Treated C24/C16

It isn’t really possible to say that C24 timber is “better” than C16, but it is possible to say C24 is “sometimes better suited” for some builds than C16. In most instances the C24 timbers are straighter and stronger than C16, they have smaller and less knots. However, C24 timber is more expensive than C16 and has to be imported from the Baltic regions. This does not help with current environmental concerns, because the UK timber industry is under increasing pressure to use home grown C16 timber.

The choice of C16 or C24 for decking is largely personal because decks rarely require planning permission. These are structural grades and C24 timber can span greater distances than C16. This should help to keep the costs down and potentially make construction of the sub-frame easier. There is a move to using C16 timbers for decking structures, particularly if the timber is UC4 pressure treated. Deck-Tape is another useful addition to the frame build; the tape helps reduce standing water on the frame and provide some durability.

The best grade of pressure treated structural timber is C24 and this C24 grade has to be imported from Northern Europe. Unfortunately, UK grown timber grows very quickly because of the warmer climate of the UK. These timbers grade to C16 but nothing higher.

Normally C24 is preferred to C16 for roofing applications because it has a higher load bearing capacity, it is usually straighter and also has fewer knots. Roofing material lists usually request C24 timbers in preference to C16, because it can be substituted for the lower specified timber.