Roof Truss Calculator (How Many Trusses)

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Enter the building length along the ridge and the truss spacing. The calculator counts trusses from one end wall to the other and separates the gable end trusses from the common ones.

Truss count calculator

How it works

  • Spaces = building length in inches ÷ spacing, rounded up
  • Trusses = spaces + 1
  • Common trusses = trusses − gable end trusses

Rounding up means the last space can be shorter than the spacing, never longer. Truss spacing, bracing and connections come from the truss design drawings, which the IRC requires for every job (R802.10).

Worked example

A 40 ft building with trusses at 24 in on center and two gable ends:

  • Length of the building (along the ridge): 40 ft
  • Truss spacing: 24 in on center
  • Gable end trusses: Two gable ends

The calculator gives:

  • Trusses in total: 21
  • Common trusses: 19
  • Gable end trusses: 2
  • Spaces: 20 spaces of up to 24 in

480 in ÷ 24 = 20 spaces, so 21 trusses: 2 gable ends and 19 commons.

Notes and limits

  • Overhangs at the gable ends (a “rake” overhang) are usually built with ladder framing or outlookers, not extra trusses.
  • Hip roofs use girder trusses and a set of hip trusses from the truss maker; count them from the truss layout, not from this tool.
  • Don’t cut, notch or alter a truss without approval from a design professional; repairs are designed, not improvised.

Frequently asked questions

How many trusses for a 24 ft building?

At 24 in on center, 13 trusses (12 spaces + 1). At 16 in, 19.

How far apart are roof trusses?

Most houses use 24 in on center. The truss design drawings set the spacing for each job.

Do I count the gable end as a truss?

Yes. Gable end trusses sit on the end walls and are counted in the total; they are a different design from the commons.

Sources

  1. International Code Council, 2021 International Residential Code, Section R802 Wood Roof Framing (rafter span tables; R802.10 wood trusses)

Last reviewed: 26 September 2026. The calculation is covered by automated tests with known answers taken from the sources above. Found an error? Please let us know.

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