What is the difference between C16 and C24 timber?

What the strength class actually measures, when the stronger grade lets you span further, and why the sizing convention catches people out.

Difficulty: beginner
Time: 12 min
Read: 3 min

Short answer

C16 and C24 are strength classes for structural softwood, with C24 stiffer and stronger. For a given joist size, C24 spans further, or lets you use a smaller section over the same span. For non-structural work the difference is irrelevant and C16 is cheaper.

Every piece of structural softwood sold in the UK carries a strength class stamp. It is the number a span table works from, so it is one of the few timber specifications that has a direct consequence.

Step by step

Understand what the class means.The C stands for coniferous, and the number relates to the characteristic bending strength in newtons per square millimetre. C24 is stronger and, more usefully, stiffer than C16.
Look for the grade stamp.Graded structural timber is marked along its length with the strength class, the species or origin, the grading body and whether it was dry or wet graded. If there is no stamp, it is not graded structural timber.
Use the class with a span table.Span tables are published by strength class, section size and spacing, for a given load. The class on its own tells you nothing without the table.
Choose C24 where the span is tight.It typically lets a joist span further, or lets you use a shallower section over the same span, which can matter where headroom or floor build-up is limited.
Choose C16 where it is adequate.It is cheaper and more widely stocked. For studwork, noggins, battens and non-structural framing, the class is not relevant at all.
Understand the sizing convention.Timber is sold by its sawn size but supplied planed, so a piece described as 47 by 100 arrives around 45 by 95. Always check the actual finished size before designing to it.
Check whether the timber is dry or wet graded.Timber graded wet, marked KD or dry when kiln dried, will shrink as it dries. In a floor, that shows up as gaps and creaks after a season.
Do not mix classes in one structural element.A run of joists should be one class. The weakest member governs the performance and it is impossible to inspect later.
Get structural work checked.Anything supporting a floor, a roof or a wall is a Building Regulations matter. Span tables are a guide; the design and any building control approval are the actual requirement.

Tips

The grade stamp runs along the length. Cutting timber does not remove the grade, but if a piece has no stamp at all, do not use it structurally.
For a floor, deflection usually governs rather than strength — which is why C24's greater stiffness matters more than its greater strength.
Sawn size and finished size differ by several millimetres. Design to the finished size, not the label.

Common mistakes

Assuming any timber of the right size will do — Ungraded timber has no known strength and cannot be used structurally. The section size is only meaningful together with a strength class.
Substituting C16 where C24 was specified — The design assumed the stiffer material. C16 in the same section over the same span deflects more, which shows as a bouncy floor and cracked finishes even if it does not fail.

If it doesn't work

New floor feels bouncy

Cause: Joists undersized, over-spanned or a lower class than assumed — Fix: Check the actual span and section against a span table for the class used. Adding noggins or sistering joists can stiffen an existing floor.

Timber shrank and joists twisted after a year

Cause: Wet-graded timber drying in service — Fix: Use kiln-dried structural timber where movement matters, and fix strutting or noggins to restrain twist.

Merchant supplied smaller sections than ordered

Cause: Sawn versus planed sizing convention — Fix: Confirm finished dimensions when ordering. It is not an error, it is the convention, but it must be allowed for.

Building control queried the timber

Cause: No visible grade stamp or the wrong class — Fix: Use stamped graded timber and keep the delivery paperwork. Unstamped timber is very difficult to justify after the event.

Questions people ask

Is C24 always worth the extra?

Only where the span or the section is tight. If C16 satisfies the span table with the section you were going to use anyway, C24 buys nothing.

What about C16 for studwork?

Perfectly normal, and the strength class is largely irrelevant in a non-loadbearing partition. Straightness matters far more there than grade.

Are there other classes?

Yes — C14, C18, C27 and hardwood D classes exist. In UK merchants, C16 and C24 are what you will actually be offered for softwood.