only. Your inputs are never added.
How the roof drainage flow calculation works
Flow in litres/second = catchment m² × rainfall mm/hour × runoff fraction ÷ 3,600. Outlet count rounds up.
Enter the design catchment and intensity required for the building and location. Gutter gradients, outlet head, blockage allowance and hydraulic verification are excluded.
Using this calculator
- Enter your own figures in the labelled inputs. Keep the currencies, units and time periods consistent.
- Choose any applicable rate profile, pattern or assumption shown for this tool. Open additional inputs when relevant.
- Review design flow litres second and outlets required, then read the stated assumptions and eligibility conditions before using the result.
Worked example
This is a static example using illustrative inputs, not facts about you or today’s date. The displayed eligibility selections and assumptions apply only to this example.
See example inputs
- Effective roof catchment (m²)
- 150
- Selected rainfall intensity (mm/hour)
- 75
- Runoff coefficient (%)
- 100
- Rated capacity per outlet (litres/second)
- 2
- Design Flow Litres Second
- 3.125 L/s
- Outlets Required
- 2