Grant, I’ve had a wooden run off roof observatory for around 12 years. The walls are tongue and groove and the roof is ply covered in roofing felt. It runs on steel angle rails with B&Q plastic wheels. The roof is ventilated around the roof line. It has been totally water proof with no condensation whatsoever – an advantage of wood – and the telescope tube never has condensation on it. The only problem is that it is so dry it gets taken over by spiders. Ladybirds also like to over winter in it. Another advantage of wood is that it’s easy to screw boards etc. to it and fix electric cables.