Metal Roofing Calculator

How the metal roof calculator works

Metal roof panels and profiled sheets go on in runs. Each run starts at the eaves and climbs to the ridge, and the runs sit side by side across the slope, each one lapping its neighbour by a rib or corrugation. That is how the calculator counts them. It works out how wide each slope is, how many runs of sheet it takes to cover that width, and how many sheets each run needs from eaves to ridge:

runs = n × ⌈A ÷ (n × S × W)⌉

k = 1 if L ≥ S, otherwise k = ⌈(S - o) ÷ (L - o)⌉

N = ⌈runs × k × (1 + w)⌉

A is the sloped roof area, n the number of roof slopes, S the slope length from eaves to ridge, W the cover width of one sheet, L the sheet length you are buying, o the end lap where one sheet overlaps the next up the slope, and w the waste allowance as a decimal. Each slope is rounded up to whole runs and each run to whole sheets. A part sheet is still bought as a sheet.

Why the count is done in runs, not by area

Dividing the roof area by the area of one sheet looks simpler. It only works when the sheets fit the roof with nothing left over, which almost never happens. On a real roof the last run on each slope is usually a part width, and if the sheets are shorter than the slope, the top sheet in each run is cut down. Both offcuts are paid for.

Take an 80 m² roof with two slopes, each 5 m from eaves to ridge, covered with 3 m sheets of 1000 mm cover and a 250 mm end lap. Each slope is 80 ÷ 5 ÷ 2 = 8 m wide, so it takes 8 runs, 16 in all. Each run needs (5 - 0.25) ÷ (3 - 0.25) = 1.73 sheets, so 2 sheets. That is 32 sheets, and with 5% waste 33.6, rounded up to 34. The area method gives 80 × 1.05 ÷ (2.75 × 1) = 30.5, or 31 sheets, which is 3 short. Change to 5 m sheets and each run is a single sheet: 16 sheets, or 17 with the waste.

A slope length is needed for this, so it is one of the inputs. It is the length measured along the roof from the eaves to the ridge, not the horizontal run. Add any overhang at the eaves you want the sheets to carry. The roof area can come from the roof area calculator, and the slope length from the roof pitch calculator.

Cover width is not the sheet width

Profiled sheets and panels are wider than the area they cover because one edge laps over the sheet beside it. The figure to enter is the cover width that the maker quotes. The Metal Sales PBR-Panel installation guide (2024) lists a 36 in (914 mm) coverage for every PBR panel, and Cladco's box profile range gives a 1000 mm cover width when lapped for both its 32/1000 and 34/1000 sheets. Standing seam panels are usually narrower than exposed-fastener panels, so the same roof takes more runs. Enter the overall sheet width by mistake and the count comes out short.

End laps and sheet length

Where one run needs more than one sheet, the upper sheet laps over the lower one. Metal Sales asks for at least 6 in (152 mm) on its PBR panel. Cladco's installation guide asks for at least 250 mm on its box profile and corrugated sheets, with the lap supported underneath by a purlin. The US side of the calculator starts at 6 in and the metric side at 250 mm. The right figure is the one from the maker of the sheet you are buying; it varies with the profile and the pitch.

Most panels can be ordered cut to the length of the slope, which does away with end laps altogether. Metal Sales gives 45 ft (13.7 m) as its maximum recommended panel length. Cladco supplies box profile up to 9 m depending on thickness (7 m in 0.5 mm for its 34/1000), tile-form sheet up to 6 m, and corrugated up to 5 m in 0.5 mm and 7 m in 0.7 mm thickness, with a two-sheet run beyond that. When the sheet length entered is at least the slope length, the calculator counts one sheet per run and shows the length to order each sheet at. Long sheets are harder to deliver and lift, so check what can reach the site.

Minimum roof slope

Metal roofing will leak if it is laid flatter than the profile allows. In the United States, section R905.10.2 of the International Residential Code, Chapter 9 (the same in the 2021 and 2024 editions) sets the minimum slope for lapped, nonsoldered-seam metal roofs at 3 in 12 (25%) without lap sealant and 1/2 in 12 (4%) with lap sealant, and for standing seam roofs at 1/4 in 12 (2%). Makers can be stricter. Metal Sales gives 1 in 12 as the minimum for PBR-Panel and recommends sealant on the sidelaps of every roof. Cladco recommends at least 4 to 5 degrees for box profile, 10 for corrugated and 12 for tile-form sheet. Elsewhere, the sheet maker's published minimum and your local building rules decide. The calculator lists these figures as a reminder; it does not check the pitch of your roof.

Worked example (US units)

A gable roof of 860 ft² has two slopes of 15 ft, and the panels are 36 in cover, ordered 16 ft long. Each slope is 860 ÷ 15 ÷ 2 = 28.67 ft wide. At 3 ft per panel that is 9.56 runs, rounded up to 10, so 20 runs in all. The 16 ft panels are longer than the 15 ft slope, so each run takes one panel cut to 15 ft. With 5% waste, 20 × 1.05 = 21 panels. At $75 a panel the estimate is $1,575. With 10 ft panels and a 6 in end lap instead, each run needs (15 - 0.5) ÷ (10 - 0.5) = 1.53, so 2 panels, and the order doubles to 42.

Hip roofs and odd shapes

The run count treats each slope as a rectangle whose width is its area divided by its slope length, which is its average width. On a gable or lean-to roof that is exact. The faces of a hip roof are trapezoids and triangles, and they are wider at the eaves than their average. For those faces the result is the number of full-length sheets that the area uses up, not the number of pieces: the runs near the hips are cut on the diagonal, and a cut sheet saves material only if its offcut can be turned round and used on the next run or face. Whether it can depends on the profile.

For a hip roof, enter the whole sloped area with 4 slopes and raise the waste allowance, often to 10 to 15%, or count the runs from a layout drawing. The same applies around valleys and dormers. The default of 5% suits a plain gable or lean-to roof.

Choosing the waste allowance

Because the run method already counts the part-width runs and the cut top sheets, the waste allowance only needs to cover damage in handling, a spare sheet or two and cuts at hips, valleys and penetrations. A simple gable roof with sheets cut to length needs little. Each extra hip, valley, rooflight or chimney adds cutting. The result rounds up to whole sheets after the allowance is added.

What this tool does not do

It counts sheets and prices them. It does not count screws, closures, ridge and verge flashings, lap tape or sealant, and it does not size purlins or check fixings for wind load. Cladco gives purlin centres of no more than 1.2 m for 0.7 mm sheet and 1 m for 0.5 mm sheet as a rough guide; the maker's span tables and a structural engineer decide the real figures. Roof work is work at height and needs proper access and edge protection.

Using this calculator with other BuildMetricLab tools

The roof area calculator gives the sloped area, the roof pitch calculator the slope length and angle, and the ridge cap calculator the ridge pieces. All BuildMetricLab tools run in your browser with no sign-up, and the formula is on the page so the maths can be checked.