12/12 roof pitch
A 12/12 roof rises 12 inches over every 12 inches of run. That one ratio fixes the angle at 45.00° and puts 1.4142 feet of rafter under every foot of run. What it does not tell you is on this page: the lengths run by run, what the 2021 IRC rafter tables list at this pitch, and which roof coverings the code will let you lay on that slope.
Rafter length, run by run
Multiply the run by 1.4142 and you have the rafter. The middle column is the width of a symmetric gable whose ridge sits over the center of the building, where the run is half the width.
| Horizontal run | Gable width | Rafter length at 12/12 |
|---|---|---|
| 4 ft | 8 ft | 5’7⅞” |
| 5 ft | 10 ft | 7’0⅞” |
| 6 ft | 12 ft | 8’5⅞” |
| 7 ft | 14 ft | 9’10¾” |
| 8 ft | 16 ft | 11’3¾” |
| 9 ft | 18 ft | 12’8¾” |
| 10 ft | 20 ft | 14’1¾” |
| 11 ft | 22 ft | 15’6⅝” |
| 12 ft | 24 ft | 16’11⅝” |
| 13 ft | 26 ft | 18’4⅝” |
| 14 ft | 28 ft | 19’9⅝” |
| 15 ft | 30 ft | 21’2½” |
| 16 ft | 32 ft | 22’7½” |
| 17 ft | 34 ft | 24’0½” |
| 18 ft | 36 ft | 25’5½” |
| 19 ft | 38 ft | 26’10½” |
| 20 ft | 40 ft | 28’3⅜” |
| 21 ft | 42 ft | 29’8⅜” |
| 22 ft | 44 ft | 31’1⅜” |
| 23 ft | 46 ft | 32’6⅜” |
| 24 ft | 48 ft | 33’11¼” |
Each length is the sloping distance over that much horizontal run and nothing more: no overhang, and no allowance for how the rafter meets the ridge. Lengths are shown to the nearest eighth of an inch.
What the 2021 IRC rafter spans mean at 12/12
Rafters are sized on the spans in the R802.4.1 tables, and a rafter span is measured along the horizontal projection of the rafter — the run, not the sloping stick. A span in 2021 IRC Table R802.4.1(3) is therefore a horizontal distance, and the rafter that covers it is longer by the 1.4142 multiplier. Below, the span each of the two tables this site carries lists for Southern pine No. 2 at 16-inch centers, with the rafter length that span works out to at 12/12.
| Rafter | R802.4.1(3) span | Rafter at 12/12 | R802.4.1(1) span | Rafter at 12/12 |
|---|---|---|---|---|
| 2×4 | 7’6” | 10’7¼” | 9’0” | 12’8¾” |
| 2×6 | 11’2” | 15’9½” | 13’6” | 19’1⅛” |
| 2×8 | 14’2” | 20’0⅜” | 17’1” | 24’1⅞” |
| 2×10 | 16’10” | 23’9⅝” | 20’3” | 28’7⅝” |
| 2×12 | 19’10” | 28’0⅝” | 23’10” | 33’8½” |
2021 IRC Table R802.4.1(3) — ground snow load 30 psf, ceiling not attached to rafters, 30 psf ground snow / 10 psf dead / L/180; 2021 IRC Table R802.4.1(1) — roof live load 20 psf, ceiling not attached to rafters, 20 psf roof live / 10 psf dead / L/180. Southern pine No. 2 at 16-inch centers in both. table source
Every species, grade, and spacing for both tables — and the other sizes — is on the rafter span pages. A rafter length in the right-hand columns is the slope length of the longest run those tables allow, not a span in its own right: the code never asks how long the stick is, only how far it reaches horizontally. Point loads, cantilevers beyond the table footnotes, unusual snow loads, or anything else outside the table's stated conditions — that's an engineer's question.
Roof coverings a 12/12 slope can carry
Chapter 9 of the 2021 IRC gives each roof covering a minimum slope, stated as a rise over 12. At 12/12, all 13 covering entries in R905 clear their minimum.
| Permitted at 12/12 | Minimum slope | Section |
|---|---|---|
| Built-up roofs | ¼:12 | R905.9.1 |
| Metal roof panels — standing seam | ¼:12 | R905.10.2 |
| Membrane roofs — modified bitumen, thermoset and thermoplastic single-ply, sprayed polyurethane foam, liquid-applied | ¼:12 | R905.11.1, R905.12.1, R905.13.1, R905.14.1 and R905.15.1 |
| Metal roof panels — lapped, nonsoldered seam with lap sealant | ½:12 | R905.10.2 |
| Mineral-surfaced roll roofing | 1:12 | R905.5.2 |
| Asphalt shingles | 2:12 | R905.2.2 |
| Photovoltaic shingles | 2:12 | R905.16.2 |
| Clay and concrete roof tile | 2½:12 | R905.3.2 |
| Metal roof shingles | 3:12 | R905.4.2 |
| Metal roof panels — lapped, nonsoldered seam without lap sealant | 3:12 | R905.10.2 |
| Wood shingles | 3:12 | R905.7.2 |
| Wood shakes | 3:12 | R905.8.2 |
| Slate shingles | 4:12 | R905.6.2 |
Minimum slopes as stated in the 2021 IRC R905 sections named on each row. chapter source
Coal-tar built-up roofs are the one covering the code takes lower still, to a ⅛:12 slope.
The R802 rules that turn on pitch
R802.4.1 rafter size. Rafters are sized on the spans in the R802.4.1 tables, and a rafter span is measured along the horizontal projection of the rafter — the run, not the sloping stick.
R802.4.4 draws its line at 3:12, and it applies only below that line — 12/12 is not under it, so the ridge, hip, and valley members over a 12/12 roof are not pushed into beam design by that section alone.
2021 IRC R802.4.1 and R802.4.4, stated here in plain words and cited so your plan reviewer can read the sections themselves. Point loads, cantilevers beyond the table footnotes, unusual snow loads, or anything else outside the table's stated conditions — that's an engineer's question. chapter source
Other pitches
Every whole-inch pitch, with its angle, multiplier, and rafter lengths: 1/12 · 2/12 · 3/12 · 4/12 · 5/12 · 6/12 · 7/12 · 8/12 · 9/12 · 10/12 · 11/12
All twelve side by side, plus the rafter-length tool: the roof pitch tables. The same multiplier turned into a panel count, a panel length and a roof area is on figuring metal roofing.
Common questions
What angle is a 12/12 roof?
45.00° off horizontal — a rise of 12 inches over 12 inches of run, or a grade of 100%. The angle follows from the ratio alone, so it is the same on every 12/12 roof regardless of span, species, or load.
How long is a rafter on a 12/12 roof?
1.4142 feet of rafter for every foot of horizontal run. Over a 12-foot run — a 24-foot-wide symmetric gable — that is 16’11⅝”; over 16 feet of run it is 22’7½”. Those lengths are plate to ridge along the slope: no overhang, and no allowance for how the rafter meets the ridge.
Can you put asphalt shingles on a 12/12 roof?
Yes. R905.2.2 sets the minimum at 2:12, and 12/12 clears both that and the 4:12 top of the section's double-underlayment band. all 13 covering entries in R905 are permitted at 12/12.
Do the IRC rafter span tables already account for a 12/12 pitch?
They do not adjust for it, and they do not need to: R802.4.1 measures a rafter span along the horizontal projection of the rafter. So the 14’2” that 2021 IRC Table R802.4.1(3) lists for a 2×8 Southern pine No. 2 at 16-inch centers is 14’2” of run — at 12/12 the stick itself is 20’0⅜” long.