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Roof truss calculator

A roof needs one truss per spacing plus one: a 40 ft building at 24 in on centre takes 21 trusses. A 24 ft Fink truss at 6/12 stands 6′ 0″ tall, with top chords 13′ 5″ long and long webs 7′ 2-9/16″.

Framing Calculators editorial teamUpdated 2 sources

Calculator

Roof truss calculator

Truss count, chord and web lengths and heel-to-peak height for a common truss.

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in 12
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Type lengths the way you read a tape: 8' 9", 7 3/4 or 7.75. Metric: 2667 mm or 2.667 m.

24′ 0″6′ 0″
Trusses21length ÷ spacing + 1
Height at peak6′ 0″top chord line over the bottom chord
Top chord14′ 6-7/16″incl. 13-7/16″ tail

Line lengths

Bottom chord
24′ 0″
Short web (third point to mid top chord)
3′ 7-1/4″2 per truss
Long web (third point to peak)
7′ 2-9/16″2 per truss
Pitch angle
26.6°

Line lengths are to the centre lines of the members. Real trusses are designed and stamped by the truss maker for your loads; use these numbers to plan and price.

How it works. Count = building length ÷ spacing, rounded up, + 1. Height = half span × pitch ÷ 12. Top chord = √(half span² + height²). Fink webs run from the bottom-chord third points to the top-chord midpoints and the peak.

How many trusses do I need?

Divide the building length by the truss spacing, round up and add one for the end. Trusses are usually set at 24 in on centre, with gable-end trusses at each end. At 16 in spacing the same 40 ft building takes 31.

Trusses for common building lengths
Building length24 in spacing16 in spacing
16 ft913
20 ft1116
24 ft1319
30 ft1624
40 ft2131
50 ft2639
60 ft3146

Truss spacing and design

Truss spacing and member sizes are set by the truss design, not chosen on site. The IRC requires wood trusses to be designed by accepted engineering practice, with metal-plate-connected trusses made to ANSI/TPI 1 and delivered with truss design drawings (R802.10)[1]. Use this calculator to plan: count, rough heights for headroom and the length of material, before the truss maker's quote.

Fink vs king post trusses

  • Fink (W) truss: the common house truss. The bottom chord is split in thirds and four webs form a W, so the chords can be lighter.
  • King post truss: a single vertical web at the centre, used on short spans such as sheds and porches.
  • Both carry the roofing; check that the pitch suits it. Shingles need at least 2/12[2].

Roof framing calculator: trusses or rafters?

Trusses are faster to set and span the whole building without interior bearing walls; rafters leave the attic open. To compare, size rafters with the rafter calculator, check the pitch with the roof pitch calculator, and count sheathing with the plywood calculator.

Frequently asked questions

How many trusses do I need for a 24 ft building?

At 24 in on centre, 24 × 12 ÷ 24 = 12, plus 1: 13 trusses.

What is the standard truss spacing?

Most roof trusses are designed for 24 in on centre; the truss design drawings state the spacing to use.

How tall is a roof truss?

Half the span times pitch ÷ 12 at the centre line: a 24 ft truss at 6/12 rises 6 ft.

Can I build my own roof trusses?

The IRC requires trusses to be engineered; site-built trusses need a design. Metal-plate trusses come from a truss maker with drawings.

Sources

  1. 2021 International Residential Code, Chapter 8: Roof-Ceiling Construction (R802 rafters) · International Code Council, accessed November 13, 2026
  2. 2021 International Residential Code, Chapter 9: Roof Assemblies (R905 minimum slopes) · International Code Council, accessed November 13, 2026