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The basics
What is Truss Span?
Knowing your span limit early is what stops a design being redrawn after the quotes come back.
Spacing and span trade against each other directly. Halving the spacing from 24 in to 16 in cuts the load each truss carries by a third, which is often enough to keep a marginal span on the smaller chord size.
Whether that is worth it comes down to arithmetic. Going from 24 in to 16 in centres means 50% more trusses. If stepping up from 2x4 to 2x6 chords costs less than that, keep the wider spacing, and it usually does.
- Chord buckling Controlled by web spacing. More web nodes mean shorter unbraced panels and more span from the same lumber.
- Deflection A truss can be strong enough and still sag visibly. Depth is the cheapest fix, and it usually governs long spans.
- Plate capacity Each joint transfers force through steel teeth. Bigger plates cost very little and often unlock the span.
- Pitch helps A steeper pitch gives more depth at mid-span, which reduces chord forces. Low-pitch roofs need heavier chords for the same span.
Want this as a PDF?
Use your browser's print function on the results. The layout is print-styled, so you get a clean one-page summary with the diagram and take-off to file with your plans.
The maths
Truss Span Formula
What span actually means
Chord strength, deflection and connector plate capacity all compete to set the maximum. Whichever runs out first wins.
Method
How to Calculate Truss Span
Span is measured, not calculated, but what you can do with it depends on three things competing to set the limit.
- 1 Measure the clear span Out-to-out of the bearing walls. This is not the same as the room width, since it includes the wall thickness at each end.
- 2 Enter your loads Snow load varies most by location and is usually what governs the design. Get the figure from your building department.
- 3 Pick the chord size Start with 2x4 and step up until the advisories clear. The tool reports the practical limit for each size.
- 4 Read the verdict A clear result means the combination is within normal practice. A warning means a bigger chord, tighter spacing or an engineer.
For short spans, plate capacity (opens in a new tab) usually governs. In the middle range it is chord buckling. At long spans deflection almost always wins (opens in a new tab): the truss passes every strength check and still sags more than the limit allows.
That is why long-span trusses get deeper rather than beefier. Depth costs a few inches of web lumber; a bigger chord costs across every lineal foot of the truss.
Worked example
A 28 ft span at 24 in centres under a 40 psf total load gives each truss a 2 ft wide tributary strip, so 80 lb per lineal foot, and 1,120 lb arriving at each bearing. Whether 2x4 chords carry that depends on species, grade and web layout, which is a design question rather than a table lookup.
Planning figures for fabricated wood trusses at 24 in on centre. Use them for feasibility, not for ordering.
| Chord size | Light load | Standard load | Heavy snow |
|---|---|---|---|
| 2x4 | 32 ft | 28 ft | 22 ft |
| 2x6 | 44 ft | 38 ft | 30 ft |
| 2x8 | 56 ft | 48 ft | 38 ft |
| 2x10 | 68 ft | 58 ft | 46 ft |
| 2x12 | 80 ft | 68 ft | 54 ft |
Note: These are planning figures for budgeting and feasibility, not a substitute for a sealed truss design. Actual spans depend on species, grade, web layout, plate sizes and deflection limits.
The tool
Why Choose Our Truss Span Calculator
This calculator is a feasibility screen first. It tells you quickly whether a span, load and lumber combination is worth pursuing at all.
- A verdict, not just numbers Combinations outside normal practice are flagged, so bad ideas die early.
- Any shape, one tool Switch truss type and see how the same span behaves across eleven profiles.
- Spacing traded against span Change the spacing and watch the load per truss drop, which is often what rescues a marginal span.
- Print for the file A clean one-page summary with the drawing, ready to attach to your plans.
More tools
Explore Related Truss Calculators
Calculators that pair with the span tool, each running the same geometry engine.
- Roof Truss Load Calculator Snow, dead and live load combined into per-truss forces and bearing reactions. Open calculator
- Roof Truss Spacing Calculator Truss count, layout and sheathing check for any on-centre spacing. Open calculator
- Gable Roof Truss Calculator Live geometry, automatic web pattern selection and a complete material take-off. Open calculator
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Answers
Frequently Asked Questions
Common questions about roof truss span calculator.
How far can a roof truss span?
A 2x4 truss spans up to about 32 ft under light residential loading, a 2x6 truss up to about 44 ft, and a 2x8 truss around 56 ft. Heavy snow loads cut every one of those figures by roughly a third. Beyond 60 ft, steel is usually the answer.
How do I measure roof truss span?
The span is the horizontal distance measured out-to-out of the bearing walls, meaning the two points the truss actually sits on. Measure the outside face of one wall plate to the outside face of the other, and do not include overhangs.
What is the maximum span for a 2x4 roof truss?
Around 32 ft under light load with a well-designed web pattern, though 28 ft is a more realistic planning figure once a normal snow load is included. Past that, deflection rather than strength becomes the limit.
Does pitch affect how far a truss can span?
Yes, substantially. A steeper pitch gives the truss more depth at mid-span, which reduces chord forces. A 12:12 truss can span noticeably further on the same lumber than a 3:12 truss, which is one reason low-pitch roofs need heavier chords.
Do I need an engineer for a long-span truss?
Any truss bought from a fabricator comes with a sealed design as part of the package. If you are building your own, most jurisdictions require an engineer's seal above roughly 30 to 40 ft, and any truss carrying floor or point loads needs one regardless of span.
What is the difference between span and spacing?
Span is how far one truss reaches across the building. Spacing is how far apart consecutive trusses sit along the building. Span is measured in feet across; spacing is measured in inches on centre along.
References
Span Tables, Codes & Sources
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 Span (engineering), 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)
- STD ANSI/TPI 1, Truss Plate Institute (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.