Guide
C16 vs C24 timber: what is the difference?
C16 and C24 are strength classes under BS EN 338: C24 timber is stronger and stiffer than C16, so it can span further or use a smaller section, which is why it is chosen for longer joists and rafters.
Written by the SK Trade team. Last reviewed 1 November 2026.
What the numbers mean
Structural softwood is graded by strength. The C stands for coniferous and the number relates to its bending strength. C24 has a higher characteristic bending strength and stiffness than C16.
When to use which
C16 is the everyday grade for joists, rafters and studs on typical domestic spans. C24 lets you span further with the same size, or use a smaller section for the same span, which can matter in loft conversions and extensions with longer spans.
Span tables decide
The size and grade you need come from span tables or a structural design, based on the span, spacing, loads and grade. Building control (building standards in Scotland) will check what is used against the design.
Grading marks
Graded timber carries a grade stamp showing its class and grading body. Keep the stamped timber where building control can see it, or keep the delivery paperwork.
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Related questions
Is your carcassing timber treated?
We supply treated carcassing as standard for structural work. Tell us if the timber is for external or ground-contact use so we supply the right treatment level.
What does CLS stand for?
Canadian Lumber Standard. It refers to the way the timber is dried, planed and finished with rounded edges.
Sources
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