condensation in garage?

my brother has just completed a garage it has a flat roof its made of 18mm chipboard with a felt roof the underside is painted with some black waterproof rubbery paint(sorry dont know the name) theres a four inch gap then fibreglass four inches thick then plasterboard ceiling there is a couple of sheets left to put up but there is a lot of condensation on the underside of the chipboard the concrete floor wasnt long down before the ceiling went up hes thinking of pulling the ceiling down can anyone please help with this problem with either drying or fixing it