What Is a Roof Truss Spacing Calculator?
Roof trusses are almost always laid out at an even on-center (o.c.) spacing — commonly 24 inches for residential roofs, though 16in or 19.2in are also used with certain sheathing and load requirements. Because a building's length rarely divides evenly into your target spacing, this calculator finds the truss count that keeps every bay at or tighter than your target, and shows you the exact resulting spacing.
This is the same rounding logic contractors use when producing a truss layout drawing: round the number of spaces up, then spread the trusses evenly across the full length.
How to Read Your Results
Trusses Needed
The total truss count for the run, including one truss at each end (gable or end wall) plus every interior truss in between.
Actual On-Center Spacing
The true spacing once the truss count is rounded to a whole number. It will always be equal to or slightly less than your target spacing — never more.
Number of Bays (Spaces)
The number of equal gaps between trusses. Truss count is always one more than the number of bays.
Real-World Example
A 45-foot-long roof with a 24-inch target on-center spacing:
| Value | Result |
|---|---|
| Building length | 45 ft (540 in) |
| Bays needed (540 ÷ 24, rounded up) | 23 |
| Trusses needed | 24 |
| Actual spacing | ≈23.5 in o.c. |
Because 540 doesn't divide evenly by 24, the layout tightens slightly to 23.48 inches on-center across 23 bays rather than leaving one wider bay at the end.
Tips for Truss Layout
- Confirm spacing against your engineered truss drawings — spacing is often set by the roof's load design, not just sheathing span.
- Standard spacings are 16in, 19.2in and 24in o.c., chosen to work evenly with 4×8 sheet sheathing.
- Girder or hip trusses may require closer spacing near valleys and hips — check your truss package layout.
- Always mark truss centerlines on the top plate before setting trusses, rather than relying on running measurements alone.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team