What is Structural Timber?
The Wood Guide · Calculating with wood
Building timberStructural timber
From the rafters in your roof to the joists under your floor. Timber that has to carry loads, and is therefore graded by what it can take.
- Strength-graded
- C classes (softwood) and D classes (hardwood)
- Strength · durability · stability
Structural timber
From the rafters in your roof to the joists under your floor. Timber that has to carry loads, and is therefore graded by what it can take.
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Structural timber in brief
A collective term for timber used in the construction of buildings and other structures: specially selected and processed to meet structural requirements.
The difference from ordinary carcassing timber is not in the tree but in the grading. A joist that has to carry a floor has been assessed for knots, sloping grain, splits and growth-ring width, and then assigned to a strength class. That way an engineer can calculate with it without handling every board.
Softwood gets C classes (C18, C24, C30), hardwood D classes (D30, D40, D50). The number stands for the characteristic bending strength in newtons per square millimetre. C24 is by far the most widely used class in house building.
Around a knot the fibres deviate, and that is exactly where a beam fails first. Large knots on the underside of a horizontal beam — where tensile stress is highest — are the main reason a board ends up in a lower class.
- What it is
- timber selected for load-bearing uses
- Softwood classes
- C18, C24, C30 …
- Hardwood classes
- D30, D40, D50 …
- Most used
- C24 in house building
- Moisture content
- usually around 18% on delivery
- Graded on
- knots, sloping grain, splits, growth-ring width
To be fair: graded structural timber is not beautiful timber. Grading looks at strength, not appearance: large knots, resin pockets and colour differences are allowed as long as they do not affect load-bearing capacity. If you want exposed beams, ask for a separate visual grade.
Five properties that decide the choice
| Characteristic | Why it counts |
|---|---|
| Strength and stability | The timber carries the load; classified by bending strength, compressive strength and modulus of elasticity |
| Durability | With weather exposure or ground contact, this determines resistance to rot, insects and fungi |
| Dimensional stability | Dried timber hardly shrinks after installation — crucial when plasterboard goes on top |
| Workability | Sawing, drilling and fixing must be possible with ordinary tools |
| Availability | Designing a structure around timber that cannot be supplied grinds to a halt on site |
Which timber goes into the structure
Spruce and pine
The workhorse: affordable, light, easy to grade. C18 to C24, and the basis of almost every Belgian roof structure.
Douglas fir and larch
Stronger and naturally more durable (class 3), so suitable for exposed or semi-outdoor structures such as carports and canopies.
SLS and CLS
North American softwood in standardised sizes, the basis of timber-frame construction. See SLS and CLS.
Oak
D classes, much stronger and more durable, and considerably more expensive. Traditional in timber framing and exposed roof trusses.
Glulam and CLT
Glued laminated timber and cross-laminated timber achieve spans no single tree can provide. See engineered wood.
Structural timber in wind and weather
As soon as a load-bearing structure stands outdoors or touches the ground, a second requirement comes in: durability. Strength class says nothing about rot resistance, and a C24 beam that rots away carries nothing any more.
Covered and dry
Ordinary C24 spruce is enough. This is use class 1 or 2.
Outdoors, not in the ground
Pressure-treated softwood, Douglas fir or larch. Use class 3.
In the ground
Pressure-treated timber in use class 4, or a naturally durable species such as robinia or oak.
Frequently asked questions about structural timber
What does C24 mean?
A strength class for softwood: the number is the characteristic bending strength in newtons per square millimetre. C24 is the most widely used class in house building.
What is the difference between C and D classes?
C classes apply to softwood (conifers), D classes to hardwood (deciduous). Hardwood reaches higher values, but it is more expensive and heavier.
Can I use ordinary carcassing timber for a load-bearing structure?
No. Ungraded timber has no known strength class, so an engineer cannot calculate with it and it does not comply with building regulations.
Does structural timber need to be pressure-treated?
Not indoors and dry. Outdoors or in ground contact, yes — or you choose a naturally durable species. See pressure-treated wood.
Structural timber in perspective
The difference from ordinary timber lies in the grading, not in the tree.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Timber-frame house under construction: softwood walls and roof trusses — L Maule, CC BY-SA 4.0 · Roof structure with timber purlins, rafters and braces — Jim Derby, CC BY-SA 3.0