It really depends on how much pitch, or slope, your garage roof has. If its not completely flat, then go for galvanised corrugated steel...available in large sheets, it lasts forever and the only awkward bit is filling in the ends of the corrugations with something. Its available second hand, too. If the roof is almost flat, then mineralised felt is probably best (but not permanent) as plastic has difficulty coping with heat expansion.
John, June 2009
