condensation forms on the barrier between hot and cold, always on the hot side, thus you are getting it on the underside of the roof sheeting I presume. This would indicate, that thankfully it is warmer inside the garage than outside.
To address this a layer of insulation is required to separate the hot and cold. A good one to use would have a vapour barrier, a sarking and hydroscopic materials.
A good product that is designed for retrofitting would be something like:
http://www.natural-building.co.uk/pa...renovation.htm
would be better and more cost effective in the long run to insulate than running a fan all day etc
JP