DIY & materials · Calculator companion · By K Imports
Roof drainage flow: practical guide
This guide explains how to use the Count.ie roof drainage flow. Calculate rainwater design flow and outlets from rated capacity. Follow the inputs, method and worked example below, then compare your own scenario.
Guide written .
Open the Roof drainage flow calculatorWhat to prepare
- Measure the actual space and record every dimension with its unit.
- Check the supplier's pack size, stated coverage and installation instructions.
- Separate the measured requirement from waste, cuts, overlaps and spare material.
Understand the inputs
The calculation method
Flow in litres/second = catchment m² × rainfall mm/hour × runoff fraction ÷ 3,600. Outlet count rounds up.
Worked example
Illustrative inputs and their result.
Example inputs
- Effective roof catchment (m²)
- 150
- Selected rainfall intensity (mm/hour)
- 75
- Runoff coefficient (%)
- 100
- Rated capacity per outlet (litres/second)
- 2
Calculated example result
- Design Flow Litres Second
- 3.125 L/s
- Outlets Required
- 2
How to interpret the result
The calculated quantity is a planning estimate, not an installation specification. Whole packs and site conditions can change the order; structural or safety-critical work needs appropriate professional guidance.
Compare alternative dimensions or pack sizes without changing the unit basis. Re-measure uncertain areas before buying, and use the manufacturer's instructions to decide a suitable allowance.
Common mistakes to avoid
- Convert millimetres, centimetres and metres before combining measurements.
- Do not add the same waste allowance at both measurement and ordering stages.
- Check irregular shapes, openings, substrate conditions and manufacturer coverage separately.
Assumptions and sources
Enter the design catchment and intensity required for the building and location. Gutter gradients, outlet head, blockage allowance and hydraulic verification are excluded.
If your case falls outside this scope, use a more suitable calculator or contact us about an unclear method.