Roof truss calculator

This roof truss calculator works out truss height, rafter length and truss count. Marcus's 24 ft garage at 6/12 needs a 6 ft tall truss and 21 trusses at 24 inches apart.

Span, pitch and length

Span is the outside width from wall to wall. Pitch is rise per 12 inches. Length is the run of the ridge, which sets how many trusses fit.

Your truss height and rafter length

Height at the peak is half the span times the pitch over 12. The top chord, or rafter, is the sloped side of that triangle plus the overhang. Lumber yards sell in 2 foot steps, so round up.

How the roof truss calculator works

Count is the building length in inches divided by the spacing, plus one for the far end. The math is the same right-triangle rule behind the Omni pitch calculator. Gable ends often get a special end truss.

When you need an engineer

Factory trusses come with a stamped drawing for your snow and wind load. Most building departments ask for that sheet before they issue a permit. Moving or cutting a truss without a repair letter can fail an inspection.

Setting trusses is work at height. OSHA requires fall protection at six feet on construction sites, per its fall protection rules.

Worked examples

1. Marcus: 24 ft span, 6/12, 40 ft long, 24 inch spacing. Height 6 ft 0 in, rafter about 14 ft 6 in, 21 trusses.
2. A 30 ft house at 8/12, 50 ft long, 24 inch spacing: height 10 ft 0 in, 26 trusses.
3. A 12 ft shed at 4/12, 16 ft long, 16 inch spacing: height 2 ft 0 in, 13 trusses.

Need the roof area as well? Use the roof size calculator or the pitch calculator.