C24 is 50% stronger and that is not what you get
The number after the C is the characteristic bending strength in newtons per square millimetre, so C24 really is fifty per cent stronger in bending than C16. It is the first thing anyone tells you and it is very nearly irrelevant to the decision you are making.
A domestic floor joist almost never fails in bending. It gets rejected because it sags, and sag is governed by stiffness, not strength. On stiffness the gap is much smaller: 11,000 against 8,000 N/mm², so 37.5%.
And it gets smaller again by the time it reaches a span. For a joist under a uniform load with a deflection limit set as a fraction of span, the arithmetic rearranges so that span cubed is proportional to stiffness. Span therefore goes as the cube root of stiffness, and the cube root of 1.375 is 1.112. C16 to C24 buys about eleven per cent more span.
That is still worth having. Eleven per cent is the difference between a 3.6 metre span and a 4 metre one, and the price gap between the two grades is usually small. But go in expecting eleven and not fifty, because sometimes a deeper C16 joist is cheaper than a shallower C24 one, and you cannot see that if you think you are buying half as much again.
Two sets of numbers for the same piece of wood
Look up C16's bending strength and you will find 16. Look it up somewhere else and you will find 5.3. Neither is a misprint.
16 N/mm² is the characteristic value from BS EN 338. It is a raw property of the timber with no safety factors in it, and Eurocode 5 applies those separately as part of the design. 5.3 N/mm² is the permissible grade stress from BS 5268-2, an older permissible-stress code that had the factors already buried inside the number.
BS 5268-2 was withdrawn in 2010 when Eurocode 5 took over. Merchants and fabricators still publish datasheets in it, so the small numbers remain everywhere, and people quite reasonably conclude that somebody is selling weak timber. Both numbers describe the same wood. What matters is never mixing them inside one calculation.
A strength class is not a species
C16 is not a type of wood. It is a guaranteed minimum performance, and several species reach it. The same log can yield pieces that grade C16 and pieces that grade C24, which is why grading is done piece by piece, by machine or by a qualified grader, and why every structural piece carries a stamp.
In practice most UK carcassing on a merchant's rack is C16 by default, and C24 is a stock item you ask for. The stamp on the timber is the only thing that makes it what it says it is, so a piece with the stamp cut off is, formally, ungraded.
TR26 is a different animal
TR26 turns up in searches beside C16 and C24 and does not belong with them. It is the grade used for trussed rafters, it is not an EN 338 C class, and you will not buy it off a rack because it comes as a finished truss from a fabricator with the design already done.
Its published figures are in the withdrawn BS 5268 system, so its bending value of 28.3 sits next to C24's 24 looking like a modest upgrade when the two are not on the same scale at all. That is the clearest illustration of why the two systems have to be kept apart.
Common questions
What is the difference between C16 and C24?
C24 has a characteristic bending strength of 24 N/mm² against C16’s 16, so 50% stronger in bending. But it is only 37.5% stiffer, 11,000 against 8,000 N/mm², and domestic joists are governed by deflection rather than bending. Since span goes as the cube root of stiffness, upgrading buys about 11% more span, not 50%.
Is C24 worth the extra over C16?
Often, because the price gap is usually small and 11% of span is a real gain when you are one joist size away from working. But go in knowing it is 11% and not 50%, because the headline number leads people to expect a jump that does not arrive, and sometimes a deeper C16 joist is cheaper than a shallower C24 one.
Why does one datasheet say C16 bending is 16 and another says 5.3?
Because there are two systems and both are describing the same timber. 16 N/mm² is the characteristic value from BS EN 338, which Eurocode 5 designs with. 5.3 N/mm² is the permissible grade stress from BS 5268-2, which has the safety factors already built into it. BS 5268 was withdrawn in 2010, but merchants still publish sheets in it, so both numbers are in circulation.
What does the C in C16 stand for?
Coniferous, meaning softwood. Hardwoods get D classes. The number is the characteristic bending strength in newtons per square millimetre, so the designation tells you the main property directly, which is unusually helpful as naming systems go.
Is C16 a species of timber?
No. It is a strength class, not a species and not a quality grade. Several species can be graded into C16, and the same species can produce C16 and C24 depending on how individual pieces grade. What you are buying is a guaranteed minimum performance, not a particular tree.
What is TR26?
The grade used for trussed rafters. It is not a C class and you will not buy it off a merchant’s rack, because it arrives as a finished truss from a fabricator. Its published figures are in the withdrawn BS 5268 system, which is why its bending value of 28.3 looks enormous next to C24’s 24 and cannot be compared with it.