Roof Truss Calculator
Calculate trusses, rafters, and sheathing. Enter values for instant results with step-by-step formulas.
Formula
Trusses = (Roof Length / Spacing) + 1
Divide roof length by truss spacing and add one. Calculate rafter length using pitch and span with Pythagorean theorem.
Worked Examples
Example 1: Simple Gable Roof
Problem:40 ft long building, 24 ft span, 6/12 pitch, 24\" truss spacing.
Solution:Trusses:\n(40 × 12) / 24 + 1 = 21 trusses\n\nRafter length per side:\nRun = 24/2 = 12 ft\nRise = 12 × (6/12) = 6 ft\nLength = √(12² + 6²) = 13.4 ft\n\nWith 1 ft overhang: 14.4 ft\n\nRoof area: 40 × 14.4 × 2 = 1,152 sq ft
Result:21 trusses, 1,152 sq ft roof area
Example 2: Steeper Roof
Problem:30 ft × 28 ft span, 8/12 pitch, 2 ft overhang.
Solution:Trusses (24\" O.C.):\n(30 × 12) / 24 + 1 = 16\n\nRafter length:\nRun: 14 ft\nRise: 14 × (8/12) = 9.33 ft\nLength: √(14² + 9.33²) = 16.8 ft\nWith overhang: 18.8 ft\n\nSheathing:\n30 × 18.8 × 2 = 1,128 sq ft\n1,128 / 32 = 36 sheets
Result:16 trusses, 36 sheathing sheets
Example 3: Shed Roof
Problem:12×16 shed, 4/12 pitch, rafters 16\" O.C.
Solution:Rafters:\n(16 × 12) / 16 + 1 = 13\n\nRafter length (12 ft span):\nRise: 12 × (4/12) = 4 ft\nLength: √(12² + 4²) = 12.6 ft\n\nWith 1 ft overhang: 13.6 ft\n\nSheathing: 16 × 13.6 = 218 sq ft\n218 / 32 = 7 sheets
Result:13 rafters, 7 OSB sheets
Frequently Asked Questions
What is truss spacing?
Standard: 24\" on-center (every 2 feet). Some use 16\" for heavy snow loads or long spans. Spacing must match rafter tables and sheathing span ratings. Closer spacing = stronger roof but more material. Most residential uses 24\" with proper sheathing.
What roof pitch should I use?
Low pitch (3/12-4/12): Modern look, less material, some roofing limits. Medium (6/12-8/12): Traditional, good shedding, balanced. Steep (10/12-12/12): Max shedding, snow/rain, more materials. Code minimums: asphalt shingles need 2/12+, metal can go lower. Climate affects choice.
How do I calculate roof pitch?
Pitch = rise over run, expressed per 12 inches horizontal. 6/12 pitch = 6 inches rise for every 12 inches run. To measure existing: 12\" level on rafter, measure vertical at 12\" mark. Pitch affects rafter length, roofing area, and material costs.
How much do roof trusses cost?
Typical: $3-8 per linear foot of span. A 30-foot span truss: $90-240 each. Complex designs (hip, valley, scissors) cost more. Includes delivery but not installation. Often cheaper than stick-framing rafters when including labor. Get quotes from local truss manufacturers.