What roof pitch means
Pitch is how many inches a roof rises for every 12 inches it runs level. A 6/12 roof (say "six-twelve") climbs 6 inches per foot. US roofers, framers and building codes all use this form. The same slope can also be written as an angle (26.6°) or a percent (50%), which is what you'll see on phone apps and outside the US. The calculator converts between all three.
Three ways to measure your pitch
- From the attic. Hold a level flat against the bottom of a rafter, mark 12 inches along the level, and measure straight up from the mark to the rafter. That number is the pitch. No ladder on the roof needed.
- From a gable end. Measure the height of the gable from the top of the wall to the peak (the rise) and half the width of the building (the run). Enter both in "Rise & run" in the same unit.
- With an angle. Lay a phone level app or an angle finder on a rafter, a fascia board or the rake trim, and read the degrees. Enter it in "Angle".
Measure on the framing or trim, not on a single shingle, since shingles lie a little flatter than the roof deck. Two readings in different spots will catch a sagging rafter.
The math, step by step
Here is what the calculator does with its opening values: a 6/12 pitch on a 24 x 40 ft gable house with 12 inch overhangs, a 1½ inch ridge board and 2x8 rafters 24 inches apart.
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1
Angle and slope
Angle = arctan(6 ÷ 12) = 26.6°. Slope = 6 ÷ 12 = 50%. Going the other way, pitch = 12 x tan(angle).
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2
Area multiplier
It is the length of the slope for one unit of level run: √(1 + (6 ÷ 12)²) = √1.25 = 1.118. Framers read the same thing off a rafter table as 13.42 inches of rafter per foot of run.
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3
Rafter length
The run is half the span: 12 ft. Take off half the ridge board, ¾ inch, for 11.94 ft. Times 1.118 is 13.35 ft. The 12 inch overhang adds 1 x 1.118 = 1.12 ft. Total: 14.46 ft, or 14 ft 5 9/16 in, so you buy 16 ft boards.
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4
Roof area
The footprint with overhangs is 42 x 26 = 1,092 sq ft. Times 1.118 = 1,221 sq ft of roof, or 12.2 squares (a roofing square is 100 sq ft). A hip roof over the same footprint and pitch has the same area.
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5
Framing list
40 ft at 24 inches on center is 21 rafters per side, 42 in all, each a 2x8x16. The ridge must be at least as deep as the rafter's sloped plumb cut: 7¼ x 1.118 = 8.1 in, so a 2x10. At $0.85 a board foot, that is $762 of rafters and $68 of ridge board: $830.
Roof pitch chart
Tap a pitch to load it. The hip and valley factor is the length of a hip or valley rafter for each foot of common run, the number to use when you measure hips, valleys and their ridge cap.
| Pitch | Angle | Slope | Multiplier | Hip/valley |
|---|---|---|---|---|
| 2/12 | 9.5° | 16.7% | 1.014 | 1.424 |
| 3/12 | 14.0° | 25% | 1.031 | 1.436 |
| 4/12 | 18.4° | 33.3% | 1.054 | 1.453 |
| 5/12 | 22.6° | 41.7% | 1.083 | 1.474 |
| 6/12 | 26.6° | 50% | 1.118 | 1.500 |
| 7/12 | 30.3° | 58.3% | 1.158 | 1.530 |
| 8/12 | 33.7° | 66.7% | 1.202 | 1.563 |
| 9/12 | 36.9° | 75% | 1.250 | 1.601 |
| 10/12 | 39.8° | 83.3% | 1.302 | 1.641 |
| 12/12 | 45.0° | 100% | 1.414 | 1.732 |
Minimum pitch for each roofing material
Water runs off a steep roof fast and backs up on a flat one, so the code sets a minimum slope for each kind of roofing. These come from Chapter 9 of the 2021 International Residential Code, which most US towns adopt in some version.
| Roofing | Minimum pitch | IRC section |
|---|---|---|
| Asphalt shingles | 2/12 (double underlayment up to 4/12) | R905.2.2 |
| Clay and concrete tile | 2½/12 (double underlayment up to 4/12) | R905.3.2 |
| Metal shingles | 3/12 | R905.4.2 |
| Mineral-surfaced roll roofing | 1/12 | R905.5.2 |
| Slate | 4/12 | R905.6.2 |
| Wood shingles and shakes | 3/12 | R905.7.1, R905.8.1 |
| Lapped metal panels | 3/12, or ½/12 with lap sealant | R905.10.2 |
| Standing seam metal | ¼/12 | R905.10.2 |
| Built-up roof | ¼/12 (coal tar ⅛/12) | R905.9.1 |
Manufacturers can be stricter than the code, and their warranty follows their own instructions. The code also has a rule tied to steepness: in places with ice dams, roofs at 8/12 and steeper need the ice barrier to reach at least 36 inches up the slope from the eave.
How pitch changes the cost of a roof
A steeper roof costs more in two ways. First, there is simply more of it: an 8/12 roof over the same house has about 7.5% more surface than a 6/12 one, and a 12/12 roof about 26% more. Second, labor climbs. HomeGuide notes that roofs steeper than 6/12 need extra safety gear and more labor, and the price per square goes up with them. Very low pitches cost more too, since they need double underlayment or a different roofing system.
