Woodworking & Carpentry · Calculator
Roof Pitch & Rafter Calculator
This roof pitch calculator converts rise and run, an x/12 pitch, or an angle into pitch, degrees, slope percent and the roof multiplier. Add your building span to get common rafter length with overhang, the board length to buy, and plumb and seat cut angles.
Your roof
How to calculate roof pitch
Roof pitch is the inches of rise for every 12 inches of level run. A 6/12 roof climbs 6 inches for each foot you move toward the ridge. Angle and slope percent describe the same thing in different units.
On the roof (the 12 inch level method): hold a 2 ft level flat, with one end touching the roof. Mark 12 inches along the level and measure straight down to the shingles. That number is your pitch.
Without getting on the roof: go into the attic and do the same thing against the underside of a rafter. Or measure from a ladder at the gable end, along the rake board. You can also use a digital angle finder and switch the calculator to Angle mode, or divide the ridge height above the wall plates by half the building span.
Roof pitch chart: 1/12 to 12/12
Every value below is computed from the formulas above. The multiplier is also the rafter length per foot of run, so a 6/12 roof needs 13.42 inches of rafter for every 12 inches of run.
| Pitch | Angle | Slope | Multiplier | Rafter per ft of run | Category |
|---|---|---|---|---|---|
| 1/12 | 4.76° | 8.3% | 1.003 | 12.04 in | Low slope: membrane roofing |
| 2/12 | 9.46° | 16.7% | 1.014 | 12.17 in | Low slope: double underlayment for shingles |
| 3/12 | 14.04° | 25.0% | 1.031 | 12.37 in | Low slope: double underlayment for shingles |
| 4/12 | 18.43° | 33.3% | 1.054 | 12.65 in | Conventional slope |
| 5/12 | 22.62° | 41.7% | 1.083 | 13.00 in | Conventional slope |
| 6/12 | 26.57° | 50.0% | 1.118 | 13.42 in | Conventional slope |
| 7/12 | 30.26° | 58.3% | 1.158 | 13.89 in | Conventional slope |
| 8/12 | 33.69° | 66.7% | 1.202 | 14.42 in | Conventional slope |
| 9/12 | 36.87° | 75.0% | 1.250 | 15.00 in | Conventional slope |
| 10/12 | 39.81° | 83.3% | 1.302 | 15.62 in | Steep slope |
| 11/12 | 42.51° | 91.7% | 1.357 | 16.28 in | Steep slope |
| 12/12 | 45.00° | 100.0% | 1.414 | 16.97 in | Steep slope |
Common roof pitches and what they are used for
- Under 2/12 (low slope): porches, additions and commercial roofs. Shingles are not allowed. Use a membrane (EPDM, TPO, modified bitumen) or standing seam metal.
- 2/12 to under 4/12: sheds, porch roofs and modern designs. Shingles are allowed, but the IRC requires double underlayment, and water sheds slowly, so ice dams are a bigger risk.
- 4/12 to 9/12 (conventional): most houses. Sheds water well, takes standard shingle installation, and is still walkable.
- Over 9/12 (steep): snow country, Cape Cods and Tudors. It sheds snow well and adds attic room, but roofers need jacks and harnesses, which raises labor cost.
| Roofing | Min slope | Notes |
|---|---|---|
| Asphalt shingles | 2/12 | Double underlayment from 2/12 up to 4/12 (R905.2.2) |
| Wood shingles | 3/12 | R905.7.2 |
| Wood shakes | 3/12 | R905.8.2 |
| Slate | 4/12 | R905.6.2 |
| Clay or concrete tile | 2-1/2/12 | Double underlayment up to 4/12 (R905.3.2) |
| Metal panels, lapped seams | 3/12 | 1/2 in 12 with lap sealant (R905.10.2) |
| Standing seam metal | 1/4 in 12 | R905.10.2 |
| Membrane (EPDM, TPO, mod bit) | 1/4 in 12 | Needs slope to drain |
How rafter length is calculated
The run of a common rafter is half the building span, measured from the outside of the wall to the center of the ridge. Multiply it by the pitch multiplier and you get the line length: the theoretical rafter length along the slope.
Two adjustments turn that into a real board. Deduct half the ridge board, since the rafter stops at the ridge face, not its center. Then add the overhang, converted from its level length to the length along the slope. Both use the same multiplier.
The calculator also adds the plumb cut offset (rafter depth x pitch / 12) before picking a board length, because a plumb cut across a sloped board needs extra length. It does not size the rafter for strength. For that, use the IRC rafter span tables (R802.4.1) for your species, grade, spacing and snow load.
Plumb cut vs seat cut
Roof pitch calculator tips and common mistakes
- Use the multiplier for materials. Roof area is the footprint (with overhangs) times the multiplier. Get the footprint from our square footage calculator, then multiply.
- Measure span outside to outside. Using the inside of the walls shortens every rafter by the wall thickness times the multiplier.
- Cut one pattern rafter first. Test fit it at both ends of the building before you cut the rest. Walls are rarely perfectly parallel.
- Use the right saw. A circular saw handles long plumb cuts on 2x10s and 2x12s. A miter saw is faster for repeat cuts on blocking and smaller rafters.
Building stairs up to a loft or deck too? The stair stringer calculator uses the same rise and run logic. Bookmark this roof pitch calculator for your next shed, porch or addition, and round rafter cuts to the nearest 1/16" on the tape.
Frequently asked questions
What is a 6/12 roof pitch in degrees?
A 6/12 roof pitch is 26.57 degrees. The angle is the arctangent of rise over run: atan(6 / 12). It is also a 50 percent slope, with a multiplier of 1.118.
How do I calculate roof pitch?
Measure the rise over a known level run, then scale it to 12 inches of run. The easy way: hold a level horizontally against the roof or a rafter, mark 12 inches along it, and measure straight down (or up) to the roof surface. That measurement in inches is the pitch, so 6 inches means 6/12.
What is the most common roof pitch?
Most pitched residential roofs fall between 4/12 and 9/12, and 6/12 is a very common choice. That range sheds water well, takes standard shingle installation, and is still walkable for roofers. Steeper roofs are common in snowy regions and on some architectural styles.
What is the minimum roof pitch for asphalt shingles?
The IRC allows asphalt shingles on slopes of 2/12 or steeper. From 2/12 up to 4/12 you must install double underlayment. Below 2/12, use a low slope system such as a membrane or standing seam metal.
How do I calculate roof area from the pitch?
Multiply the flat footprint of the roof, including the overhangs, by the pitch multiplier. A 1,500 square foot footprint at 6/12 is 1,500 x 1.118 = 1,677 square feet, or about 16.8 squares of shingles before waste.
How do you calculate rafter length?
Multiply the rafter run (half the building span) by the pitch multiplier to get the line length to the ridge center. Subtract half the ridge thickness (times the multiplier) and add the overhang times the multiplier. A 24 foot span at 6/12 with a 1-1/2 inch ridge and 12 inch overhang gives a rafter of 14' 5-9/16".
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