C24 timber explained: strength classes, moisture and grading

C24 timber is the strength class British importers ask for by name, and the reason is arithmetic, not fashion. A C24 joist carries more than a C16 joist of the same section, so it spans further or lets you use a smaller one.

Strength class and appearance grade are independent systems. A board can be C24 structurally and plain to look at, or handsome and only C16. Specify both, plus moisture content, or a third of the order is left to chance.

What does C24 timber mean?

Strength classes are defined in BS EN 338. The number is the characteristic bending strength in newtons per square millimetre. The classes a UK buyer meets:

• C16: 16 N/mm² bending, 8.0 kN/mm² modulus of elasticity, 310 kg/m³ characteristic density
• C18: 18 N/mm², 9.0 kN/mm², 320 kg/m³
• C24: 24 N/mm², 11.0 kN/mm², 350 kg/m³ — the standard structural specification
• C30: 30 N/mm², 12.0 kN/mm², 380 kg/m³ — for longer spans and heavier loads

Stiffness matters as much as strength in floors, where deflection usually governs. That is why the modulus of elasticity is the column to read when you compare timber strength class C24 with C16 in C24 timber span tables.

Is C24 timber better than C16?

For the same section, yes, and the difference comes down to where the tree grew. Conifers grown in the UK reach harvest in around forty years, with wide growth rings and larger knots, and grade predominantly to C16. Nordic conifers take sixty years or more; slow growth gives tight rings and small knots, and a high proportion grades to C24.

That is why British buyers import Nordic timber for longer spans: not sentiment, but the yield of the higher class. For short spans and non-critical members C16 is adequate and usually cheaper.

Visual or machine grading

Visual grading assesses each piece by eye: knot size and position, fissures, slope of grain, distortion and ring width. The Nordic rules, INSTA 142, use T classes that map onto the C classes: T0 is C14, T1 is C18, T2 is C24 and T3 is C30.

Machine grading measures physical properties instead: bending stiffness, resonant frequency, density by X-ray or machine vision. It is more accurate, yields more of the higher classes, and the highest classes can only be assigned by machine. Both routes sit under BS EN 14081, which also governs the grade mark and the CE or UKCA marking on structural timber.

Every graded piece is stamped with the strength class, the grading standard, the species or source and the grader. Check it against the delivery note before the pack is broken: C16 against a C24 order is a recalculation, not a conversation.

Kiln dried timber moisture content

Structural timber for the UK trade is supplied kiln dried, typically to around 20% moisture content. Timber shrinks and swells almost entirely across the grain — more tangentially than radially, and effectively not at all along its length — so a section fixed too wet loses dimension as it dries, and one fixed too dry can swell.

Joinery, linings and flooring need a lower moisture content than structural framing, because they end up in a heated, dry interior. Let packs acclimatise before fixing, and store them off the ground and covered.

Appearance grades, sizes and treatment

• Appearance grades: A (subdivided A1–A4), then B, C and D — A for visible surfaces, D at the bottom
• Sawn: straight from the saw, dimensions varying as it dries
• Regularised: machined to a consistent depth so the nominal size holds
• CLS or ALS: eased-edge planed sections to standard target sizes
• Treatment: specify the use class — use class 4 for timber in ground contact

Machining reduces the section slightly, so the engineer must know which form is supplied; on long spans that difference stops being academic.

PEFC and FSC chain of custody

PEFC and FSC certify that the forest is managed to the scheme's criteria and that the timber can be traced from forest to product. Neither says anything about strength or moisture. What they do is speed up legality and deforestation risk assessment, useful for your own timber due diligence, though never a replacement.

What to specify when you order

• Strength class: C16, C24 or C30, and whether grading is visual or machine
• Grade stamp showing the class and the source
• Moisture content on delivery, and confirmation the timber is kiln dried
• Sawn, regularised or CLS/ALS target sizes
• Treatment and use class for the exposure
• PEFC or FSC chain of custody, and the country of origin

With those six lines in the enquiry, quotations become comparable. Without them the cheapest quote is usually a different product from the dearest, and the gap only shows when the pack is opened on site — joists, cladding or a round log package.

• BS EN 338: C16 is 16 N/mm² and 310 kg/m³; C24 is 24 N/mm², 11.0 kN/mm² and 350 kg/m³.
• UK-grown conifer matures in about 40 years and grades mostly C16; Nordic timber yields far more C24.
• Grading is visual or by machine; both sit under BS EN 14081, which also covers the marking.
• Nordic INSTA 142 visual classes map across: T0 is C14, T1 is C18, T2 is C24 and T3 is C30.
• Structural timber for UK trade is kiln dried, typically to around 20% moisture content.
• PEFC and FSC prove management and traceability — never strength, moisture or fitness for a span.