In reply to Kes:
Regarding the damp issue, I've had a wall in my garage for about 5 years now. It's on the north side of the house so never gets any sun and is unheated but there's no issue with damp and the wood isn't treated.
Garage moisture levels must be higher than in a centrally heated house but nowhere near enough to rot timber if there's adequate ventilation and the roof doesn't leak.
One possible exception might be that old garage floors may not have a functioning damp proof memrane so timbers resting on the floor could potentially rot.
I suspect any dampness is specific to an individual garage rather than a general problem.