Roof Truss Calculator
Calculate roof trusses or rafters needed with sheathing estimates. Determine quantities for any roof span, pitch, and spacing.
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: (40 × 12) / 24 + 1 = 21 trusses Rafter length per side: Run = 24/2 = 12 ft Rise = 12 × (6/12) = 6 ft Length = √(12² + 6²) = 13.4 ft With 1 ft overhang: 14.4 ft Roof 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.): (30 × 12) / 24 + 1 = 16 Rafter length: Run: 14 ft Rise: 14 × (8/12) = 9.33 ft Length: √(14² + 9.33²) = 16.8 ft With overhang: 18.8 ft Sheathing: 30 × 18.8 × 2 = 1,128 sq ft 1,128 / 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: (16 × 12) / 16 + 1 = 13 Rafter length (12 ft span): Rise: 12 × (4/12) = 4 ft Length: √(12² + 4²) = 12.6 ft With 1 ft overhang: 13.6 ft Sheathing: 16 × 13.6 = 218 sq ft 218 / 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.
Trusses vs rafters - which is better?
Trusses: Pre-fabricated, engineered, faster installation, span longer without support, more economical for standard designs. Rafters: Stick-framed on-site, allows attic space, better for custom/complex roofs. Most modern homes use trusses except custom designs.
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.
What size trusses do I need?
Determined by span, spacing, pitch, and loads (snow, wind). Truss manufacturer engineers based on your specs. Common residential span: 24-30 feet. Trusses can span 60+ feet for commercial. Never modify trusses - engineering is specific to design.
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.
Can I store trusses on-site?
Stack flat on level blocking, not directly on ground. Support at panel points (not mid-span). Cover to protect from weather. Don't stand trusses upright unsupported - they can fall/warp. Lift trusses properly (use multiple people or equipment). Install as soon as practical.
Do I need engineering for trusses?
Truss manufacturer provides engineering (included in cost). They stamp and seal drawings. This satisfies most building codes. Custom stick-framed roofs or modifications may require separate structural engineering. Truss manufacturer won't stamp modified trusses.
What sheathing for roof?
OSB: Most common, economical, adequate for most applications. Plywood: More moisture-resistant, better for exposed overhangs. Thickness: ½" for 24" spacing is common, ⅝" for better performance. Use rated sheathing for structural strength, not just boards.
How many people to install trusses?
Minimum 3-4 people for safety. Light trusses (under 30 ft): can hand-set with crew. Longer/heavier: use crane or boom truck. Plan installation day carefully - trusses must be braced properly as installed. Weather should be calm (wind can blow trusses over).
Background & Theory
Roof trusses or rafters support the roof covering and transfer loads to walls. Proper spacing and sizing ensure structural integrity.
Truss vs Rafter:
| Feature | Trusses | Rafters |
|---|---|---|
| Installation | Faster | Slower |
| Span | Longer | Limited |
| Attic space | Limited | Full |
| Cost | Lower | Higher |
| Customization | Limited | Flexible |
Common Pitches:
| Pitch | Angle | Use |
|---|---|---|
| 3/12 | 14° | Low slope |
| 4/12 | 18° | Minimal |
| 6/12 | 27° | Standard |
| 8/12 | 34° | Steep |
| 12/12 | 45° | Very steep |
Spacing:
24\" O.C. standard
16\" for heavy snow/wind
Rafter Length:
Length = √(Rise² + Run²) + Overhang