Rafter Calculator
Calculate rafter dimensions, cut angles, and roof geometry. Get precise measurements for common rafters, hip rafters, and shed roof framing.
⚠️ Professional Design Required
This calculator provides estimates for educational purposes. Always consult with a licensed structural engineer or architect before construction. Local building codes and load requirements vary.
Roof Dimensions
Calculation Results
Bird's Mouth Dimensions
Bird's mouth notch sits on top plate. Standard seat cut is 3.5" for 2×4 wall plate.
Cutting Guide
1. Ridge Cut (Top)
Cut plumb at 26.57° to meet ridge board
2. Bird's Mouth (Seat)
Located 80.50" from ridge along rafter
- • Seat cut: 3.50" horizontal
- • Plumb cut: 1.75" vertical
3. Tail Cut (Fascia)
Cut at 63.43° or plumb depending on fascia style
Pitch ↔ Angle Converter
Common Roof Pitches
Rafter Basics
Roof Pitch
Expressed as rise over run (e.g., 6:12 means 6" rise for every 12" horizontal run). Steeper roofs shed water better but require more material.
Common Rafter
Runs from ridge board to wall plate perpendicular to the ridge. Most roofs primarily use common rafters.
Bird's Mouth
Notch cut in rafter to sit on wall plate. Provides solid bearing surface and keeps rafter from sliding off.
Material Sizing
Use 2×6, 2×8, 2×10, or 2×12 depending on span and load. Consult span tables for your specific design loads.