Your truss, drawn to scale
Fink web
Need a roofing contractor for this project?
Take the span, pitch and truss count you just worked out to a roofing pro. They handle the installation, the replacement or the repair, and can price the finished roof rather than just the frame.
- Roof installation
- Roof replacement
- Roof repair
Planning estimates only. Always confirm against a sealed engineered truss design before ordering or building.
- Calculators
- 17 Calculators
- Truss shapes
- 11 Truss shapes
- Data stored
- 0 Data stored
- Forever
- Free Forever
The basics
What is a Barn Truss?
A barn truss is chosen for clear span and cheap enclosure, and the gambrel version is chosen when the loft is worth paying for.
The gambrel profile exists for exactly this reason. On a 32 ft span, a gambrel roof gives roughly 20 ft of full-height loft width where a 6:12 gable gives almost nothing usable.
What you must not do is treat that loft as free floor space. Hay, feed and equipment are heavy, and a working hay loft is a designed floor system rather than a truss bottom chord. Both the bottom chord and the collar tie have to be designed for it before fabrication.
- Clear span, no interior posts The whole point of a truss barn. Equipment moves without dodging columns.
- Wide spacing cuts truss count Eight or twelve foot centres mean far fewer trusses, offset by purlins and heavier members.
- Unheated means more snow Codes apply a thermal penalty to unheated buildings, so a barn carries meaningfully more snow than the house beside it.
- Post connections are designed The truss-to-post detail is hardware, not toe-nails, and the post embedment carries a large point load.
Snow load on an unheated barn
Heated buildings lose enough heat through the roof to melt some snow. An unheated barn does not, and design standards apply a thermal factor to account for it. Use your local ground snow figure rather than a national average.
The maths
Barn Truss Formula
Pole barn load and geometry
A truss at 12 ft centres carries six times the load of the same truss at 24 in centres. Everything downstream scales with it.
Method
How to Calculate a Barn Truss
Barn trusses are calculated the same way as house trusses, then scaled for the much larger strip of roof each one carries.
- 1 Set the span Pole barns typically run 24 ft, 30 ft, 36 ft, 40 ft or 48 ft between post lines.
- 2 Choose the profile Gambrel for maximum loft, gable for maximum clear span at the lowest cost.
- 3 Set the truss spacing 8 ft or 12 ft on centre onto posts is standard for pole barns. The calculator flags the purlins this requires.
- 4 Add the snow load Barns are usually unheated, which means no heat loss to melt snow. Use the full ground snow load for your area.
The pole barn system (opens in a new tab) concentrates load. Instead of dozens of trusses sharing the roof onto continuous walls, you have a handful of trusses each carrying an 8 ft or 12 ft wide strip of roof down to a single post.
That changes everything downstream. The truss is heavier, the bearing connection to the post is designed hardware rather than a toe-nail, and the post and its embedment have to carry a large point load. This is where DIY pole barn plans most often fall short.
Worked example
A 36 ft barn truss at 8 ft centres carries an 8 ft wide strip rather than the 2 ft a house truss carries. Under the same design load that is four times the load per truss, which is why barn trusses use deeper chords and why the post below needs designing rather than assuming.
What each truss spacing implies for the secondary framing above it.
| Truss spacing | Purlin size | Purlin spacing | Typical use |
|---|---|---|---|
| 4 ft o.c. | 2x4 flat | 24 in | Light residential-style barns, sheathed decks |
| 8 ft o.c. | 2x6 on edge | 24 in | Standard pole barn with steel roofing |
| 10 ft o.c. | 2x6 on edge | 24 in | Larger equipment barns |
| 12 ft o.c. | 2x8 on edge | 24 in | Wide-bay barns, check purlin deflection carefully |
Note: Purlin spacing is set by the roofing panel's span rating rather than by the truss spacing. Check the panel manufacturer's table.
The tool
Why Choose Our Barn Truss Calculator
Pole barns space trusses in feet, not inches, and that one difference changes every number downstream. This calculator handles it.
- Wide spacing supported Presets run out to 8 ft on centre, with the tributary width scaled accordingly.
- Purlin implication flagged When spacing exceeds what sheathing spans, the tool says purlins are needed rather than staying silent.
- Gambrel or gable profile Switch between the loft-friendly gambrel and the cheaper gable and compare the take-off.
- Unheated snow reminder Barns carry more snow than the house beside them, and the tool prompts you to use the local ground figure.
More tools
Explore Related Truss Calculators
Calculators that pair with the barn tool, each running the same geometry engine.
- Gambrel Roof Truss Calculator Two slopes, one cut list. Works for 12 ft, 16 ft, 24 ft and 40 ft gambrel spans. Open calculator
- Steel Roof Truss Calculator Light-gauge or structural steel, with weight take-off and long-span checks. Open calculator
- Roof Truss Cost Calculator Per-truss price, total package, labour, delivery and cost per square foot. Open calculator
Get a free roofing quote
Roof installation, replacement and repair, quoted by a contractor who can visit the site. Bring your numbers from above and skip the guesswork on the rest of the job.
- Free quote
- No obligation
- Fast callback
Answers
Frequently Asked Questions
Common questions about barn roof truss calculator.
How far apart should pole barn trusses be?
8 ft on centre is the most common, matching a typical post spacing. 12 ft is used for wide-bay barns and needs heavier purlins. Anything wider starts requiring a purlin system heavy enough that another truss would have been cheaper.
How far can a barn truss span?
Wood barn trusses commonly span 24 to 60 ft. Past 60 ft you are generally into steel, though heavy-timber and engineered-lumber trusses can go further at a price.
What is a barn style roof truss?
Usually a gambrel, the four-sloped profile with a steep lower slope and shallow upper slope that creates loft space. The term is also used loosely for any wide-span truss on a pole building, including plain gables.
Do pole barn trusses need to be engineered?
In nearly all jurisdictions, yes. The combination of long span and wide spacing puts large concentrated loads onto individual posts, and the truss-to-post connection is a designed hardware detail. Most suppliers provide sealed drawings with the package.
How much do pole barn trusses cost?
A 40 ft gable barn truss typically runs several hundred dollars each depending on region, pitch and snow load. At 8 ft centres a 60 ft long barn needs nine trusses, so get a current quote rather than working from a published range.
Can I add a hay loft to an existing barn truss?
Not without a structural assessment. A roof truss bottom chord is not a floor, and hay is one of the heaviest storage loads there is. The loft has to be designed in, or added as a separate framed floor with its own supports.
References
Barn & Pole Building 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 Barn, Wikipedia (opens in a new tab)
- WIKI Purlin, Wikipedia (opens in a new tab)
- STD ANSI/TPI 1, Truss Plate Institute (opens in a new tab)
- STD American Wood Council, NDS and span tables (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.