A worked example that turns a pitched roof’s plan footprint into effective design area using the applicable pitch allowance.
Sizing rainwater gutters and pipes starts from how much rain a roof actually collects, and a common slip is to use the sloping surface area of a pitched roof instead of its plan area. This worked example shows the principle.
The idea. Rain falls roughly vertically, so what matters for run-off is the area the roof covers when seen from above, its plan or footprint area, not the longer distance up the slope. A pitched roof and a flat roof of the same footprint collect about the same vertically falling rain.
Step 1 - the footprint. Take one pitched roof slope that covers a building 8 metres long, with a horizontal span, measured flat from eaves to ridge, of 3 metres. The plan area of that slope is 8 x 3 = 24 square metres.
Step 2 - both slopes. A typical dual-pitch roof has two slopes draining to two gutters. If both are the same, each gutter serves 24 square metres of plan area, so calculate each gutter’s effective design area from its own 24 m² footprint and pitch, rather than assigning the whole roof to each gutter. The 24 m² is an intermediate plan area, not the final pitched-roof design area.
Step 3 - why not the slope length. Measuring up the actual slope would give a number larger than 3 metres and would give the sloping surface area. That is not the plan-area input to the stated design method: start with the horizontal footprint, then apply its pitch allowance.
Step 4 - wind-driven allowance. Use the pitch allowance for the chosen design method. In the Approved Document H example below, a 45-degree slope uses a factor of 1.50: 24 × 1.50 = 36 m² effective area for each gutter. This allows for wind-driven rain; it does not replace the footprint calculation.
Step 5 - carry it forward. Combine the effective design area, after the pitch allowance, with the selected rainfall intensity to calculate design flow. Then check the gutter, outlet and downpipe capacities for their actual layout.
Approved Document H's pitch allowance
Approved Document H in England, Part H3, Table 1 gives the factors for turning a pitched roof's plan area into its effective design area:
- Flat roof: plan area, unchanged
- Pitched roof at 30 degrees: plan area x 1.29
- Pitched roof at 45 degrees: plan area x 1.50
- Pitched roof at 60 degrees: plan area x 1.87
- A wall, or a roof pitched over 70 degrees, draining onto a roof or gutter below: 0.5 x its elevational area
A valley gutter collects run-off from two roof slopes at once, so it is sized for the combined effective area of both, not just one. Because it gathers more flow and debris than an eaves gutter and is harder to inspect, a blocked or overwhelmed valley is more likely to let water into the building rather than spill harmlessly at the eaves.