Hi
i have replaced a few of the asbestos roofs on these type of garages.
If it is aluminium roof it should only be the joints that are leaking or on a cold day you will get condensation.
If me and the roof panels sound i would re silicone all the joints. If gone porous go to your nearest demolition yard and get re-placement corrugated sheets should only set you back about £60 and a box of roofing screws will set you back about £25.00(screw fix) And you will need some wood to go on the cross beams about £25.00 whole job should be unde £150.
Marley
August 2008