21/12 Roof Pitch
A 21/12 roof rises 21 inches for every 12 inches of horizontal run. That is a 60.26° angle, a grade of 175.0% and a roof area multiplier of 2.016, so every square foot of plan area becomes 2.016 square feet of roof surface.
Rafter length for a 21/12 pitch
Multiply the horizontal run by 2.016. Every foot of run needs 24.19 inches of common rafter. Add the overhang (horizontal overhang × 2.016) and deduct half the ridge board thickness.
| Building span | Run (½ span) | Common rafter | Ridge rise |
|---|---|---|---|
| 12 ft | 6 ft | 12 ft 1 in | 10 ft 6 in |
| 16 ft | 8 ft | 16 ft 1½ in | 14 ft 0 in |
| 20 ft | 10 ft | 20 ft 1¾ in | 17 ft 6 in |
| 24 ft | 12 ft | 24 ft 2¼ in | 21 ft 0 in |
| 28 ft | 14 ft | 28 ft 2½ in | 24 ft 6 in |
| 32 ft | 16 ft | 32 ft 3 in | 28 ft 0 in |
| 36 ft | 18 ft | 36 ft 3¼ in | 31 ft 6 in |
| 40 ft | 20 ft | 40 ft 3¾ in | 35 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 | 4.03 m | 3.50 m |
| 6 m | 3 m | 6.05 m | 5.25 m |
| 8 m | 4 m | 8.06 m | 7.00 m |
| 10 m | 5 m | 10.08 m | 8.75 m |
| 12 m | 6 m | 12.09 m | 10.50 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 21/12
Roof surface area = plan area × 2.016. Use the roof calculator for multi-section roofs, waste and material costs.
| Plan area | Roof surface | Squares (+10%) | Shingle bundles |
|---|---|---|---|
| 1,000 sq ft | 2,016 sq ft | 22.2 | 67 |
| 1,500 sq ft | 3,023 sq ft | 33.3 | 100 |
| 2,000 sq ft | 4,031 sq ft | 44.3 | 134 |
| 2,500 sq ft | 5,039 sq ft | 55.4 | 167 |
| 3,000 sq ft | 6,047 sq ft | 66.5 | 200 |
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 | 161.2 m² | 177.4 m² |
| 100 m² | 201.6 m² | 221.7 m² |
| 150 m² | 302.3 m² | 332.6 m² |
| 200 m² | 403.1 m² | 443.4 m² |
| 250 m² | 503.9 m² | 554.3 m² |
Plan area is the footprint measured to the outside of the eaves. Multiply by 2.016 for the sloped surface.
Roofing materials for a 21/12 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 21/12 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 21/12 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 21/12 roof cannot be walked. All work needs roof jacks or staging plus a personal fall arrest system.
21/12 roof pitch FAQ
What is a 21/12 roof pitch in degrees?
A 21/12 pitch is 60.26°. It rises 21 inches per 12 inches of run, so the angle is arctan(21 ÷ 12). As a percentage that is a slope of 175.0%, and in metric it rises 1,750 mm per metre.
What is the roof pitch multiplier for 21/12?
The 21/12 multiplier is 2.016. Multiply the roof's plan (footprint) area by 2.016 to get the sloped surface area. A 2,000 sq ft plan area, for example, is 4,031 sq ft of roof before waste.
How do I calculate rafter length for a 21/12 pitch?
Multiply the horizontal run by 2.016. Each foot of run needs 24.19 inches of rafter, so a 24 ft wide gable (12 ft run) needs a common rafter of about 24 ft 2¼ in before adding the overhang and deducting half the ridge board. Hip and valley rafters on a regular hip roof need 27.00 inches per foot of common run.
Can you walk on a 21/12 roof?
A 21/12 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 21/12 pitch?
Under the US International Residential Code a 21/12 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.