The system ends where water is discharged safely—not at the bottom of the downpipe.
1. Divide the roof into connected catchments
Use horizontal projected area and trace which portion drains to each valley, gutter and outlet. Complex roofs can concentrate flow at a short gutter section even when total area seems modest.
Record overflow paths and roof transitions. Leaves, snow, ice and wind-driven rain may create conditions outside a simple uniform-rainfall calculation.
2. Select a local design rainfall intensity
Rainfall intensity depends on location, duration and selected risk. Use the source required by local rules and document the value. Annual rainfall is not a substitute for a short-duration design intensity.
The peak-flow calculator converts one uniform intensity over one connected area into litres per second. It does not size a gutter profile or outlet.
3. Check every restriction along the path
Evaluate gutter cross-section and fall, valleys, strainers, outlets, bends, downpipes and underground connections using the applicable method. The smallest or most easily blocked component may control capacity.
EPA moisture guidance calls for sizing gutters and downspouts for maximum runoff using roof area, pitch and rainfall intensity. Provide a visible and safe overflow route when practical.
EPA · Moisture control guidance ↗4. Keep discharge away from vulnerable building areas
Confirm where each downpipe and overflow releases water. Ground should manage runoff without sending it toward foundations, entrances, neighbouring property or an unsuitable wastewater system.
DOE Building Science describes gutters, grading and free-draining foundation measures as parts of one water-management approach. Inspect the system during real rain and after seasonal debris or freeze events.
DOE Building Science · Foundation drains, gutters and downspouts ↗Trace roof water through every component
| Stage | Input to record | Failure to inspect |
|---|---|---|
| Roof catchment | Connected projected area | Missed valley concentration |
| Design storm | Intensity and duration | Annual average used |
| Gutter and outlet | Geometry and fall | Outlet controls capacity |
| Downpipe | Diameter, bends and blockage | Nominal size assumed |
| Discharge | Route and receiving capacity | Water returns to foundation |
Worked example
Peak runoff from one roof catchment
A connected 120 m² roof receives a design intensity of 100 mm/h with a runoff factor of 90%.
Flow = 120 × 100 × 0.90 / 3600 = 3.00 L/s
Use an applicable hydraulic sizing method for the gutter, outlet and downpipe, then verify the discharge route.
What to have ready
- Roof drainage plan by outlet
- Local design rainfall intensity
- Gutter, outlet and downpipe data
- Overflow and final discharge route
Common mistakes
- Using annual rainfall as peak intensity.
- Sizing only the downpipe.
- Discharging roof water beside the foundation.
Reference check: 5 September 2026. Sources include US public agencies. Principles are general; regulations and design values must be checked where you live.Worked examples use stated hypothetical inputs.