A relative of mine has black mould on the underside of the roof in a concrete bolt together garage built about 8 years back.
One possible cause is the tumbler is in there and not vented which clearly needs sorting out. It has apparently been operated with the side garage door open, but I'd guess the steam went upwards into the roof space rather than
10 foot sideways!
That being said, the amount of mould is surprisingly significant - with patches of it under every corrugated roof sheet in the whole garage. Ventilation is actually pretty good through the ridge plate.
Could the steam from the tumbler be responsible for ALL this do you think, or should I be looking for other causes? Not clear how long the tumbler has been there or when the problem started but it sounds like it could be 2 years+ I doubt the tumbler - a mini one - has been used regualry.
Any recommendations for good mould remover/killers?
TIA Midge.