Planning a metal roof
Metal roofing is sold as long panels that run from the ridge down to the eave. Unlike shingles, which are counted in squares, panels are counted one by one and usually cut to length at the factory. Getting the count and lengths right avoids a second delivery and leftover panels that cannot be returned.
How the calculation works
- Panels per side = eave length ÷ panel coverage width, rounded up.
- Panels needed = panels per side × number of roof sides.
- Panel length = rafter length from ridge to eave, plus the overhang.
- Roof area = eave length × rafter length × number of sides. One roofing square is 100 sq ft.
- Trim: the ridge cap runs along the ridge, the eave trim along each gutter edge, and the rake (gable) trim up both sloped edges of each side.
Worked example
A gable roof with a 40-foot eave and 16-foot rafters including the overhang, covered with exposed-fastener panels of 36-inch coverage:
- Panels per side: 40 × 12 ÷ 36 = 13.3, rounded up to 14.
- Panels needed: 28, each 16 feet long.
- Roof area: 40 × 16 × 2 = 1,280 sq ft, or 12.8 squares.
- Ridge cap: 40 ft. Eave trim: 80 ft. Rake trim: 32 ft in total.
- Screws: at 30 per panel, about 840.
Coverage width is the key number
Panels overlap their neighbors by one rib, so the width they cover is smaller than their full width. Common profiles:
| Panel type | Typical coverage |
|---|---|
| Exposed-fastener (R-panel, PBR) | 36 in |
| Corrugated | 24 to 34 in |
| Standing seam | 12 to 18 in |
Always use the coverage width from the manufacturer’s data sheet.
Things to add to your order
- Trim lengths usually come in 10- or 12-foot pieces that overlap a few inches, so divide the totals by about 9.5 or 11.5 feet to count pieces.
- Closure strips under the ridge cap and at the eave keep out insects, birds and wind-driven rain.
- Underlayment, typically synthetic, under the whole roof.
- Valleys, hips and dormers need extra panels and trim; this calculator assumes simple rectangular sides.
Screws and fastening
Exposed-fastener panels are screwed through the flat of the panel or the rib into the purlins or deck, in a set pattern of several screws across each panel at every purlin line. The number per panel depends on the profile, the panel length and the wind zone; 80 screws per 100 sq ft is a common first estimate. Use screws with neoprene or EPDM washers and do not overtighten them, which crushes the washer and causes leaks.
Safety
Metal panels are slippery, especially when wet or dusty, and their edges are sharp. Work with fall protection, wear gloves, and never walk on panels on a steep roof without a harness and anchor.