18/12 Roof Pitch
A 18/12 roof rises 18 inches for every 12 inches of horizontal run. That is a 56.31° angle, a grade of 150.0% and a roof area multiplier of 1.803, so every square foot of plan area becomes 1.803 square feet of roof surface.
Rafter length for a 18/12 pitch
Multiply the horizontal run by 1.803. Every foot of run needs 21.63 inches of common rafter. Add the overhang (horizontal overhang × 1.803) and deduct half the ridge board thickness.
| Building span | Run (½ span) | Common rafter | Ridge rise |
|---|---|---|---|
| 12 ft | 6 ft | 10 ft 9¾ in | 9 ft 0 in |
| 16 ft | 8 ft | 14 ft 5 in | 12 ft 0 in |
| 20 ft | 10 ft | 18 ft 0¼ in | 15 ft 0 in |
| 24 ft | 12 ft | 21 ft 7½ in | 18 ft 0 in |
| 28 ft | 14 ft | 25 ft 2¾ in | 21 ft 0 in |
| 32 ft | 16 ft | 28 ft 10¼ in | 24 ft 0 in |
| 36 ft | 18 ft | 32 ft 5½ in | 27 ft 0 in |
| 40 ft | 20 ft | 36 ft 0¾ in | 30 ft 0 in |
Gable roof, imperial. Rafter length is measured along the slope from wall plate to ridge centre line, before eave overhang and ridge board deduction.
| Building span | Run (½ span) | Common rafter | Ridge rise |
|---|---|---|---|
| 4 m | 2 m | 3.61 m | 3.00 m |
| 6 m | 3 m | 5.41 m | 4.50 m |
| 8 m | 4 m | 7.21 m | 6.00 m |
| 10 m | 5 m | 9.01 m | 7.50 m |
| 12 m | 6 m | 10.82 m | 9.00 m |
Gable roof, metric. Rafter length is measured along the slope from wall plate to ridge centre line, before eave overhang and ridge board deduction.
Roof area at 18/12
Roof surface area = plan area × 1.803. Use the roof calculator for multi-section roofs, waste and material costs.
| Plan area | Roof surface | Squares (+10%) | Shingle bundles |
|---|---|---|---|
| 1,000 sq ft | 1,803 sq ft | 19.8 | 60 |
| 1,500 sq ft | 2,704 sq ft | 29.7 | 90 |
| 2,000 sq ft | 3,606 sq ft | 39.7 | 119 |
| 2,500 sq ft | 4,507 sq ft | 49.6 | 149 |
| 3,000 sq ft | 5,408 sq ft | 59.5 | 179 |
Plan area includes eave overhangs. Bundles assume 3 bundles per square (100 sq ft) with 10% waste; hips, valleys and dormers need more.
| Plan area | Roof surface | With 10% waste |
|---|---|---|
| 80 m² | 144.2 m² | 158.6 m² |
| 100 m² | 180.3 m² | 198.3 m² |
| 150 m² | 270.4 m² | 297.5 m² |
| 200 m² | 360.6 m² | 396.6 m² |
| 250 m² | 450.7 m² | 495.8 m² |
Plan area is the footprint measured to the outside of the eaves. Multiply by 1.803 for the sloped surface.
Roofing materials for a 18/12 pitch (US & Canada)
Minimum slopes from the 2021 International Residential Code. Canadian codes and manufacturers use similar limits for shingles and metal. Minimum pitch by material →
| Roofing system | At this pitch | Code section |
|---|---|---|
| Asphalt shingles | Meets the 2/12 minimum | IRC R905.2.2 |
| Clay & concrete tile | Meets the 2½/12 minimum | IRC R905.3.2 |
| Metal roof shingles | Meets the 3/12 minimum | IRC R905.4.2 |
| Slate & slate-type shingles | Meets the 4/12 minimum | IRC R905.6.2 |
| Wood shingles & shakes | Meets the 3/12 minimum | IRC R905.7.2 / R905.8.2 |
| Standing-seam metal panels | Meets the ¼/12 minimum | IRC R905.10.2 |
| Lapped (exposed-fastener) metal panels | Meets the ½/12 minimum | IRC R905.10.2 |
| Mineral-surfaced roll roofing | Meets the 1/12 minimum | IRC R905.5.2 |
| Membranes (TPO, EPDM, modified bitumen, built-up) | Allowed, but normally used on low-slope roofs | IRC R905.9–R905.13 |
Roofing materials for a 18/12 pitch (Australia & UK)
Typical manufacturer minimums. Check the specific product, sheet thickness, rafter length and exposure before ordering.
| Roofing system | At this pitch | Notes |
|---|---|---|
| Concealed-fix steel deck (e.g. Lysaght Klip-Lok 700) | Meets the typical 1° minimum | 1° applies to thicker sheet; 0.42 BMT is generally 2°. |
| Trapezoidal steel sheet (e.g. Lysaght Trimdek) | Meets the typical 2° minimum | — |
| Corrugated steel sheet (e.g. Lysaght Custom Orb) | Meets the typical 5° minimum | — |
| Interlocking concrete or terracotta tiles | Meets the typical 15° minimum | Profile dependent; many UK interlocking tiles need 17.5°–22.5°. |
| Natural slate | Meets the typical 20° minimum | Typical for larger slates; smaller slates and exposed sites need steeper pitches. |
| Plain tiles (clay or concrete) | Meets the typical 35° minimum | Some products allow 30°. |
| Flat-roof membranes (EPDM, single-ply, felt) | Usually reserved for flat and low-slope roofs | Designed for a finished fall of about 1:80 (0.7°) or more. |
Is a 18/12 roof steep?
Very steep roofs are mostly architectural: A-frames, Gothic and Tudor styles, and dormers. Expect a steep-slope labour surcharge on every quote.
A 18/12 roof cannot be walked. All work needs roof jacks or staging plus a personal fall arrest system.
18/12 roof pitch FAQ
What is a 18/12 roof pitch in degrees?
A 18/12 pitch is 56.31°. It rises 18 inches per 12 inches of run, so the angle is arctan(18 ÷ 12). As a percentage that is a slope of 150.0%, and in metric it rises 1,500 mm per metre.
What is the roof pitch multiplier for 18/12?
The 18/12 multiplier is 1.803. Multiply the roof's plan (footprint) area by 1.803 to get the sloped surface area. A 2,000 sq ft plan area, for example, is 3,606 sq ft of roof before waste.
How do I calculate rafter length for a 18/12 pitch?
Multiply the horizontal run by 1.803. Each foot of run needs 21.63 inches of rafter, so a 24 ft wide gable (12 ft run) needs a common rafter of about 21 ft 7½ in before adding the overhang and deducting half the ridge board. Hip and valley rafters on a regular hip roof need 24.74 inches per foot of common run.
Can you walk on a 18/12 roof?
A 18/12 roof cannot be walked. All work needs roof jacks or staging plus a personal fall arrest system. OSHA (29 CFR 1926.500) treats any roof steeper than 4/12 as a steep roof for fall protection purposes.
What roofing materials can go on a 18/12 pitch?
Under the US International Residential Code a 18/12 pitch meets the minimum slope for asphalt shingles, clay & concrete tile, metal roof shingles, slate & slate-type shingles, wood shingles & shakes, standing-seam metal panels, lapped (exposed-fastener) metal panels and mineral-surfaced roll roofing. Always check the manufacturer's installation instructions and your local code.