How is roof pitch calculated?
Roof pitch is the rise in inches over 12 inches of horizontal run, so divide any measured rise by its run and multiply by 12. The angle is the arctangent of rise over run, and the percent grade is rise over run times 100. All three describe the same slope; roofers and framers usually say pitch, while product data sheets often give degrees or percent.
The rafter multiplier is the length along the slope for each unit of horizontal run. Multiply the rafter run plus the overhang by it to get the common rafter length, before deducting half the ridge thickness and allowing for the plumb cut and tail. Our roof pitch and slope guide explains how pitch shows on a roof framing plan.
Roof pitch to degrees chart
Table: common roof pitches with angle, percent grade and rafter multiplier (calculated).
| Pitch | Angle | Percent grade | Rafter multiplier |
|---|---|---|---|
| 2/12 | 9.46° | 16.7% | 1.014 |
| 3/12 | 14.04° | 25.0% | 1.031 |
| 4/12 | 18.43° | 33.3% | 1.054 |
| 5/12 | 22.62° | 41.7% | 1.083 |
| 6/12 | 26.57° | 50.0% | 1.118 |
| 7/12 | 30.26° | 58.3% | 1.158 |
| 8/12 | 33.69° | 66.7% | 1.202 |
| 9/12 | 36.87° | 75.0% | 1.250 |
| 10/12 | 39.81° | 83.3% | 1.302 |
| 12/12 | 45.00° | 100.0% | 1.414 |
What is the minimum roof pitch for each roof covering?
The 2021 International Residential Code (IRC) sets minimum slopes by roof covering. Asphalt shingles need at least 2/12 under Section R905.2.2, with double underlayment from 2/12 up to 4/12. Clay and concrete tile need at least 2-1/2/12 under Section R905.3.2, and metal roof shingles need at least 3/12 under Section R905.4.2. Manufacturer instructions can be stricter, so check them too.
Underlayment rules change with pitch as well; see roof underlayment requirements and ice and water shield.
How do you measure the pitch of an existing roof?
- Hold a level horizontally against the underside of a rafter or on the roof surface.
- Mark 12 inches along the level from where it touches the roof.
- Measure straight up or down from that mark to the rafter or roof surface. That is the rise.
- Enter the rise with a run of 12 to read the pitch directly.
Rafter sizes for a given span come from the code tables, not from pitch alone. See the rafter span chart and parts of a roof for the members involved.
Plan the framing under your roof
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Open the framing studioFrequently asked questions
How do you calculate roof pitch?
Roof pitch is the vertical rise in inches for every 12 inches of horizontal run. Measure the rise over any known run, divide rise by run and multiply by 12. A roof that rises 3 inches over 6 inches of run has a 6/12 pitch, about 26.6 degrees.
What is a 6/12 roof pitch in degrees?
A 6/12 roof pitch is about 26.57 degrees, because the angle is the arctangent of 6 divided by 12. As a percent grade it is 50 percent. The common rafter multiplier for a 6/12 pitch is about 1.118, so each foot of run needs about 1.118 feet of rafter.
What is the minimum roof pitch for asphalt shingles?
The 2021 IRC Section R905.2.2 allows asphalt shingles on roof slopes of 2/12 or steeper. From 2/12 up to 4/12, the IRC requires double underlayment applied under Section R905.1.1. Metal roof shingles need at least 3/12 under Section R905.4.2.
How do you calculate rafter length from roof pitch?
Multiply the rafter's horizontal run, usually half the building span, plus the overhang projection by the pitch's rafter multiplier. The multiplier is the square root of pitch squared plus 144, divided by 12. The result is the length along the slope before the ridge and plumb-cut deductions.
Results are preliminary layout figures for planning. The building code edition and local amendments adopted where you build govern, and the building official has the final say.