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Last Autumn i replaced my shed roof using plywood covered with new felt. almost imediately it sprouted black and white mould all over the inner surface. Probably due to condensation forming on it during cold weather. It does have a decent air gap all around between the roof and the walls too (or perhaps thats the problem?). So its now had some wood preserver sponged over it but now im wondering wheather varnishing it would be a good idea or not or would it be wiser to apply some ronseal woodstain instead which says it can breath? I have a tin of both varnish or ronseal so which would likely work better to stop the mould coming back... Thanks
A permeable finish to allow the ply to breathe would be my preferred route.
Given that you have an air gap all around the roof line and condensation is forming on the new ply, I would be tempted to introduce a couple of vents just above floor level to help the air move. The warm humid air in the shed rises above the ventilation and hangs around next to an uninsulated wood roof. Air movement will be your friend.
HTH
That makes sense thanks sa.