[parson]

I've found that the best material to use is 18mm exterior ply, as opposed to planks of shiplap or t+g.

I would have thought t+g would have been fine as long as you had enough incline and also the toungue facing up into the groove. If not then some silocon sealer along grooves and spotted onto the offending knots should do the trick.

[Bantymad]

i have a 6x4 shed formy bantys and painted bitumen on roof then felt and again with bitumen, inside i went round all the overlaps and knots with silicone sealer and also painted inside and out with creocote, the legal stuff!! dry as a bone in there. i got the silicone sealer from in-exess for a pound a tube, as opposed to 5 quid at other places.