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Roof Truss Count Calculator

Enter the roof length and truss spacing to find how many trusses you need.

How it is calculated

The tool converts the roof length to inches, counts how many spacings are needed to cover it, rounding up, and adds one for the first truss: trusses = ceil(length_ft × 12 ÷ spacing_in) + 1. Length is in feet and the on-center spacing is in inches, so a 2 ft spacing must be typed as 24.

When the length is an exact number of spacings, the last truss lands on the end of the roof. When it is not, for example 31 ft at 24 in, the final bay is shorter than the spacing and the tool rounds up, so that end still gets a truss. Both inputs must be above zero. The tool counts trusses only; it does not allow for extra trusses at gable ends, hips, dormers or girders, and it does not check the spacing against any building code or truss manufacturer limit.

Worked example

A roof is 31 ft long and the trusses are placed 24 in on center. Length in inches = 31 × 12 = 372 in. Spacings = 372 ÷ 24 = 15.5, which rounds up to 16. Trusses = 16 + 1 = 17, so the last, short bay still has a truss at the end. For a 40 ft roof at 24 in, 480 ÷ 24 = 20 exactly, so 21 trusses. At 16 in spacing the same 40 ft roof needs 480 ÷ 16 = 30 spacings, so 31 trusses.

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Common mistakes

  • Typing the spacing in feet (2) or centimetres (61) instead of inches. The field is in inches, so 24 is a typical value.
  • Entering the roof width or rafter length instead of the length along which the trusses are spaced.
  • Reading spacing as the clear gap between trusses. It is measured from the centre of one truss to the centre of the next.
  • Expecting the count to include gable-end, hip or girder trusses. Those special trusses are ordered separately.

About this calculator

Roof trusses are prefabricated triangular frames spaced at regular intervals — commonly 24 inches on-center for residential roofs — to support the roof deck and distribute loads down to the walls. The truss count is essentially the roof length divided by the spacing, plus one extra truss to cap both ends: Trusses needed = (Roof length ÷ spacing, rounded up) + 1, so a length that is not a whole multiple of the spacing still gets a truss at the far end.

Builders, roofers, and framing contractors use this figure early in a project to get accurate material quotes and schedule truss delivery, since trusses are usually custom-ordered from a manufacturer with lead times of days to weeks — underestimating the count mid-build can stall the whole framing schedule.

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Comments (1)

  • Editorial team

    Spacing is the biggest lever on the order, so run the same roof at a few values before you phone the supplier. A 40 ft roof needs 31 trusses at 16 in, 26 at 19.2 in and 21 at 24 in. Which spacing is allowed depends on the truss design, sheathing and local code, so the manufacturer or an engineer decides, not this count. Do you tend to quote at the spacing you expect or at the tightest one the supplier offers?

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