Schedule underlay, gutters and downpipes separately

One roof-area number cannot order every roof-related material. Underlay uses coverage and laps; gutters follow eaves; downpipes follow particular routes. Make separate lines so each quantity can be checked against the drawing or supplier schedule.

Check the roll's effective width

An illustrative roll is 1.5 m wide and 30 m long. With a 0.15 m side lap, a simplified repeating effective width is 1.35 m, giving 40.5 m² per roll before end laps and cutting. A 76 m² requirement therefore needs at least two rolls by this simplified area method. Lay out the runs to check end laps, first-strip treatment and short offcuts.

Trace gutter runs

A rectangular building does not necessarily need gutters on every side. Mark only the designed gutter lines, then measure each run. Add stop ends, outlets, corners and brackets as separate items from the selected system; they cannot be inferred reliably from total length alone.

Draw downpipe routes

For an illustrative route with 2.8 m of vertical drop and 0.5 m of straight offset, the straight pieces total 3.3 m before cutting and fitting details. Bends, connectors and overlaps affect the stock-length purchase.

Separate quantity from capacity

Gutter and downpipe lengths do not size a drainage system. Catchment, rainfall, overflow arrangements and system capacity need their own design. Use the calculator only after the relevant routes and components have been selected.

Found a measurement or calculation issue? Send the example and your correction.