New house has a brick built workshop with a much neglected flat roof, felt over OSB. The roof is seriously mouldy in places - I can ram my wrecking bar right through it. Pitch is about 1 in 40. The timbers, at least the ones I've checked, have a fair covering of mould in places, but appear otherwise sound.

The rough plan is:

Strip it back to the rafters.
Scrape off mould where necessary
Treat timbers with something (what?? Bearing in mind I want to share airspace with these timbers later, though I suppose chucking up some plasterboard is alright if I have to).
Cover with OSB 3, tarp cover while I wait for non-freezing dry weather, then EDPM.