Calculate the water first, use verified component capacity second, and then draw levels and overflow paths that can be checked on site.
1. Draw every connected roof catchment
Mark ridges, valleys, changes of pitch and the direction of flow. Assign horizontal projected area to each gutter section and outlet; do not divide total roof area equally unless the geometry truly does so.
Valleys and short intense storms can concentrate flow at one point. Record the path from the roof surface to the final discharge before selecting hardware.
2. Match design flow to declared capacity
Use a local short-duration design rainfall and an explicit runoff factor. Add any planning margin visibly, then divide by tested capacity for the complete outlet or downpipe arrangement and round up.
Nominal pipe diameter alone is not a capacity rating. The gutter profile, water depth, outlet shape, bends and connection details can control the result.
3. Set the gutter from one repeatable datum
Choose high and low points, multiply the specified fall by the run length and record both offsets from one level datum. Equal support spacing should not exceed the system limit after rounding.
Check fascia straightness, outlet position, joints and thermal movement before fixing. Extra supports may be required at corners, joints, outlets or high-load locations.
4. Design for blockage and visible overflow
Leaves, ice and debris can remove an outlet from service. Provide a visible overflow route that does not send water into the building, behind cladding or beside a vulnerable foundation.
Inspection and cleaning access are part of capacity. A system that cannot be safely maintained should not rely on perfect long-term flow sharing.
5. Verify the receiving system
Follow every downpipe through surface channels, soakaway, tank or storm connection. Confirm that the receiving route can accept the design flow and that overflow cannot return toward the house.
Use applicable drainage rules and competent design for complex roofs, internal gutters, siphonic systems or any arrangement where overflow could enter occupied space.
Roof-drainage design record
| Item | Record | Do not assume |
|---|---|---|
| Catchment | Projected area and flow direction | Equal area per outlet |
| Rainfall | Local intensity and duration | Annual average |
| Outlet | Tested capacity and configuration | Diameter alone |
| Set-out | Datum, high point, low point and spacing | Fascia is level |
| Overflow | Visible safe route | Outlets never block |
Worked example
One 120 m² roof catchment
The local design intensity is 100 mm/h, runoff factor 90%, planning margin 20% and each selected outlet is rated at 1.50 L/s for the intended arrangement.
Peak runoff = 120 × 100 × 0.90 / 3600 = 3.00 L/s Design flow = 3.00 × 1.20 = 3.60 L/s Outlets = round up(3.60 / 1.50) = 3
Place the three outlets from the real catchment geometry, verify gutter capacity and draw a safe overflow and discharge route.
What to have ready
- Catchment drawing with valleys and outlet groups
- Local design rainfall source
- Tested gutter and outlet product data
- Datum, fall and support set-out
- Maintenance, overflow and final discharge route
Common mistakes
- Using annual rainfall instead of design intensity.
- Treating pipe diameter as a complete capacity rating.
- Dividing roof area equally without tracing flow.
- Ending the design at the downpipe foot.
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.