Roof Truss Calculator
This counts how many roof trusses a building needs at a given spacing. It is for laying out the job on paper, not for checking that a roof is strong enough.
- Free
- Works offline after your first visit
- Saves on this device
- No sign-up
- Updated
How it works
Divide the length of the building by the spacing, round down, and add one, so both end walls get a truss. Spacing is measured from centre to centre of each truss.
Many framers use 24 inches, or 61 centimetres, as the usual starting point. Your truss supplier or engineer will give the spacing your design calls for.
Worked example
A building 40 ft long, with trusses every 24 in, needs 21 trusses, one at each end included.
Questions
Does this check the roof strength?
No. Strength depends on the span, snow and wind loads, and the lumber, which an engineer or the truss maker works out.
Why add one?
Trusses sit at both ends as well as in between. That is one more truss than the number of gaps.
What if the length does not divide evenly?
The last gap is shorter. Ask your truss supplier how they place the last truss, since they may adjust the spacing a little.
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