To turn this pitch into shingles, underlayment, ridge cap and an installed price, take the roof area and pitch into the roofing calculator. For a full board-foot list of the rest of the framing, the lumber calculator takes any mix of sizes and lengths.
Rafters: what the length doesn't tell you
The rafter length here is the line length along the top edge, from the ridge to the tail of the overhang. To lay out a real rafter you also need the plumb cut at the ridge (the angle of the pitch), the birdsmouth notch where it sits on the wall, and the tail cut. A speed square has the pitch marks for all three. The calculator adds about 6 inches when it picks a board length, to leave room for those cuts.
- Size comes from the span tables. Whether a 2x8 can span your run depends on the species and grade of the lumber, the spacing, the snow load and whether there is a ceiling attached. Use the rafter span tables in IRC section R802, or ask your building department.
- The ridge must be deep enough. The IRC requires the ridge board to be at least as deep as the cut end of the rafter, which is deeper than the rafter itself on a sloped cut. The calculator picks the ridge size that way.
- Don't skimp on the birdsmouth. Common practice is to cut no deeper than about a third of the rafter depth, so the tail keeps its strength.
- Cut one, test it, then copy it. Cut a pattern rafter, try it at both ends of the building, and use it to mark the rest.
- Structural work needs a permit. New roof framing, a new pitch or a new addition almost always needs one, and many towns want stamped drawings for trusses.
Working safely on a sloped roof
OSHA's construction rules call any roof steeper than 4/12 a steep roof, and require guardrails, safety nets or a personal fall arrest system for workers on it. Homeowners are not bound by those rules, but the physics is the same. Measure from the attic or the ground when you can. If you must go up, stay off wet, frosty or very hot shingles, use a harness tied to a proper roof anchor at 6/12 and steeper, and leave two-story and steep roofs to a crew that has the gear.
Common questions
How do I calculate roof pitch?
Measure how many inches the roof rises over 12 inches of level distance. Hold a level against the roof or a rafter, mark 12 inches along it, and measure straight down or up to the roof surface. A 6 inch rise over 12 inches is a 6/12 pitch. For any rise and run, pitch = rise / run x 12.
What is a 6/12 roof pitch in degrees?
A 6/12 pitch is 26.6 degrees, a 50% slope. Its area multiplier is 1.118, so every 100 square feet of footprint is about 112 square feet of roof. A 4/12 roof is 18.4 degrees and a 12/12 roof is 45 degrees.
What is the minimum roof pitch for asphalt shingles?
The International Residential Code allows asphalt shingles on slopes of 2/12 or steeper. From 2/12 up to 4/12, the underlayment must be doubled. Below 2/12 you need a low-slope roof such as a membrane or a standing seam metal roof.
How do I calculate rafter length from pitch?
Take the run, which is half the building width on a gable roof, minus half the ridge board thickness. Multiply it by the pitch multiplier, then add the overhang times the same multiplier. A 24 ft wide building with a 6/12 pitch, a 1 1/2 inch ridge board and a 12 inch overhang needs rafters about 14 ft 5 9/16 in long, so 16 ft boards.
What is a roof pitch multiplier?
It is the number you multiply the flat footprint by to get the sloped roof area: the square root of 1 plus (rise / 12) squared. It is 1.054 for a 4/12 pitch, 1.118 for 6/12, 1.202 for 8/12 and 1.414 for 12/12.
What roof pitch is considered steep?
OSHA's fall protection rules call any roof steeper than 4/12 a steep roof, and 4/12 or less a low-slope roof. Roofers usually charge more above about 6/12, and from 8/12 they need roof jacks or a harness to work safely.
Sources and assumptions
Pitch, angle, multiplier, hip factor and rafter length are trigonometry. Code values are from the 2021 IRC; check the edition your town has adopted. The lumber price is a US retail figure from October 2026 and can be edited.
- Minimum slopes, double underlayment and the 8/12 ice barrier rule: 2021 International Residential Code, Chapter 9 (ICC), sections R905.1.2, R905.2.2, R905.3.2, R905.4.2, R905.5.2, R905.6.2, R905.7.1, R905.8.1, R905.9.1 and R905.10.2.
- Ridge board depth and rafter span tables: 2021 IRC, Chapter 8, sections R802.3 and R802.4.
- Steep and low-slope roof definitions: OSHA 29 CFR 1926.500, and fall protection on steep roofs in 1926.501.
- Pitch and labor cost: HomeGuide, roof replacement cost (November 2025).
- Lumber: $0.85 per board foot for SPF framing lumber, the same default as our lumber calculator, from LatestCost, 2x8 lumber prices (October 2026). Board feet use nominal sizes.
- Our assumptions: a simple gable roof; overhang measured level and the same at the eaves and rakes; a 1½ inch ridge board; 6 inches added for cuts before rounding up to an even board length (8 to 20 ft); ridge boards bought in 16 ft lengths; no extra rafters for gable overhang framing. In metric mode, timber lengths step by 0.6 m and spacing is 300, 400 or 600 mm.