Truss Spacing Calculator
Roof trusses are pre-engineered structural members that form the roof frame. Ordering the correct number is straightforward once you know the building length and the on-center (OC) spacing. The standard formula is: number of trusses = (building length in feet / spacing in feet) + 1. The result is rounded up to the next whole truss to ensure complete coverage. This accounts for one truss at each end of the building plus all intermediate trusses. Common residential truss spacings are 12 inches, 16 inches, and 24 inches on center. At 24 inches OC (2 ft), standard 7/16-inch OSB sheathing is typically sufficient for roof decking. At 48 inches OC or wider, thicker sheathing or additional purlins are usually required. Truss spacing must match your engineering plans and local building code requirements. Always obtain an engineered truss design for any project requiring a building permit, as trusses must be sized for local snow, wind, and seismic loads.
Truss count formula
Spaces = Length (ft) / Spacing (ft)
Trusses = Ceiling(Spaces) + 1
Worked example
Building: 40 ft. Spacing: 24 in (2 ft) OC.
- Spaces = 40 / 2 = 20 spaces
- Trusses = 20 + 1 = 21 trusses
At 16 in OC
- Spaces = 40 / 1.333 = 30 spaces
- Trusses = 30 + 1 = 31 trusses
Frequently asked questions
How many trusses do I need for a 40-foot building?
For a 40-ft building with trusses spaced 24 inches (2 ft) on center: (40 / 2) + 1 = 21 trusses. At 16 inches on center: (40 / 1.333) + 1 = 31 trusses. Always round up and add the end trusses. This calculator handles the math for common spacings.
What is the standard spacing for roof trusses?
The most common residential truss spacing is 24 inches on center. Many manufactured homes and some site-built homes use 16 inches on center. Agricultural and commercial buildings sometimes use 4 ft, 6 ft, or 8 ft spacing with heavier trusses. Check your structural plans for the required spacing.
Is the number of trusses the same as the number of spaces?
No. The number of trusses is one more than the number of spaces. A building with trusses every 2 ft along a 24-ft length has 12 spaces but 13 trusses (one at each end plus one at each intermediate point). The formula is (Length / Spacing) + 1.
Do I need to add gable end trusses separately?
The formula (Length / Spacing) + 1 includes both end trusses (gable trusses). If your design uses a different gable end detail, such as a fly rafter instead of a gable truss, you may need to subtract the end trusses and order them separately. Confirm with your truss supplier.
How does truss spacing affect cost and material?
Wider spacing (24 in OC) uses fewer trusses, which reduces truss cost but requires thicker sheathing and may require larger ceiling joists or furring to carry the load between trusses. Closer spacing (16 in OC) uses more trusses but can support standard 7/16 or 1/2 inch OSB sheathing. Your structural engineer or truss designer specifies the appropriate spacing.
Sources
- Structural Building Components Association (SBCA): Truss design standards.
- International Residential Code (IRC): Table R802.4.1 rafter span tables.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. Truss design must be performed by a licensed engineer for permitted projects.