use a bit of gg4 or 3 (4 if you are in a hardwater area or 3 in a soft water). Mix it with water in a spray bottle and spray on roof. If the tiles are big enough to squeegee, try that but if they are too small buff them with a microfibre (highly recommend the blue Microfibres from windowcleaningwarehouse)
Just a thought........ if it is 3 times a year they should be pretty clean anyway so you might get away with just a dry buff with the microfibre! if not, try a MF that is slightly damp with gg4 and then dry buff
If they are smokers they will definately need to be sprayed
Oh, and gg4/3 available from windowcleaningwarehouse.
Hope this helps
Adie