Roof Pitch Guide: How to Calculate Rise, Run, Rafter Length & Pitch Angle
Roof pitch = rise ÷ run (expressed as X:12). A 6:12 pitch rises 6 inches for every 12 inches of horizontal run. Rafter length = √(rise² + run²).
Understanding Pitch Notation (X:12 Format)
In the US, roof pitch is conventionally expressed as 'X in 12' or 'X:12', meaning X inches of vertical rise for every 12 inches of horizontal run. The 12 is standard because it simplifies measurement on a framing square — one foot of run. Common pitch categories: Low pitch (1:12–3:12) — requires specialized low-slope roofing (TPO, modified bitumen,
Calculating Rafter Length
The rafter is the hypotenuse of a right triangle formed by the rise (vertical leg) and run (horizontal leg). The run for a common rafter equals half the building width (from the ridge to the exterior wall top plate). Step-by-step example: Building width = 28 feet, pitch = 6:12. Run = 28/2 = 14 feet. Rise per 12" run = 6", so total rise = (14 × 6)/1
Converting Between Pitch, Angle & Percentage
Different trades and contexts use different slope expressions. Architects often specify degrees; contractors use X:12; civil engineers and roofing calculators may use percentage slope. Conversions: Pitch X:12 → Degrees: θ = arctan(X/12). Degrees → Pitch: X = tan(θ) × 12. Percentage slope: S% = (rise/run) × 100 = (X/12) × 100. Examples: 5:12 = arcta
Roofing Materials by Pitch
Material selection depends critically on pitch — minimum slope requirements are dictated by drainage physics and are codified in most building codes. 0:12–2:12 (flat to low slope): Built-up roofing (BUR), EPDM rubber, TPO, PVC, or modified bitumen. Standing seam metal works down to 1:12 with proper seaming. 2:12–4:12: Roll roofing, some metal panel
Frequently Asked Questions
How do I measure my existing roof pitch?
Use a level and a tape measure. Hold a 12-inch level horizontally against the roof surface, starting from the roof surface (not the attic rafter). Measure the vertical distance from the end of the level to the roof surface. That measurement in inches is your rise for a 12-inch ru
How does pitch affect roofing material quantities?
Roofing quantities use 'squares' (1 square = 100 sq ft of roof area). The roof area always exceeds the building footprint due to slope. Multiply the footprint area by the pitch multiplier: 4:12 × 1.054, 6:12 × 1.118, 8:12 × 1.202, 10:12 × 1.302, 12:12 × 1.414. A 2,000 sq ft footp
What is the minimum roof pitch for snow country?
Most building codes in snow regions require a minimum 3:12 pitch, and roof structures must be engineered for the local ground snow load (typically 20–100 psf in mountain states). Steeper pitches shed snow more effectively — 6:12 and above shed snow well; below 3:12, snow accumula
How do I calculate the ridge board length?
The ridge board runs horizontally at the peak of the roof, parallel to the building's length. For a simple gable roof: Ridge length = Building length − (2 × rafter run × 0) = building length for a full gable. For a hip roof: Ridge length = Building length − Building width (the ri
Can I change my roof pitch without rebuilding the structure?
Significantly changing roof pitch requires structural work — new rafters, ridge board repositioning, and typically new sheathing. It's a major renovation comparable in cost to a full roof replacement plus framing. Minor adjustments (adding a steeper overlay pitch for aesthetic or