Ventilation for roof of mineral felted brick building.

It’s sounds like an outbuilding? Install air bricks at ground level obviously not completely touching the ground and then install some at the top of the wall, this will allow air to move and circulate round and will help eliminate moisture levels without putting holes in the roof there no need to do this if you haven’t insulated the roof space you just creating unnecessary weak points in the roof. It sounds like the damp course hasn’t been installed in foundations etc or has failed, if you think water is penetrating the walls you can use a chemical solution like storm cream this is a temporary measure that needs reapplying every 10 years or so. I will say it is strange mould is growing on the OSB are you sure it isn’t leaking slowly? Does it puddle? Have the joins of the bitumen membrane been bleed out? Hope this helps Regards Frankie BT Roofing Services

It sounds like a case of , the technical term is “ sweating it’s tits off “, if there’s no leaks it’s all condensation, I’ve seen a single brick out building, concrete floor and we fitted new boards and GRP fibreglass roof , the full 3.5 m length of boards where covered in black mould, when cooking the steam all went out kitchen window into out building, when cold outside all the time heating is on or cooking etc all moisture goes to cold outside wall and roof