Effective area of a pitched roof (worked example)

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 size each gutter and downpipe for its own 24 square metres rather than the whole roof.

Step 3 - why not the slope length. Measuring up the actual slope would give a number larger than 3 metres and would oversize the system, because the rain is collected over the flat-projected span, not the slope distance.

Step 4 - wind-driven allowance. Some design methods add part of the vertical elevation to allow for wind blowing rain onto the slope. Whether and how much to add depends on the method in the current standard, so treat that as a separate, confirmed step rather than assuming it.

Step 5 - carry it forward. Once you have the effective plan area, you combine it with a rainfall intensity to get the design flow the gutter and downpipe must carry. Use the plan area you calculated and the method in the current guidance to size the components.