Shed was put up in 2011 but this year it has started getting really bad mould on the inside of the roof. The building itself is standard shed airtight, ie gaps all over the place and the felt is the stuff that came on it, is in a good visible state but always felt quite thin. Any thoughts on what is causing it? Every now and then I use a fan heater in there if I’m fiddling with bikes but beyond that its just left. If the felt on the roof is letting water through and getting the boards underneath damp, then would that be enough to cause the mould?
I’m thinking of putting a new layer of felt over the top of the existing layer to try and help further waterproof the roof or do I need to take the existing off, cover it whilst the boards dry out and then do a new layer on the roof? Never had mould in a shed before so somewhat baffled.