Durability is the ability of a species to resist decay either naturally or through preservatives. The Class is based on the ability of the heartwood (inner part of the tree) to resist fungal decay. The sapwood (the living outermost portion of the tree)is considered not durable and should not be used for external projects without preservative.
BS EN 350 has 5 classes of durability they are:-
Class 1 to 3 can be left as untreated timber, a natural ageing process will accrue e.g. Cedar will turn a grey colour. Class 4 and 5 will need to be treated with preservatives.
For further information see www.greenspec.co.uk/building-design/cladding-durability-quality
Is among the most popular softwoods used today it's natural resistance to decay and moisture absorption means it can typically be installed without treatment. Rated BS EN 350 class 3 moderately durability (see below) and is subject to little movement when installed.
Home grown Larch is a viable cladding alternative, as well as the first choice for many. It's a harder timber making it more resistant to impact damage and is more suited than Cedar for structural applications, such as support columns, door frames, window frames.
Douglas Fir will need to be treated please see www.treatex.co.uk