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The basics
What is a Wood Truss?
Timber wins on cost, weight and buildability at every span a house actually needs. Its weaknesses are all about water.
Trusses are fabricated at a higher moisture content (opens in a new tab) than they settle at in service. As they dry, the bottom chord can arch slightly in winter, lifting the ceiling away from interior partition walls and cracking the drywall joint. This is normal timber behaviour rather than a defect.
The fix is detailing rather than structure: use drywall clips near partitions, and never fasten a truss bottom chord down to an interior wall top plate. Every genuine timber truss failure, as opposed to this cosmetic movement, traces back to water getting in and staying in.
- Roughly 30% less lumber Against the equivalent rafter and ceiling joist roof, for the same span and load.
- Goes up in a day A truss roof is set and braced in a day where stick framing takes most of a week.
- Clear spans the building No interior load-bearing walls, so partitions can go wherever the plan wants them.
- Needs to stay dry Ventilation and leak-free detailing are what make a timber roof last indefinitely. Wet wood rots at the plates first.
Connector plates need full bearing
The pressed steel plates at every joint transfer the entire member force through their teeth. A plate that is lifted, rusted or backed out of the wood has lost capacity. Inspect them on delivery and reject anything damaged in transit.
The maths
Wood Truss Formula
Board foot take-off
Board feet per lineal foot: 2x4 is 0.67, 2x6 is 1.00, 2x8 is 1.33, 2x10 is 1.67 and 2x12 is 2.00.
Method
How to Calculate a Wood Truss
A timber take-off starts from member lengths and ends in board feet, which is the unit lumber yards actually price in.
- 1 Set the span and pitch The calculator flags when your span exceeds the practical limit for the chord size selected.
- 2 Choose your species SPF, Southern Yellow Pine, Douglas Fir-Larch or LVL. Each has a different density, cost and set of design values.
- 3 Pick the chord size 2x4 for most residential spans, 2x6 beyond that, 2x8 and up for long spans and heavy loads.
- 4 Read the take-off Board feet, lineal feet and weight per truss, with your waste allowance already applied.
A stick-framed rafter (opens in a new tab) carries load in bending along its full length, so it has to be deep enough to resist that bending everywhere. A truss converts the same load into tension and compression along short straight members, and axial forces need far less material than bending does.
That is why a 2x4 truss outspans a much deeper rafter, why a truss roof uses roughly 30% less lumber overall, and why trusses went from novelty to near-universal in residential construction within a generation.
Worked example
A 24 ft gable truss with 2x4 chords contains roughly 85 lineal feet of lumber. At 0.67 board feet per foot that is about 57 board feet per truss, and around 100 lb of weight. Across 21 trusses with 5% waste the job runs close to 1,250 board feet.
How the common truss species compare on the properties that matter.
| Species | Density | Relative cost | Best for |
|---|---|---|---|
| SPF (Spruce-Pine-Fir) | 28 lb/ft³ | Baseline | The default residential truss lumber across North America |
| Southern Yellow Pine | 37 lb/ft³ | Around +12% | Higher design values, holds plates well, heavier to handle |
| Douglas Fir-Larch | 34 lb/ft³ | Around +15% | Strongest common softwood, favoured on the west coast |
| LVL / engineered | 45 lb/ft³ | Around +85% | Girder trusses, long spans, anywhere stability matters |
Note: Design values for each species and grade are published by the American Wood Council and are what a truss designer actually works from.
The tool
Why Choose Our Wood Truss Calculator
Lumber yards quote in board feet, so this calculator ends where the order begins.
- Board feet, not guesses Taken from the drawn member lengths, with your waste allowance applied.
- Four species to compare SPF, Southern Yellow Pine, Douglas Fir-Larch and LVL, each with its own density and cost factor.
- Weight per truss The number that decides whether two people can set it or you need a machine.
- Chord size sanity check Spans past the practical limit for the section you picked are flagged before you order.
More tools
Explore Related Truss Calculators
Calculators that pair with the wood tool, each running the same geometry engine.
- Steel Roof Truss Calculator Light-gauge or structural steel, with weight take-off and long-span checks. Open calculator
- Roof Truss Weight Calculator Per-truss and total roof weight from a real member take-off. 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 wood roof truss calculator.
What size lumber is used for roof trusses?
2x4 for the majority of residential trusses, 2x6 for longer spans, heavier snow loads and most girder trusses, and 2x8 or larger for attic truss bottom chords and long-span applications. The right size depends on span, load, spacing, species and grade together.
How many board feet are in a roof truss?
A 24 ft gable truss with 2x4 chords contains roughly 85 lineal feet of lumber, which at 0.67 board feet per foot is about 57 board feet. The calculator gives the exact figure from the geometry, including waste.
What is the best wood for roof trusses?
SPF is the default and is right for most residential work: light, inexpensive and adequate. Southern Yellow Pine and Douglas Fir-Larch offer higher design values where span or load demands it, at more cost and more weight.
How much does a wood roof truss weigh?
A typical 24 ft residential 2x4 truss weighs 90 to 110 lb, which two people can carry. A 40 ft 2x6 truss runs 250 to 350 lb and needs mechanical lifting.
How long do wood roof trusses last?
Indefinitely if they stay dry and ventilated. Trusses from the 1960s are still in normal service. Every wood truss failure traces back to water: a roof leak, a blocked vent or a bathroom fan discharging into the roof space.
Can I use treated lumber for trusses?
Rarely, and only where the exposure requires it. Treated lumber is heavier, wetter when delivered, and the chemistry is corrosive to standard connector plates, so stainless or heavy galvanised plates become necessary and fabricators charge for them.
References
Timber Truss 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 Lumber, Wikipedia (opens in a new tab)
- WIKI Board foot, 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.