Roof Truss Calculator
Count the trusses and bracing needed for a building of any length.
What you'll need to order
- roof trusses
- hurricane ties
- lateral bracing lumber
- truss clips
Prices vary by region and haul distance. Get quotes from at least two local suppliers.
How this calculator works
trusses = (building length × 12 ÷ spacing) + 1
The "+1" accounts for the truss at both ends — a 40 ft building at 24 in spacing has 20 spaces but 21 trusses. Gable-end trusses differ from common trusses and are usually ordered separately.
Switch the site to metric — the control is in the footer — and your inputs, your results and the tables below follow. The formula, the constants, the guidance text and the sources stay in US units on purpose: that is how the standards they come from are written, and restating them would detach each figure from the document you are meant to be able to check it against.
Where these numbers come from
- ICC International Residential Code 2021, R802.10 — wood truss design, bracing and spacing requirements
Standards are revised on a cycle. The edition above is the one this figure was checked against — confirm it against the edition your jurisdiction has adopted.
Worked example
With building length 40 ft, truss spacing 24 in, truss span (building width) 30 ft, this calculator returns 21 .
Trusses needed for buildings of common length
Click through any row to load those measurements. The address bar keeps them, which makes the result something you can paste into a message.
| Job | Size | Trusses needed |
|---|---|---|
| Two-car garage | 24 ft × 24 in × 24 ft | 13.00 |
| Small house | 40 ft × 24 in × 28 ft | 21.00 |
| Pole barn | 60 ft × 48 in × 40 ft | 16.00 |
| Long shop building | 80 ft × 24 in × 30 ft | 41.00 |
| Shed | 12 ft × 24 in × 12 ft | 7.00 |
Gable-end trusses are built differently from common trusses and are ordered separately.
Frequently asked questions
How many trusses do I need?
Divide the building length in inches by the spacing, then add one for the end truss. A 40 ft building at 24 inch centres needs 21 trusses.
What spacing should roof trusses be?
Twenty-four inches on centre is standard for residential work and lets you use 5/8 inch sheathing. Sixteen inch spacing is specified for heavy snow loads or where thinner sheathing is used.
Can I cut or modify a roof truss?
Never, without an engineer's approval. A truss is an engineered system in tension and compression — cutting any web or chord destroys its capacity, even if it looks like a redundant piece.
Do trusses need bracing?
Yes. Permanent lateral bracing along the bottom chord and web members is part of the design, not optional. Most truss collapses during construction happen because temporary bracing was inadequate.