The roof to wall band: fascia, gutter run, soffit area and the downpipes that drain it.
Foundation, roof and structure. 5 questions answered below, each with the arithmetic behind it and the limits of what the tool decides. Open the Soffit, Fascia and Gutter Estimator tool.
Rainwater goods are ordered by the metre and by the fitting, and both come from the roof edge rather than from the roof area. The estimator works from the eaves perimeter, adds the overhang to produce soffit area, puts downpipes where the runs justify them, and returns an order that a merchant can fill. The three lines behave differently. Gutter and fascia follow the perimeter directly. Soffit follows the perimeter multiplied by the overhang depth, so a deeper overhang costs more without changing the roof. Downpipes follow the roof plan area and the spacing you set, which is why a long low roof can need fewer than a compact steep one of the same area. Keeping the three apart is what stops one being scaled from another.
A roof plan of twelve by nine metres with a six hundred millimetre overhang has an eaves perimeter of forty two metres. Gutter and fascia are forty two metres each, with four corners, two stop ends and, at a spacing of ten metres, five outlets. Soffit is forty two metres multiplied by zero point six, which is twenty five square metres of board. Downpipes at a spacing of eight metres around the perimeter give six, each with its brackets and its shoe. The roof area never entered the gutter order, which is the mistake this page exists to prevent.
Measure the eaves perimeter, which is the wall line plus the overhang on every side, then add the fittings the run needs: corners, stop ends, outlets and joints. The estimator takes the plan dimensions and the overhang, returns the perimeter, and counts the fittings from your geometry. Gutter is not scaled from roof area, because a shallow roof and a steep roof of the same area have the same gutter and very different areas.
From the roof plan area, the perimeter and the spacing you set. Each downpipe serves a length of gutter, so a long run needs more outlets, and the catchments either side are what the pipe has to carry. The estimator places them around the perimeter at the spacing you choose and counts the pipe, the brackets and the shoe for each. Whether that spacing is adequate is a drainage calculation that depends on the rainfall intensity where you are building.
The overhang depth multiplied by the eaves perimeter. A six hundred millimetre overhang on a forty two metre perimeter is twenty five square metres of soffit board, and the same roof with a three hundred millimetre overhang is half of that. The area is a flat measure of a sloping surface only where the soffit is horizontal, which most are, and where it is raked the depth you enter is the width of the board rather than a vertical drop.
It should, when both are measured from the same plan. The fascia runs around the same eaves line that the roof edge follows, so the numbers agree once the overhang is included in both. Where they disagree, the usual cause is a roof estimate built from the roof surface at the pitch and a fascia measured from the wall line, which differ by the overhang and by the pitch.
By the length of gutter each downpipe serves. A common guide is one outlet per twelve to fifteen metres of gutter on a simple roof, closer on a valley or where two roofs meet, and always at the low end of a fall. The estimator takes your spacing and counts the pipe, but the adequacy of that spacing depends on the rainfall the roof will actually see, which varies enough by region that it is a design input rather than a default.