C24 timber
Plain English. Practical detail. UK context.
C24 is a strength class for structural softwood under BS EN 338. It groups timber by specified structural properties; it is not a species, a visual finish or a guarantee of durability outdoors. A C24 stamp does not by itself tell you that a piece is suitable for a particular joist, lintel or deck.
Last reviewed: (editorial review against the public sources listed below).
What the grade means on a job
Structural design uses characteristic properties such as strength and stiffness along with member size, span, support and load. C24 can offer advantages over a lower strength class where the design requires them, but changing the class does not remove the need to check the whole member. The designer’s specification may also include service conditions, moisture limits and treatment. Order against those requirements rather than using the strongest-sounding label as a universal answer.
Check the delivered timber
Look for the appropriate grading and product information and confirm dimensions and quantity. Nominal sizes and finished dimensions can differ. Store timber supported, protected from prolonged wetting and able to dry appropriately; poor storage can introduce distortion and moisture problems before installation. Keep product records where the structure needs traceability. If a piece is badly damaged, heavily split or distorted, do not assume its original grade stamp resolves the concern. Ask the supplier or designer about acceptance.
Common mistakes
Strength class and preservative treatment are different specifications. Untreated C24 does not automatically suit ground contact. Cutting large notches or holes for services can reduce a designed member’s capacity, regardless of grade. Do not substitute a smaller C24 joist for a larger specified member without checking the design. Similarly, doubling pieces together without a designed connection is not necessarily equivalent to a single engineered member. Structural modifications require more than matching the total width of timber.
Alternatives and applicable guidance
C16 may be adequate for a properly designed application. LVL, glulam and engineered I-joists offer different properties and installation requirements; they are not interchangeable lengths of ordinary softwood. BS EN 338 covers strength classes and BS EN 14081 covers strength-graded structural timber within its scope. Timber structures are commonly designed using Eurocode 5 and its relevant UK provisions. Approved Document A is a starting point for structural compliance in England, with different national guidance elsewhere. Follow the project specification and any manufacturer’s restrictions on drilling or notching.
Frequently asked questions
Is C24 always better than C16?
Not in every practical sense. The correct choice depends on the design and use, not the number alone.
Is all C24 pressure treated?
No. Treatment and strength grading are separate attributes.
Can I drill service holes anywhere?
No. The permitted locations and dimensions depend on the member and design. Engineered products often have specific rules.
Sources and further reading
Public sources and further reading. Sources checked: 8 November 2026. Consult the original for details and later changes.