Rafter Calculator

Roof pitch describes the steepness or slope of a roof. It's typically expressed as a ratio of rise over run:

• Rise: The vertical height the roof gains over a horizontal distance
• Run: The horizontal distance, typically measured as 12 inches in the US
• Example: A 6/12 pitch means the roof rises 6 inches for every 12 inches of horizontal run
• Common pitches: 4/12 (18.43 degrees), 6/12 (26.57 degrees), 9/12 (36.87 degrees), 12/12 (45 degrees)

You can also measure pitch in degrees using an angle finder or digital level placed on the roof surface.

A birdsmouth cut is a notch cut into the bottom of a rafter where it sits on the top plate of the wall:

• Seat cut: The horizontal portion that rests on the top plate
• Plumb cut: The vertical portion that sits against the wall
• Purpose: Creates a stable connection between the rafter and wall, preventing the rafter from sliding
• HAP (Height Above Plate): The remaining depth of the rafter above the birdsmouth, typically should be at least 2/3 of the rafter depth

Building codes typically limit the birdsmouth depth to no more than 1/3 of the rafter depth to maintain structural integrity.

The ridge board height is measured from the top of the wall plates to the top of the ridge board:

• Total Rise: Calculate using the formula: Run x Pitch (rise per foot)
• Ridge adjustment: Subtract half the ridge board thickness from the rafter run to account for the ridge
• Wall plate height: Add this to the total rise if measuring from the floor
• Standard ridge boards: Use 2x material (1.5" actual) or engineered lumber for longer spans

The ridge should be at least as deep as the plumb cut of the rafter for proper bearing surface.

Rafter spacing depends on several factors including roof loads, rafter size, and local building codes:

• 16" on center: Most common for residential construction, provides strong support for most roofing materials
• 24" on center: Acceptable for some applications, often with larger rafters or lower loads
• 12" on center: Used for heavy loads like tile roofs or areas with high snow loads
• Factors to consider: Snow load, roofing material weight, span length, and local code requirements

Always consult local building codes and a structural engineer for specific load requirements in your area.

Roof overhang (also called eaves) protects walls from water and provides shade:

• Standard residential: 12-18 inches is typical for most homes
• Minimal overhang: 6-8 inches for modern or budget designs
• Extended overhang: 24-36 inches for additional protection or architectural style
• Climate considerations: More overhang in rainy climates, less in windy areas
• Structural note: Longer overhangs may require additional support (outriggers or lookouts)

Consider your local climate, architectural style, and maintenance access when choosing overhang length.