Untreated timber, used in various building frames & construction work, untreated timber distinguishes itself by the absence of preservatives and chemicals, making it a more "natural" solution. 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.