Treated - tanalised timber is used for external building frames exposed to the elements, C24 treated - tanalised timber is used for external building frames exposed to the elements, this timber is sprayed with chemicals and preservatives no significantly extend the material's life and protect it from rot, wear, and pesky creepy crawlies. what exactly is C24 timber? C24 timber is one of the highest quality woods available. it, like C16 timber, is kiln-dried to reduce moisture content and has fewer defects in terms of both characteristics and appearance. This strengthens and fortifies it.