Timber Grading Explained: What UK Sawmills Mean by C16 and C24

Walk down the structural timber rack at any builders' merchant and you will see the same two codes stamped on the ends: C16 and C24. They are not species names, not treatment codes, and not a maker's brand. They are strength classes, and they tell a designer, a building control officer and anyone reading a span table roughly how much load that piece of timber can be expected to carry.

What a strength class actually means

The C stands for conifer — softwood, in everyday language. The number is the characteristic bending strength in newtons per square millimetre (N/mm²). A piece graded C24 has a characteristic bending stress of 24 N/mm²; C16 has 16 N/mm².

"Characteristic" is doing real work in that sentence. It is not a promise about the individual length in your hand. Strength varies from board to board, even within one log. Testing a large batch of pieces produces a spread of results, and the characteristic value is set near the lower end of that spread, with further factors applied in the Eurocode design rules. The class describes the batch, not the stick.

Bending strength is only the headline figure. Every class also carries values for stiffness, compression and tension, and they move together. C16 has a mean modulus of elasticity of about 8 kN/mm²; C24 is about 11 kN/mm². That gap in stiffness is why a C24 joist deflects noticeably less than a C16 joist of the same size and span — and it is often stiffness, not bending, that decides whether a floor feels solid underfoot. Softwood classes run from C14 up to C50, though availability above C24 falls away quickly in the UK. Hardwoods use a separate D series.

Visual grading: knots, grain and judgement

Visual strength grading is exactly what it sounds like. A trained grader inspects each piece against a written set of limits. The main things they weigh up:

Knots — size, position and clustering. A large knot near an edge, in the tension zone, matters far more than the same knot near the centre.
Slope of grain — how far the fibres run out of line with the length. Steep grain loses strength quickly.
Fissures, checks and splits, especially running along the length.
Wane and bark — missing wood at the corners reduces the effective section.
Distortion — bow, spring, twist and cup.
Decay, insect damage, resin pockets and rate of growth, the last saying something about density.

In the UK, softwood visual grading follows BS 4978, which sorts timber into GS (general structural) and SS (special structural). Species tables in BS EN 1912 then convert those grades into classes. For the softwoods commonly grown and imported here, GS generally lands at C16 and SS at C24, but the mapping is species-specific, so the paperwork settles it. Visual grading is a professional judgement made thousands of times a day, which is why graders are trained, tested and audited, and why the mill's certificate matters as much as the stamp.

Machine grading: measuring the wood, not just looking at it

Machine grading takes some of that judgement out of the equation. Grading machines work in different ways: some bend each piece under a known load and measure the deflection; some use X-rays to read density; some work acoustically, tapping the piece and reading how it responds. The mill establishes by testing which machine readings correspond to which class for a given species and size. After that, the machine sorts every piece.

Machine-graded timber is marked with the class alone — C16, C24 or similar — without a GS or SS letter. Because a machine measures stiffness directly, it can often pull more C24 out of a parcel of logs than a visual grader could, and it does so consistently. The trade-off is that a grading line is a serious investment, so not every mill has one.