Roof Truss Calculator
Count roof trusses by building length and spacing.
Enter the building length and your truss spacing to get the number of roof trusses required. The estimate adds one truss because both end walls need one.
How it works
Divide the building length (in inches) by the on-center spacing to get the number of spaces, then add one for the final truss. Trusses are commonly set 24 inches on-center; 16-inch spacing is used for heavier loads.
Worked example
For a 40 ft-long building at 24 inches on-center: 40 ft is 480 inches, ÷ 24 = 20 spaces, plus one truss to close the final bay and one already at the start — the rule is spaces + 1, giving 21 trusses. Drop to 16-inch spacing for a heavier snow or tile load and the same building needs 480 ÷ 16 + 1 = 31 trusses.
What changes your estimate
- Spacing drives everything. 24" on-center is the residential standard; 16" is used for heavy roof loads or long spans. Tighter spacing means more trusses but lighter individual members.
- Gable vs. hip roofs. A simple gable uses uniform common trusses. Hip roofs need shortened trusses and jacks near the corners — count those separately.
- End trusses differ. The two gable-end trusses are often a different type (vertical webs for siding attachment), so confirm them with your supplier.
- Always confirm with an engineer. Truss layout and load rating should be signed off for your local snow/wind code — this estimate is for planning the order, not for structural sign-off.
Frequently asked questions
How many roof trusses do I need?
Divide building length in inches by the spacing and add one. A 40-ft building at 24-inch spacing needs about 21 trusses.
What is standard truss spacing?
24 inches on-center is most common for residential roofs; 16-inch spacing is used for heavier loads or certain sheathing.
Does this count gable-end trusses?
The +1 covers the end truss. Gable-end trusses are a different (vertical-web) type — confirm your design with the truss supplier.
Is this calculator free and private?
Yes. Every CubicCabin calculator runs entirely in your browser, so the numbers you enter are never uploaded or stored. There's no sign-up, no limits, and it keeps working even if you lose connection after the page loads.