The condensation is caused when warm air carrying lots of moisture meets the cold surface of the metal roof. To reduce the condensation you might try:
Insulate or at least double-line the inside of the garage roof.
Keep the garage cooler - try not to let as much warm, humid air in there.
Arrange for better ventilation and or leave the garage door(s) open when you can.
If you keep your car in the garage allow it to cool down before putting it away.
Peccavi, June 2009
