How to Use This Calculator
Roof pitch describes how steeply a roof rises. In residential framing, pitch is expressed as a ratio: “rise over run,” typically stated per 12 inches of horizontal run. A 6:12 pitch means the roof rises 6 inches for every 12 inches of horizontal distance. This calculator converts between pitch ratios, angles in degrees, and rafter lengths.
Enter your rise and run in inches, and the calculator gives you the pitch angle in degrees, the slope percentage, and the multiplier for calculating rafter lengths. For a common rafter spanning a 20-foot building width with a 6:12 pitch, you’d calculate half the span (10 feet) as your run, find the rise (10 × 6/12 = 5 feet), then use the Pythagorean theorem or the calculator’s rafter length feature to get your cut length.
Different pitches affect water drainage, wind resistance, and framing labor. A 4:12 pitch is common in areas with moderate rain and light snow. Steeper 8:12 or 10:12 pitches shed water and snow faster, essential in heavy-snow regions, but require more material and are harder to frame safely. Flatter 2:12 to 3:12 pitches suit arid climates and are easier to walk, but demand extra insulation and careful flashing work.
Formula
Pitch Angle (in degrees): Angle = arctan(rise / run)
Where rise and run are in the same units (typically inches per 12 inches of run).
Slope Percentage: Slope % = (rise / run) × 100
Rafter Length (using Pythagorean theorem): Rafter length = √(rise² + run²)
Where rise and run are the vertical and horizontal distances (in feet or inches—use consistent units).
Common pitch multipliers (for 12” run):
- 2:12 = 1.02 (angle 9.5°)
- 4:12 = 1.05 (angle 18.4°)
- 6:12 = 1.12 (angle 26.6°)
- 8:12 = 1.20 (angle 33.7°)
- 10:12 = 1.30 (angle 40.0°)
- 12:12 = 1.41 (angle 45.0°)
When to Use This
You need this before you touch a saw. Pitch determines the angle of every rafter, the spacing of shingles, and the feasibility of the job. A 12:12 pitch roof (45-degree angle) demands fall protection and is slow to frame. A 2:12 pitch requires special underlayment and extra flashing; it’s flat enough that water can pool if not perfectly sloped.
Snow loads, rain exposure, and wind uplift all depend on pitch. A 4:12 roof in a heavy-snow area is asking for a collapsed roof—carpenters in those regions frame 8:12 or steeper. Know your climate, check your local codes, and frame accordingly. The pitch affects not just framing but also the building’s performance over decades.
Code References
- IRC R905.2: Roof covering requirements vary by pitch; some materials require minimum pitches
- IRC R905.2.8.1: Asphalt shingle minimum pitch is 4:12 (some low-pitch products allow 2:12)
- IRC R802: Wood roof framing prescriptions based on snow load, wind speed, and span
- Local snow load and wind speed maps: Determine required truss ratings and member sizing
Frequently Asked Questions
Can I use the same pitch multiplier for all rafter lengths?
Yes, as long as all rafters on your roof have the same pitch. The multiplier (derived from the Pythagorean theorem) converts the horizontal run to rafter length at that angle. If you have a hip roof with different pitches on different sides, calculate each separately.
How do I measure pitch on an existing roof?
Use a level and a tape measure. Place the level on the roof parallel to the ridge. Measure up 12 inches along the slope, then measure the vertical rise at that point with a tape measure held plumb. That vertical rise per 12 inches is your pitch. Example: 6 inches up per 12 inches out = 6:12 pitch.
What’s the minimum pitch for water drainage?
Building code usually specifies 4:12 for asphalt shingles, 3:12 for metal roofing, and 2:12 for low-slope built-up or modified bitumen. Anything flatter than 2:12 requires special calculation for water accumulation and ice damming. In heavy-rain climates, steeper is better; in arid climates, flatter is acceptable.
Do I need to adjust rafter length for overhang?
Yes. Calculate the slope length from the interior wall to the ridge, then add the overhang dimension (usually 12 to 24 inches) projected along the slope, not horizontally. If your overhang is 18 inches horizontal on a 6:12 pitch, the slope distance is 18 × 1.12 = 20.16 inches. Add that to your main rafter length.
Why does the rafter length multiplier change with pitch?
Because the rafter follows the slope of the roof, not a horizontal line. The longer the vertical rise relative to the run, the longer the hypotenuse (the rafter). The multiplier is simply the Pythagorean relationship: if rise = 6 and run = 12, then rafter = √(36 + 144) = √180 ≈ 13.42. Divide by the run (12) and you get 1.12—the multiplier for 6:12 pitch.