C24 timber is a high-strength structural softwood widely used in UK construction. The “C” stands for coniferous (typically spruce, pine or fir), while “24” refers to its strength class — meaning it can withstand 24 N/mm² of bending strength.
It is kiln-dried to a moisture content below 20%, improving stability, durability and structural performance. Compared to lower grades such as C16, C24 is more carefully selected and graded to meet stricter structural standards.
What Is C24 Timber Used For?
Thanks to its strength and consistency, C24 timber is commonly used in structural applications, including:
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Carcassing and general framework
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Studwork
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Floor joists and beams
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Roofing structures
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Flooring systems
Because it typically contains fewer knots and defects than C16, C24 is also suitable for exposed structural features such as visible beams.
Is C24 Timber Treated?
All C24 timber is kiln-dried as part of the grading process. It may then be supplied:
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Untreated (for internal use), or
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Pressure treated (for external or damp environments)
Most C24 timber sold in the UK is treated to protect against moisture, rot, mould and insect attack. Treated C24 is ideal for outdoor and load-bearing projects.
What Sizes Are Available?
C24 timber comes in a wide range of sizes, commonly sold by the metre. Popular dimensions include:
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47 x 100mm to 47 x 225mm
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75 x 100mm to 75 x 225mm
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100 x 100mm and 100 x 150mm
Additional sizes and lengths are often available depending on supplier stock.
How Is C24 Timber Graded?
To achieve a C24 rating, timber must meet strict grading criteria:
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Moisture Control – Must be dried to 20% moisture content or below.
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Strength Testing – Assessed for structural performance.
Grading is carried out either by machine testing or by visual inspection from a certified grader. Approved timber is stamped with its strength class. If timber is cut or resized, it must be regraded before structural use.
Can C24 Be Used Outdoors?
Yes — provided it is pressure treated. Treated C24 is suitable for decking, sheds, pergolas, garden buildings and other outdoor frameworks. Applying protective coatings can further extend its lifespan.
C16 vs C24: What’s the Difference?
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Strength: C24 offers greater load-bearing capacity (24 N/mm² vs 16 N/mm²).
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Quality: C24 has fewer knots and defects.
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Stability: Less prone to warping or cracking.
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Cost: C24 is more expensive due to higher grading standards.
C24 isn’t always necessary; many domestic projects can use C16. The right choice depends on structural demands, budget and whether the timber will be visible.
For advice on choosing the right timber for your next project, call 0121 706 0404 or email sales@johnsonstimber.co.uk.