Coverage width is not panel width
A panel described as 36 inches covers 36 inches. The sheet itself is wider, because the ribs at each edge overlap the neighbouring panel. Order by coverage and the arithmetic works; order by sheet width and you come up short by one panel every ten or twelve. Standing seam is the same idea with different numbers, commonly 16 or 18 inches of coverage, and the sheet is wider again because the seam legs are formed up out of the material.
The count across a plane is the width divided by the coverage, rounded up, which almost always leaves an overrun. The last panel gets ripped down. What is worth checking is how much you are throwing away: if the overrun is nearly a full panel width, a different starting point or a product with a slightly different coverage can save you a panel per plane on both sides of the roof.
Panel length is the number to get right
Most suppliers roll exposed-fastener and standing seam panel to length, which is the significant advantage metal has over shingles on a simple roof: one piece from eave to ridge, no horizontal joints anywhere. The length you order is the slope length plus the projection past the eave, and the slope length comes from the horizontal run and the pitch. The multiplier is the square root of one plus the pitch over twelve, squared, which is 1.054 at 4/12, 1.118 at 6/12 and 1.202 at 8/12. Fourteen feet of run at 4/12 is 14.76 feet up the slope, and with two inches of overhang the panel is 14.93 feet.
Measure the run rather than trusting the plan. Walls are not always where the drawings say, ridges are not always centred, and a roof that has been altered can have two different runs on two sides. If the two sides differ, run this page twice with one plane each.
Where the fastener count comes from
The screws-per-square figure defaults to 80, which is a working number for exposed-fastener panel on purlins with a typical pattern, and it is the kind of figure that varies by more than you would like. The honest way to get it is to look at the fastening pattern in the installation instructions, count the screws on one panel, and multiply by the number of panels. A panel screwed in every rib at every purlin takes many more than one screwed in every other rib. Wind exposure changes the pattern too, and in high-wind regions the required pattern is denser and is set by the manufacturer instructions together with whatever your local authority requires. Nothing here substitutes for either.
Stitch screws at side laps, screws at end laps, and the closure strips and sealant tape that go with them are all extra and are not in the count above.
Metal roofs and standing on them
This has to be said without hedging. Falls from roofs kill and permanently disable people every year, and the risk has nothing to do with how big the job is. A metal roof is more slippery than a shingle roof, in every condition, and it is dramatically worse with dew, frost, dust or a film of pollen on it. Panels also flex underfoot and can hide where the purlins are. A steep metal roof, a wet metal roof or a high metal roof is not a do-it-yourself job. This page can compute material and it cannot make a roof safe to walk. If you are estimating a metal roof for a building you cannot safely get onto, use the numbers to buy intelligently and have the installation done by someone equipped for it.
What to ask the supplier before ordering
Four questions settle most of what this page cannot: the exact coverage width of the profile, whether they roll to length and what the maximum shippable length is, the minimum roof slope the profile is rated for, and what closure and sealant the profile needs at the eave and ridge. If you are comparing against asphalt for the same roof, the roofing squares calculator gives the shingle side of that comparison, and the roof pitch calculator will confirm the pitch you are feeding into both. Tearing off what is up there first is sized by the roof tear-off debris calculator.
Questions people ask
How do I measure the slope length if I cannot get on the roof?
Measure the horizontal run and the pitch, and let the multiplier do the rest, which is what the default mode here does. The run on a simple gable is half the building width including the overhang, measured on the ground along the gable end. The pitch can be taken at the gable end with a level and a tape against the rake board, without leaving the ladder or the ground on a low eave. That combination is more accurate than most people expect, because the multiplier is not very sensitive to small pitch errors: being half a step out at 4/12 changes the length by about a quarter of a percent.
Should I order panels to length or use stock lengths?
To length, wherever the supplier offers it and the panels can be transported and handled. A single piece from eave to ridge has no horizontal joint, and horizontal joints in metal roofing are where leaks start, because the lap has to shed water by geometry and sealant rather than by gravity alone. The limits are practical: very long panels are hard to carry up without kinking, they oil-can more visibly, and freight gets awkward past a certain length. Ask the supplier what they will ship.
Why does the calculator show overrun on the last panel?
Because a roof width is rarely an exact multiple of the panel coverage. If the width is 40 feet and the coverage is 36 inches, you need 13.33 panels, so you buy 14 and rip the last one to 4 inches of coverage. That is normal. What the note flags is the case where the overrun is nearly a full panel, which suggests the layout can be adjusted, or that the ripped panel from one plane could serve as the starter on the other. Whether that works depends on the profile, since some panels have a distinct male and female edge that cannot be reversed.
Can I put metal over the existing shingles?
Sometimes, and it depends on the panel system, the condition and number of the existing layers, whether the deck is sound, and what your local building department permits. It is not a decision this page makes. What is worth knowing is that going over shingles hides the deck, so any rot or delamination stays hidden and any fastener you drive is going into wood you have not looked at. If the roof is old enough to be replaced, that is also the one opportunity in thirty years to see the deck.
Does this handle hips, valleys and dormers?
Only crudely. It models identical rectangular planes, which covers a gable roof and many sheds and outbuildings well. A hip roof has trapezoidal and triangular planes where every panel is a different length, and a dormer breaks a plane into pieces with their own flashing. For those, break the roof into planes on paper, run the rectangular parts here, and treat the triangles as needing a panel-by-panel takeoff. The trim fields will still give you a reasonable ridge, hip and valley count if you enter the measured lengths.