What is the difference between C16 and C24 timber?

Anyone who’s been involved with construction projects using carcassing timber (kiln-dried softwood) has likely encountered the terms ‘C16’ and ‘C24’. But while these descriptions are pretty widespread, not everybody knows what they mean, nor the difference between them, which is why we wanted to clear up the confusion with this handy little guide. 

For starters, it’s important to know that both of these terms refer to the strength grade of a piece of carcassing timber. These grades are set by the British Standards Institution in the BS 5268 compendium of codes relating to the structural use of timber and are worked out by taking various measurements that indicate the strength of the wood. This involves assessing the effect of any knots, shakes (splits along the grain), the slope of this grain, and woodworm holes, all of which can lower its strength. The ideal piece of timber has a straight grain with no knots, shakes or woodworm holes.

According to this system, the ‘C’ grading simply means conifer, the type of tree the timber is made from, while the number denotes its strength. The higher the number, the better the quality of timber, which means, yes, C24 timber is superior to C16. While there are 12 strength grades in the BS 5638, C16 and C24 standards are by far the most common, which is the reason they’re our focus here. But why exactly is the C24 a better quality timber, and what are the differences between them in practice?

What does C16 timber mean?

C16 timber is the most common type of carcassing timber in the UK, as it is incredibly versatile and cost-effective. The reason it’s on the cheaper side of things is that the C16 grade allows for some defects. For example, grain deviations and wanes may affect strength, while sap stains and uneven surfaces can have an aesthetic impact. Despite the potential strength defects, C16 timber is still incredibly robust and has the bending, compression and density to be used for most construction projects, and its most common applications include internal construction projects such as walls, floors and roof joists. 

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What does C24 timber mean?

C24 timber is also widely used in the UK, though not as much as C16 due to its higher price, which is largely down to its superior strength and appearance. This is also because the timber is imported into the UK, where it’s grown in climates that force a slower growth rate and a tighter grain. Our main country of import is Sweden, with Finland and the Baltic States just behind. C24 timber is most commonly used for external construction projects like decking and shed bases.

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What are the differences between C16 and C24 timber in practice?

Below is a table demonstrating the specific measurements of both types of timber, all of which indicate C24’s superior strength.

Should I go for C16 or C24 timber?

Without trying to sound like we’re sitting on the fence too much, it’s difficult to say which grade of carcassing timber to use, and the choice will ultimately come down to your own preferences and the needs of your construction project. Both perform well in general, even if C24 timber is of a higher quality and often better suited to external construction work. And, of course, C16 timber makes sense for those on a budget, while, for larger-scale projects and those where appearance is more important, C24 could be the best choice.