Roof Slope Calculator
Enter the roof rise and run to calculate pitch, angle, grade, and rafter length. You can also add total roof span to estimate one-side rafter length from the same slope.
Tip: Common roof pitch is often shown as X-in-12. For example, 6-in-12 means 6 units of rise for every 12 units of run.
What is roof slope?
Roof slope describes how steep a roof is. It compares vertical change (rise) to horizontal distance (run). Builders, architects, and homeowners use slope to determine drainage performance, roofing materials, structural loads, and visual style.
You’ll see roof slope expressed in several ways:
- Ratio: rise:run (example: 1:2)
- Pitch: X-in-12 (example: 6-in-12)
- Angle: in degrees (example: 26.57°)
- Grade: percentage (example: 50%)
How this roof slope calculator works
Input values
You provide rise and run in any consistent unit (feet, inches, meters, etc.). As long as both inputs use the same unit, the slope calculations are accurate.
Calculated outputs
- Slope ratio: rise/run
- Pitch (X-in-12): (rise ÷ run) × 12
- Roof angle: arctan(rise ÷ run)
- Grade: (rise ÷ run) × 100
- Rafter length: √(rise² + run²)
- Slope factor: rafter length ÷ run
Why roof pitch matters
1) Water and snow shedding
Steeper roofs generally drain faster and shed snow better. Low-slope roofs can work, but they need the right membrane systems, drainage details, and maintenance plans.
2) Material compatibility
Some roofing products require minimum slope values. For instance, certain shingles may not be recommended below a specific pitch. Always check manufacturer specs.
3) Structural design
Pitch affects rafter length and the way loads are transferred. As slope changes, framing geometry changes too, impacting material quantity and installation complexity.
4) Curb appeal and interior volume
Roof slope has a major visual effect. It can also influence attic space, vaulted ceilings, and overall building proportions.
Common roof slope ranges
- Low-slope: under ~2:12 (special systems typically required)
- Conventional residential: about 4:12 to 9:12
- Steep-slope: above 9:12
- Very steep: above 12:12 (safety and installation complexity increase)
Example calculation
Suppose your roof has a rise of 6 ft and run of 12 ft:
- Slope = 6/12 = 0.5
- Pitch = 0.5 × 12 = 6-in-12
- Angle = arctan(0.5) = 26.57°
- Grade = 0.5 × 100 = 50%
- Rafter length = √(6² + 12²) = 13.42 ft
Tips for accurate measurements
- Measure rise and run from the same reference points.
- Keep units consistent across all entries.
- Double-check dimensions before ordering materials.
- When in doubt, verify with a laser level or digital angle finder.
Important note
This calculator is for planning and educational use. Local building code requirements, snow/wind loads, material installation rules, and engineered design criteria may vary by location. For final construction decisions, consult licensed professionals and current code documents.