47x225 TR26 kiln-dried (KD) untreated structural timber - TR26 ('TR' standing for 'trussed rafter') timber is a specific grade of timber designed for roof trusses, engineered joists and metal web joists. It's machine graded to ensure it's of the highest quality, kiln-dried to around 20% or less, and made almost exclusively from Scandinavian spruce or pine