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Metal Roofing Calculator Guide
Panels, length, ridge cap and screws — worked from the footprint and the pitch, because a roof is always bigger than the building under it.
Pitch multiplier = √(1 + (rise/12)²)
A 4/12 pitch gives √(1 + 0.111) = 1.054, so a 1,200 sq ft footprint is 1,265 sq ft of roof.
Panels = roof width ÷ panel coverage width — rounded up.
Coverage Width, Not Panel Width
This is the mistake that leaves people a panel short. A sheet sold as "36 inch" often measures 38–39 inches and covers 36 once the rib laps over its neighbour.
The coverage width is the number that matters. Divide by the sheet width instead and you'll be short — and the last panel on a roof is the one you can't improvise.
The Roof Is Bigger Than the Building
A 40×30 building doesn't have 1,200 sq ft of roof. At 4/12 it has about 1,265, and at 12/12 it has nearly 1,700 — the same footprint, 40% more material.
Pitch is why two identical-looking houses take very different quantities. Measure the pitch rather than guessing it: a level, a ruler and 12 inches of run will tell you in a minute.
Panels Run Eave to Ridge
Metal roofing panels are cut to run the full slope in one piece — that's most of the point of the product, since a continuous panel has no horizontal seams to leak through.
Panel length is the rafter length plus your overhang. On a big roof that can mean panels longer than a standard truck, which is a delivery problem worth discovering before order day rather than on it.
Screws Are Consumable and Directional
Roughly 80 per square is a working figure, and it climbs with exposed-fastener panels, closer purlin spacing and high-wind detailing.
They also have a service life. The neoprene washer that makes the seal degrades in UV long before the steel does, so an exposed-fastener roof needs its screws checked and eventually replaced — which is the honest trade against a standing-seam roof that hides its fasteners and costs more upfront.
What This Doesn't Cover
Valleys, dormers, hips, chimneys and skylights all add material and a lot of labour. A complex roof is not a big simple roof — it's a different job, and the cut waste around every penetration is real.
Related: saltbox roof calculator, DIY shed cost calculator, framing calculator.
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