Roof Sheet Calculator: How Many Roofing Sheets Do I Need?

Profiled roofing sheets are sold by the number of sheets and their length, not by the square metre, so the order comes down to two counts: how many sheets fit across the roof at their cover width, and how many rows you need from eaves to ridge once end laps are taken off. Enter your roof below for box profile or corrugated sheets, cut lengths, fixings and flashing runs. The key figure is cover width: a sheet always covers less than its full width because each one overlaps the next.

How the calculator works

Sheets across the roof. The roof width is divided by the sheet's cover width and rounded up to a whole sheet. Cover width is the width a sheet adds once it overlaps its neighbour, so a box profile sheet sold as 1000mm cover is physically wider than 1000mm. Rounding up means the last sheet usually overhangs the verge a little or is trimmed along its length.

Sheet length. The length to cover is the slope plus the eaves overhang. If that fits in one sheet, each run is a single sheet. If not, the run is split into rows that overlap by the end lap, and with cut-to-length sheets the rows are made equal and rounded up to the nearest 10mm. With fixed stock lengths every sheet is ordered at the stock length and the top row is trimmed.

End laps. 150mm is a common starting figure on steeper roofs, but manufacturers ask for bigger laps, and sometimes sealant tape in the lap, as the pitch gets shallower. Take the figure from the sheet manufacturer's installation guide for your pitch. A single sheet from eaves to ridge avoids the lap altogether, which is why cut-to-length sheets are popular on garages and outbuildings.

Fixings. The count is sheets across, times purlin lines (one at the eaves plus one per purlin spacing up the slope), times fixings per sheet at each purlin. Manufacturers set the pattern, often a fixing in every second or third trough, with more at the eaves, ridge and end laps; side lap stitching screws are extra. Use their figure if it differs from the default.

Before you order

• Measure the slope along the rafter, not the horizontal span. The roof area calculator and roof pitch calculator turn a span and pitch into a sloping length.
• Check the sheet's minimum pitch. Profiled sheets have a lowest pitch they are designed for, and it differs by profile and lap detail.
• Order the trims at the same time: ridge flashing on a duo pitch, a top or wall flashing on a mono pitch, verge flashings for both sides of each slope and eaves fillers to close the profile.
• For a roof on a home, or anywhere that needs Building Regulations sign-off, ask a roofer to confirm purlin sizes and fixing pattern for the wind exposure of your site.