Roof Truss Calculator
Enter your building length and truss spacing to get the number of roof trusses you need.
What your result means
Trusses is the total count, including the one extra at the end since a length never divides evenly by the spacing. Order gable-end trusses separately, since they're framed differently, and grab one spare. Anything load-bearing should be engineered, not eyeballed.
How to use this calculator
- Measure the building length (the dimension trusses sit across).
- Standard residential truss spacing is 24 inches on center.
- Tighter 16 in OC spacing is used for heavy loads (snow, tile roofs).
- Read the truss count.
- Order 1 extra truss as a spare.
The formula
Convert the building length to inches, divide by the spacing, and round up for the number of bays. Add one for the truss at the end.
Worked example
Say your building is 40 ft long with trusses 24 inches on center. That's 480 inches divided by 24, which is 20, plus one end truss, so 21 trusses. Order one more as a spare and figure the gable ends separately.
Truss spacing by load
| Spacing | When to use |
|---|---|
| 24 in OC | Standard residential |
| 16 in OC | Heavy snow, tile roofs, or per span tables |
| 12 in OC | Very heavy loads (rare residential) |
Tips & gotchas
- Standard spacing is 24 inches on center.
- Tighten to 16 inches on center for heavy snow, tile roofs, or when the span tables call for it.
- Always add one truss for the end, since a length never divides evenly by the spacing.
- Order quantity is length in inches divided by spacing, plus one.
- Order gable-end trusses separately. They're framed differently from the common trusses.
Frequently asked questions
How far apart should roof trusses be?
24 inches on center is the residential standard. Drop to 16 for heavy loads.
How many trusses do I need?
For a 40 ft building at 24 inches on center, 21. Run your length through the calculator for the exact count.
Can homeowners install trusses themselves?
Setting trusses takes a crane and a crew. It's not a DIY job beyond small structures.
Can I modify a truss to add storage?
No. A truss is an engineered unit, and cutting or modifying it voids the engineering. Use attic-storage trusses instead.
How much does a roof truss cost?
$50 to $500 each depending on span. A typical 30 ft truss runs $100 to $150.
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