Rafter Length
Calculate rafter length using roof rise and run or pitch. Free online construction calculator for roof truss dimensions, roof angle, and pitch with charts.
About This Calculator
The Rafter Length Calculator helps builders, architects, roofers, and DIY homeowners determine the exact sloping length of roof rafters using either the roof rise and horizontal run or the roof pitch ratio. Accurate rafter dimensions are essential for cutting rafters, ordering materials, estimating costs, and ensuring structural safety.
The calculation is based on the Pythagorean theorem: rafter length = √(rise² + run²). The rise is the vertical height from the wall top plate to the ridge board, and the run is the horizontal distance from the wall to directly under the ridge (half the total span). Alternatively, enter the roof pitch as a ratio (e.g. 6:12 means 6 inches of rise per 12 inches of run), and the calculator derives the rise automatically from the pitch and run. The roof angle in degrees is also computed using the arctangent of rise over run.
This tool supports two input modes: Rise & Run and Pitch & Run. Results include rafter length in feet and inches, roof pitch as an X:12 ratio, roof angle in degrees, and total rise. Interactive charts visualize how rafter length changes with different run distances and roof pitches, making it easy to compare options during the planning phase.
Regional Notes
India: Common roof pitches range from 4:12 to 8:12. RCC flat roofs are also prevalent, but pitched roofs with clay or concrete tiles use 6:12 as a standard. Rafter spacing is typically 16 inches OC for tile roofs. Follow the National Building Code (NBC) of India for structural requirements.
United States: The International Residential Code (IRC) governs rafter sizing and spacing. Asphalt shingle roofs commonly use 4:12 to 6:12 pitch with 24-inch OC spacing for 2×6 rafters. Snow loads in northern states may require steeper pitches (8:12 to 12:12) and closer spacing.
United Kingdom: Building Regulations Part A specifies minimum rafter sizes. Common pitches range from 30° to 45° (roughly 7:12 to 12:12), with 400 mm or 600 mm rafter centers. The UK's frequent rain and snow make steeper pitches more common than in warmer climates.
Frequently Asked Questions
How do I calculate rafter length?
Rafter length is calculated using the Pythagorean theorem: rafter length = √(rise² + run²). Enter the roof rise and horizontal run, or the roof pitch and run, and the calculator instantly returns the rafter length in feet. The rise is the vertical height from the top of the wall to the roof ridge, and the run is the horizontal distance from the wall to directly under the ridge.
What is the formula for rafter length?
The rafter length formula is: rafter length = √(rise² + run²). Using roof pitch, the formula becomes: rafter length = run × √(pitch² + 1), where pitch is the rise divided by the run expressed as a ratio (e.g. 6:12 = 0.5). This is derived directly from the Pythagorean theorem since the rafter forms the hypotenuse of a right triangle with the rise and run as the two legs.
What is the difference between rafter length and span?
The span is the total horizontal distance between the exterior walls that the roof covers. The run is half the span — the horizontal distance from one wall to directly under the ridge. The rafter length is the sloping distance from the wall top plate to the ridge board. For example, a 24-foot span with a 6:12 pitch has a 12-foot run and a rafter length of approximately 13.42 feet.
What is a standard roof pitch for houses?
Standard residential roof pitches range from 4:12 to 9:12. A 4:12 pitch (18.4°) is common for low-slope roofs in moderate climates, while 6:12 (26.6°) is one of the most widely used pitches in North America and India for residential homes. Steeper pitches like 8:12 (33.7°) and 9:12 (36.9°) are common in areas with heavy snowfall such as the UK and northern US.
How far apart should roof rafters be spaced?
Roof rafters are typically spaced 16 inches or 24 inches on center (OC), depending on local building codes, roof load, lumber size, and climate. In India, 16-inch spacing is common for clay tile roofs. In the US, the International Residential Code (IRC) permits 24-inch spacing for standard asphalt shingle roofs with 2×6 rafters. In the UK, rafters are often spaced at 400 mm or 600 mm centers per Building Regulations.
How do I calculate the number of roof trusses needed?
To calculate truss count, divide the total roof length (in inches) by the on-center spacing (typically 24 inches), then add 1 for the end truss. Formula: truss count = (roof length × 12 / spacing) + 1, rounded up. For example, a 40-foot roof with 24-inch spacing needs (40 × 12 / 24) + 1 = 21 trusses. Total cost = truss count × price per truss plus installation labor.
What is the difference between a rafter and a roof truss?
A rafter is a single sloped structural beam that extends from the roof ridge to the exterior wall. A roof truss is a prefabricated framework of multiple members (top chords, bottom chords, and webs) connected in triangular shapes. Trusses are stronger, can span longer distances, require less on-site labor, and are factory-engineered for specific loads. Rafters are cut and assembled on-site and are more common in custom or smaller roof designs.
What size lumber is used for rafters?
Common rafter lumber sizes include 2×6, 2×8, and 2×10. A 2×6 rafter typically spans up to 12 feet, 2×8 up to 16 feet, and 2×10 up to 20 feet for standard roof loads. Always consult local building codes — the International Residential Code (IRC) for the US, National Building Code (NBC) for India, and Building Regulations Part A for the UK specify minimum rafter sizes based on span, spacing, and load.