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The basics
What is Roof Truss Load?
Getting the load right early is what makes every downstream number, from chord size to footing size, trustworthy.
Uniform snow load (opens in a new tab) rarely causes collapses. Drifting does. Wherever a roof meets a taller wall, a parapet, a roof step or a valley, snow piles into a narrow band that can reach several times the uniform load.
Drift depends on specific local geometry and no general calculator handles it. If your roof has any level change at all, that condition needs designing. This is one of the clearest cases where a table cannot substitute for an engineer.
- Exposure A windswept site sheds snow; a site sheltered among trees or buildings holds more of it.
- Heated or not Unheated buildings carry a thermal penalty, which catches out garages, barns and workshops.
- Roof slope Steep slippery roofs shed snow, so a sloped-roof factor reduces the load above roughly 30 degrees.
- Loads belong at panel points A concentrated load applied mid-panel puts that member into bending it was never designed for.
Drift is the real danger
Snow piling against a taller wall, at a roof step, or in a valley can reach several times the uniform load over a narrow band. Any level change in a roof needs a drift check that no general calculator can do for you.
The maths
Truss Load Formula
From psf to bearing reaction
Snow and roof live load are not added together. The design uses whichever governs.
Method
How to Calculate Roof Truss Load
Load calculation is two conversions: pounds per square foot into pounds per lineal foot, then into a reaction at each bearing.
- 1 Enter the ground snow load Get your local figure from the building department. It varies enormously between locations and is not something to estimate.
- 2 Set the dead loads Top chord dead load covers sheathing and roofing; bottom chord dead load covers ceiling and insulation.
- 3 Set span and spacing Spacing sets the tributary width, meaning how wide a strip of roof each truss picks up.
- 4 Read the reactions The bearing reaction is what the wall, header or post below must carry. The rest of your structure is sized from it.
Ground snow load (opens in a new tab) is converted to a roof load through factors covering exposure, whether the building is heated, how important the structure is, and roof slope. A steep, slippery roof sheds snow and carries less; a sheltered, unheated building carries more.
Two consequences are worth knowing. An unheated building attracts a thermal penalty because it loses no heat to melt snow from underneath, so a detached garage carries more snow than the heated house beside it. And ground snow varies enormously over short distances (opens in a new tab) in mountainous regions, so a national map is not a substitute for your local figure.
Worked example
A 30 ft truss at 24 in centres under a 40 psf total load carries a 2 ft strip, so 80 lb per lineal foot. Across 30 ft that is 2,400 lb total and 1,200 lb at each bearing, with a maximum moment of 9,000 ft-lb. Every figure here is illustrative; your design loads must come from local code.
Where the psf figures in a residential roof design come from.
| Component | Applied to | Notes |
|---|---|---|
| Roof covering | Top chord | Asphalt shingles are light, concrete tile is several times heavier |
| Sheathing | Top chord | Thicker panels add proportionally |
| Truss self-weight | Both chords | Higher for long spans and steel |
| Ceiling and insulation | Bottom chord | Depends on finish and insulation depth |
| Roof live load | Top chord | Maintenance traffic, does not combine with full snow |
| Snow | Top chord | Location-specific, usually governs in cold regions |
| Storage live load | Bottom chord | Zero unless specified before fabrication |
Note: Specific psf values are jurisdiction-specific. Take yours from local code rather than a general table.
The tool
Why Choose Our Truss Load Calculator
Most load calculators stop at pounds per square foot. This one carries the number through to the reaction the wall below has to hold.
- The whole load path From psf, through tributary width, to pounds per lineal foot, to the reaction at each bearing.
- Sloped snow handled Ground snow is converted for pitch, so a steep roof does not carry a flat roof figure.
- Governing case picked Snow and roof live load are compared rather than added, which is how the design actually works.
- Honest about drift The tool states that drift needs designing, instead of implying a uniform figure covers it.
More tools
Explore Related Truss Calculators
Calculators that pair with the load 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 Spacing Calculator Truss count, layout and sheathing check for any on-centre spacing. Open calculator
- Roof Truss Weight Calculator Per-truss and total roof weight from a real member take-off. Open calculator
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Answers
Frequently Asked Questions
Common questions about roof truss load calculator.
How do you calculate roof truss load?
Add the dead loads to the governing live or snow load to get total psf. Multiply by the tributary width, which is spacing divided by 12, for the uniform load in pounds per lineal foot. Multiply that by the span for the total load, and halve it for the reaction at each bearing.
How much weight can a roof truss hold?
Only what its design specifies. A standard residential truss is designed for a combination of roof live or snow load plus dead loads, with bottom chord live load usually zero unless storage was requested. The truss drawing is the only reliable source.
What is the snow load on a roof truss?
It is your ground snow load converted through factors for exposure, heating and importance, then reduced for pitch above roughly 30 degrees. The ground figure is location-specific and comes from your building department.
Can I hang things from roof trusses?
Light items distributed along the bottom chord, yes. Anything concentrated, such as a hoist, a heavy fan or storage racking, needs to be checked against the truss design and hung from a designed point, never from a web member mid-panel.
What load do I use for a garage or barn?
The same dead and snow loads as a house, but with the thermal factor raised because the building is unheated. Add storage live load on the bottom chord if you intend to use the space above, and specify it before fabrication.
Why do roofs collapse under snow when they were built to code?
Usually drift rather than uniform load. Snow piling against a taller wall, at a roof step or in a valley can reach several times the uniform design load over a narrow band. Blocked drains and missing permanent bracing are the other common contributors.
References
Load Standards & References
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 Structural load, Wikipedia (opens in a new tab)
- WIKI Truss, Wikipedia (opens in a new tab)
- CODE International Code Council, model building codes (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)
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.