How much roof will “100 metres of steel sheet” actually cover?

The same sheet might have an overall width of 1.173 m. In that case, 100 linear metres have an overall area of 117.3 m². This is the same material measured on a different basis. A price per linear metre is useful only if you know the width. Comparing two profiles by length alone can give a misleading result.

Cover width is the width a sheet contributes to the finished roof after allowing for the side lap. Overall width includes the part used in the lap. Check which width the supplier uses to calculate the price per square metre. Applying a gross-area unit price to the effective cover area gives the wrong total.

Ordering 100 m² of effective cover does not guarantee enough sheets to finish a 100 m² roof. Full-width sheets may need trimming, and end laps use extra material. Converting the units puts quotations on the same basis. A separate cutting plan determines how much to buy.

Take two rectangular slopes, each 10 × 5 m. With a cover width of 1.128 m, each slope needs nine sheets across its 10 m width. Eight cover only 9.024 m, whereas nine cover 10.152 m. The order therefore contains 18 sheets, each 5 m long. That is 90 linear metres and 101.52 m² of effective sheet cover.

This illustration leaves out openings, end laps and adjustments at the roof edges. It is not a finished order list. It shows how a 100 m² roof of this shape can take less than 100 linear metres of sheet. Sheet width and layout matter as much as the combined area of the slopes.

For illustration, assume approximately €9.61 excluding VAT per square metre of effective cover. The table uses the same profile and the same unrounded rate for all three quantities. The example prices cover the sheet alone. Flashings, fixings, delivery and installation are additional.

The price difference comes from buying different quantities at the same rate. When comparing suppliers, first convert their prices to the same units. Then check whether they have used the same sheet layout.

Specify the number of sheets in each length. An order for 100 linear metres could mean 20 sheets of 5 m each or a completely different set of lengths. Choose lengths that suit both the roof and the delivery vehicle. Label each group by roof slope to make checking and installation easier. For long sheets, agree where to set them down after unloading. Limited space may require different lengths or a revised delivery plan.

Include the exact profile, thickness, coating and colour. List flashings, fasteners and any specified moisture-control measures. Agree the unloading arrangements. One incorrect length can mean a second delivery even when the total metres on the invoice are correct. Check the piece list as carefully as the price.

File the cutting plan with the profile designation and width. If one sheet needs replacing years later, a note saying “100 metres purchased” is of little use. You will need dimensions, edge geometry and finish. Photographs showing the sheet layout and notes on unusual junctions can help identify a replacement.

Before extending a roof, check that the same cross-section is still available. Similar profile codes do not guarantee the same cover width. New sheets may also differ in colour from the weathered roof. A clear specification makes it easier to judge whether a single sheet can be replaced or a larger section needs attention.