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Roof area × the profile's extra surface × coats ÷ the spread rate on the can, plus 10%. A 30 × 40 ft roof at 4/12 in R-panel is 1,374 sq ft to paint, so two coats at 300 sq ft per gallon take 11 gallons. Corrugated adds about 23% surface, standing seam about 19%.
Roof paint = footprint × pitch factor × the panel's surface factor × coats × 1.10 ÷ the spread rate on the can. Profiled metal has more surface than its covered width: about 22.7% more for 2.67 in × 7/8 in corrugated, 9.2% for small 1¼ in corrugation, 8.6% for R-panel ribs and 18.75% for 16 in standing seam with 1½ in seams. Direct-to-metal paints list 150–400 sq ft per gallon per coat — about 300 for Valspar's Metal Building Paint. A 30 × 40 ft footprint at 4/12 is 1,265 sq ft of roof and 1,374 sq ft to paint in R-panel, so two coats take 11 gallons: two 5-gallon pails and a gallon.
Gallons = roof footprint × pitch factor × the surface factor of the panel profile × coats × 1.10 ÷ the spread rate printed on the paint. The surface factors are geometry: the paint follows every wave and rib, so profiled metal has more surface than its covered width.
| Profile | Factor | Assumed shape |
|---|---|---|
| Flat surface (shingles, flat sheet, smooth deck) | 1.000 | No profile |
| Corrugated 2.67 in × 7/8 in | 1.227 | Sine wave, 2.67 in pitch, 7/8 in deep |
| Corrugated 1¼ in × ¼ in (small wave) | 1.092 | Sine wave, 1¼ in pitch, ¼ in deep |
| R-panel / PBR (ribs every 12 in) | 1.086 | Ribs 1¼ in high, 1 in across the top, 3½ in at the base, 12 in apart |
| Standing seam, 16 in panels, 1½ in seams | 1.188 | Both faces of a 1½ in seam every 16 in |
| Paint | Sq ft per gallon |
|---|---|
| Valspar Metal Building Paint (waterborne acrylic, semi-gloss) | about 300 |
| Rust-Oleum 3800 System DTM acrylic enamel | 150–270 |
| Benjamin Moore High Performance Quick Dry Acrylic DTM (HP3000) | 300–400 |
| Footprint | Flat surface | Corrugated 2.67 in × 7/8 in | Corrugated 1¼ in × ¼ in | R-panel / PBR | Standing seam |
|---|---|---|---|---|---|
| 600 sq ft | 5 gal | 6 gal | 6 gal | 6 gal | 6 gal |
| 1,000 sq ft | 8 gal | 10 gal | 9 gal | 9 gal | 10 gal |
| 1,500 sq ft | 12 gal | 15 gal | 13 gal | 13 gal | 14 gal |
| 2,000 sq ft | 16 gal | 19 gal | 17 gal | 17 gal | 19 gal |
| 3,000 sq ft | 24 gal | 29 gal | 26 gal | 26 gal | 28 gal |
Roof area × the panel's surface factor × coats ÷ the spread rate on the can. A 30 × 40 ft footprint at 4/12 is 1,265 sq ft of roof; in R-panel (about 1.09 × the flat area) that is 1,374 sq ft to paint, so two coats at 300 sq ft per gallon plus 10% is 11 gallons: two 5-gallon pails and a gallon.
Usually 150 to 400 sq ft per coat, and the can says which. Valspar's Metal Building Paint lists about 300 sq ft per gallon, Rust-Oleum's 3800 DTM acrylic 150–270 and Benjamin Moore's HP3000 DTM 300–400. Rusty, chalky or rough metal soaks up more, so use the low end.
Yes. The paint covers the full wave, not the flat width. Treated as a sine wave, 2.67 in × 7/8 in corrugated has about 23% more surface than its covered width, small 1¼ in × ¼ in corrugation about 9%, and R-panel ribs about 9%.
No. Paint goes on a few mils thick and restores colour and rust protection on sound metal. Elastomeric and silicone roof coatings go on many times thicker, at 50–100 sq ft per gallon per coat, to waterproof and reflect heat — use the roof coating calculator for those.
Prime bare steel, rust spots and new galvanized metal with a metal primer the paint maker approves; a sound, clean factory finish usually only needs washing and scuffing. Count the primer as one more coat at its own spread rate.
Waterproofing rather than repainting? The roof coating calculator sizes elastomeric and silicone coatings.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.