Order timber for a joist run, a stud wall or a roof, and you'll quickly see two grades on every spec sheet: C16 and C24. They look identical on the stack, they come in the same sizes, and C24 costs a bit more per length. So does the difference actually matter on your job?
Often it does. Here's a straight answer — and why every structural length we sell is C24.
The short version
- C16 is the general-purpose structural grade. It's fine for lightly loaded work: non-load-bearing stud walls, noggins, battens, short spans and framing where a span table confirms it's enough.
- C24 is stronger and stiffer. It lets you span further, drop a section size, or open up joist centres — and it's often what the drawings, the engineer or building control will ask for on floors, roofs and extensions.
If you're not sure which one your job needs, C24 is the safer buy. You'll never fail an inspection for using a higher grade than specified; you can for using a lower one.
What the numbers actually mean
C16 and C24 are strength classes set out in BS EN 338. The "C" stands for coniferous (softwood), and the number is the characteristic bending strength in N/mm². In plain terms: a C24 joist can take roughly 50% more bending stress than a C16 joist of exactly the same size before it reaches its design limit.
Just as important is stiffness. C24 has a noticeably higher modulus of elasticity than C16, which means it deflects less under load. On a floor, that's the difference between a solid feel underfoot and a bouncy one — and deflection, not breaking, is very often what limits how far a joist can span.
Every graded length should be stamped with its class. If there's no stamp, treat it as ungraded and don't use it for anything structural.
Head-to-head comparison
| C16 | C24 | |
|---|---|---|
| Bending strength | 16 N/mm² | 24 N/mm² |
| Stiffness | Lower — more deflection under load | Higher — stiffer, flatter floors |
| Span for the same section | Shorter | Longer |
| Appearance | More knots and visual defects allowed | Fewer, smaller knots; generally cleaner |
| Typical cost | Slightly lower per length | Slightly higher per length |
| Best for | Non-load-bearing partitions, noggins, battens, short or lightly loaded spans | Floor and ceiling joists, rafters, flat roof joists, load-bearing studs, extensions, anything specified on drawings |
When C16 is fine
C16 does a perfectly good job where loads are light and spans are short:
- Non-load-bearing stud partitions — internal walls that only carry their own weight and the plasterboard on them
- Noggins, bracing and blocking — short lengths between joists or studs
- Battens and grounds — for fixing boards, cladding or ceilings
- Short spans that a span table confirms — small porches, sheds, decking frames and similar
When you need C24
C24 earns its slightly higher price anywhere the timber is carrying real load or covering a real distance:
- Floor and ceiling joists — especially over longer runs, where stiffness stops the floor feeling bouncy
- Roof rafters and flat roof joists — which carry roof coverings, insulation and snow load
- Load-bearing stud walls — supporting floors or roofs above
- Extensions and loft conversions — where the structural calculations or drawings will usually specify C24
- When you want a smaller section — C24 can sometimes let you use the next size down compared with C16 on the same span, which saves headroom and weight
The golden rule: if your drawings, calculations or building control officer specify C24, you must use C24. Swapping down to C16 to save a few pounds a length can mean failing an inspection and replacing the lot.
Which section size for which job?
Grade is only half the decision — the depth of the timber matters just as much. As a rough guide to where each of our C24 sections usually ends up:
| Section | Typical uses |
|---|---|
| 47mm x 100mm (4" x 2") | Stud walls, wall plates, rafters on smaller roofs, framing |
| 47mm x 125mm (5" x 2") | Rafters, ceiling joists, short floor spans |
| 47mm x 150mm (6" x 2") | Flat roof joists, floor joists on moderate spans, deeper rafters |
| 47mm x 175mm (7" x 2") | Floor joists on longer spans, extension roofs |
| 47mm x 200mm (8" x 2") | Long-span floor joists, larger extensions, heavier roof structures |
Exactly how far any section will span depends on the grade, the joist centres (typically 400mm or 600mm), and the load it carries. Always check the TRADA span tables or ask a structural engineer before you order — it's far cheaper than getting it wrong.
What "regularised" and "treated" mean
Regularised timber has been machined to a consistent width and depth along its whole length, so every joist in a run sits level. That means less packing, less planing and a flatter floor or ceiling when the boards go on. Ours is kiln-dried with eased edges for safer handling on site.
Treated timber has been pressure treated against fungal decay and insect attack. That matters for anything in a roof, a floor void or an external wall, where moisture and poor ventilation can shorten the life of untreated timber.
Why we only stock C24
We made a simple call: every structural length we sell — from 47mm x 100mm up to 47mm x 200mm — is C24, regularised and treated. It covers every job C16 would do, it's what most drawings and engineers ask for, and it means you never have to worry about the wrong grade turning up on site. The price difference per length is small; the cost of replacing under-specified timber isn't.
The bottom line
C16 is fine for light, short, non-structural work. For joists, rafters, load-bearing walls and anything on a drawing, C24 is the grade to buy — stronger, stiffer, and far less likely to cause problems at inspection.
Not sure which section your span needs? Give our team a call on 07862352788 — we deliver C24 timber across the UK and can help you work out the right size and quantity for your job.