Metal Roofing Calculator
Last reviewed How we calculate
Metal roofing is ordered by the panel, and each panel is usually cut to the exact length of the roof from eave to ridge. That makes the order simpler than shingles in one way and less forgiving in another: the count depends on the coverage width of the panel, and the length has to be right before anything is made.
Panels, fasteners and ridge cap for a metal roof
Panels, panel length, fasteners and ridge cap for a metal roof, from the eave length, slope length and panel coverage.
How to measure
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Panels to order Each one cut to the panel length below.
- Panels per side
- Panel length
- Total length of panel
- Roof area covered
- Roofing squares
- Panel fasteners
- Ridge cap pieces
This assumes full-length panels on rectangular roof sides. Hips, valleys, dormers and skylights all add cut panels and trim, and every manufacturer has its own lap and fastening rules. Give your measurements to the supplier as well: most will produce a cut list, and theirs is the one to order from.
How the calculation works
Divide the eave length by the coverage width of one panel and round up: that is the number of panels on one side of the roof. Multiply by the number of sides for the total. Every panel is cut to the slope length from eave to ridge, which is the figure the supplier needs from you. Fasteners are estimated from the roof area and the number per square in the manufacturer fastening schedule, and ridge cap from the ridge length and the effective length of one piece.
A worked example
A simple gable with 40 ft eaves, a 16 ft slope length and panels covering 36 in each needs 14 panels per side, because 13 would leave the last foot uncovered. That makes 28 panels at 16 ft, covering 1,280 sq ft. At 80 fasteners a square that is 1,024 fasteners, and a 40 ft ridge takes 4 pieces of cap at 10 ft each after laps.
Coverage width is the number that matters
Panels are described by their full width, but they overlap at the side seam, so each one covers less than it measures. Using the full width in the sum is the most common way to end up a panel short on each side. The coverage width is printed on every manufacturer data sheet.
Where this method is wrong
It assumes rectangular roof faces. On a hip roof the panels on each face are cut to different lengths, and valleys and dormers add cut panels, trim and flashing that a simple count cannot capture. Most metal roofing suppliers will prepare a cut list from your measurements or drawings. Order from theirs, and use this to check it is in the right region.
Frequently asked questions
How long can a metal roof panel be?
Panels are usually made to the full slope length, so each one runs from eave to ridge without a joint. Very long panels are limited by transport and handling, so check with the supplier for long roofs.
Can metal roofing go over shingles?
Sometimes, where the local code and the panel manufacturer allow it and the deck underneath is sound. It often needs battens or a new underlayment first, so check both before planning it.
How many screws does a metal roof need?
It depends on the panel, the wind zone and the purlin spacing, and the manufacturer fastening schedule sets it. Around 70 to 90 fasteners per square is common for exposed-fastener panels.