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The basics
What is Truss Spacing?
Spacing is where a small decision multiplies across the whole building, in both directions.
Structurally, 24 in on centre works for the vast majority of residential roofs. The times it does not are heavy snow regions, tile and slate roofing, and spans at the very top of a chord size's range.
In all three cases, 16 in centres buys capacity more cheaply than upsizing every truss. Whether that is true for your job is a straight arithmetic comparison: 50% more trusses against one step up in chord size.
- 16 in on centre Heavy snow, tile roofing, or where a stiffer ceiling plane matters. Half again as many trusses.
- 24 in on centre The default. Matches sheathing, matches insulation batts, matches almost everything else.
- 48 in and wider Pole barns and commercial buildings. Requires purlins between the trusses and much heavier trusses.
- End frames count The trusses at each end are gable-end frames, which cost more and are often a separate line item.
Lay out from one end
Mark your truss layout from a single end of the building and never accumulate measurements truss by truss. Small errors compound, and by the far end your sheathing no longer lands on a chord.
The maths
Truss Spacing Formula
Truss count and tributary width
The plus one exists because you need a truss at both ends of the run, including both gable-end frames.
Method
How to Calculate Truss Spacing
Truss count is simple division plus one, but the spacing you divide by has consequences well beyond the count.
- 1 Enter the building length The dimension the trusses run along, perpendicular to the span.
- 2 Choose the spacing 24 in on centre is the North American standard. 16 in is used for heavy loads, 19.2 in gives five bays per 8 ft sheet.
- 3 Read the truss count The count is length divided by spacing plus one, because you need a truss at both ends of the run.
- 4 Check the sheathing note Standard roof sheathing spans 24 in. Wider spacing means thicker decking or a purlin system.
Twenty-four inch centres became standard for a sheathing reason rather than a structural one. A 4 ft by 8 ft panel (opens in a new tab) divides evenly into 24 in bays, giving a truss under every panel edge with no cutting and no waste.
Once trusses move further apart than sheathing can span, you need a secondary framing layer. In a pole barn at 8 ft centres, purlins (opens in a new tab) span between the trusses and the roofing fastens to those, which is why a pole barn truss carries several times the load of a house truss.
Worked example
A 40 ft building at 24 in centres needs (480 ÷ 24) + 1 = 21 trusses, each carrying a 2 ft wide strip of roof. Switch to 16 in centres and you need 31 trusses, each carrying 1.33 ft. Same total roof load, spread across more members.
Number of trusses required, including the frame at each end of the building.
| Building length | 16 in o.c. | 19.2 in o.c. | 24 in o.c. | 48 in o.c. |
|---|---|---|---|---|
| 16 ft | 13 | 11 | 9 | 5 |
| 24 ft | 19 | 16 | 13 | 7 |
| 32 ft | 25 | 21 | 17 | 9 |
| 40 ft | 31 | 26 | 21 | 11 |
| 48 ft | 37 | 31 | 25 | 13 |
| 60 ft | 46 | 38 | 31 | 16 |
Note: 19.2 in on centre divides an 8 ft sheet into exactly five bays, which is efficient but uncommon and easy to get wrong on site.
The tool
Why Choose Our Truss Spacing Calculator
Spacing is a decision that multiplies across the whole building, so this calculator shows both sides of it at once.
- Exact truss count Building length divided by spacing, plus the frame at the far end that people forget.
- Tributary width shown How much roof each truss actually picks up, which is what changes when you tighten the spacing.
- Sheathing check Spacing wider than standard decking spans is flagged, with purlins named as the fix.
- Presets that match reality 12, 16, 19.2 and 24 inches plus the wide pole-barn spacings, rather than a free-text box.
More tools
Explore Related Truss Calculators
Calculators that pair with the spacing tool, each running the same geometry engine.
- Roof Truss Span Calculator Check any span against chord size, species, pitch and load in one pass. Open calculator
- Roof Truss Load Calculator Snow, dead and live load combined into per-truss forces and bearing reactions. Open calculator
- Roof Truss Cost Calculator Per-truss price, total package, labour, delivery and cost per square foot. Open calculator
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Answers
Frequently Asked Questions
Common questions about roof truss spacing calculator.
How far apart should roof trusses be?
24 in on centre is standard for residential roofs in North America and matches sheathing modules exactly. 16 in is used for heavy snow, tile roofs and long spans. Pole barns run 4 to 12 ft on centre with purlins between.
How many trusses do I need?
Divide the building length in inches by the spacing, then add one for the truss at the far end. A 40 ft building at 24 in on centre needs 21 trusses, including both gable-end frames.
Can roof trusses be 4 feet apart?
Yes, but only with purlins between them, and the trusses must be designed for the much larger tributary area. This is the pole barn system, and it is not something you can do with house trusses designed for 24 in centres.
Does closer truss spacing let me span further?
It helps. Halving the spacing from 24 in to 16 in reduces the load on each truss by a third, which can keep a marginal span on a smaller chord. Whether it is economic depends on whether 50% more trusses costs less than upsizing the chords.
What spacing do I need for a metal roof?
Metal panels are usually screwed to purlins rather than solid decking, so the truss spacing can be wide. The purlin spacing is what matters, and it comes from the panel manufacturer's span table.
Should the end trusses be included in the count?
Yes, and remember they are gable-end frames rather than common trusses, so they are different construction at a different price. Always order them as a separate line item.
References
Spacing Sources & Standards
Related calculators on this site, plus the standards and reference works the figures on this page are based on. External links open in a new tab.
Authoritative external references
- WIKI Purlin, Wikipedia (opens in a new tab)
- WIKI Truss, Wikipedia (opens in a new tab)
- STD American Wood Council, NDS and span tables (opens in a new tab)
- ORG SBCA, truss handling and bracing guidance (opens in a new tab)
- CODE International Code Council, model building codes (opens in a new tab)
Codes and standards are revised on their own cycles. Confirm the current edition adopted in your jurisdiction with your local building department before relying on any figure.