C16 vs C24 timber: which grade do I need?
C16 and C24 turn up on almost every structural timber list. They’re strength classes from the British and European standard BS EN 338, and the number tells you the grade, not the size.
What the numbers mean
The number is the characteristic bending strength in newtons per square millimetre. C16 is rated at 16 and C24 at 24, so C24 is the stronger and stiffer grade. Each length is stress graded, by machine or by eye, and stamped with its class.
Where each is used
C16 is the everyday grade for general carcassing, such as studwork, noggins and many floor and roof timbers on shorter spans. C24 is often specified where spans are longer or loads are higher, and it’s common in trussed rafters and engineered roofs. C24 usually costs more than C16 of the same size.
Which do I need?
If the job is structural, your engineer, architect or building control sets the grade and size, usually from span tables. Order what the drawings say. Swapping C24 for a larger C16 section, or the other way round, is a decision for whoever signed off the design, not the person ordering.
Ordering tips
- Put the grade on the order next to each size, for example 47 × 150 C24
- Say whether it needs to be treated, for external or ground-contact use
- Check the lengths, so you’re not jointing joists that should run in one piece
- Check the grade stamp on delivery if the grade matters for your job
This is a general guide, not a specification. Your builder, architect, building control or the manufacturer will confirm exactly what your project needs.
Get it priced
Send your timber list with sizes and grades, and Allan will price it, delivered.
Common questions
Is C24 stronger than C16?
Yes. C24 has a higher strength and stiffness rating than C16, so the same size of C24 can usually span further or carry more. Your engineer or building control will say which grade the design needs.
Can I use C16 instead of C24?
Only if whoever designed the structure agrees. A C16 joist may need to be deeper, or set closer together, to do the same job as C24. Don’t substitute without checking.
Is CLS timber C16 or C24?
It can be either. CLS describes the size and planed finish, not the strength. It’s commonly sold as C16, and C24 CLS is available. Check the grade if it matters.