I have a compton precast outbuilding which is not a full size garage but made of the same materials, concrete walls, and corrugated metal roof located on wooden rafters.
For years I have struggled to stop the damn thing from dripping, over my bikes and tools and other precious belongings, in the Autumn and Winter and have tried alsorts (not the liquorice type).
This year to save me from tears (thanks George and Andrew) I came up with a plan, which I am pleased to say has so far worked.
And it goes like this (cheers Chubby).
First I staples plastic sheeting to the rafters close to the roof, then using battens I formed a frame attached to the rafters, this allowed enough room for loft insulation material to be put in place next. It was tricky to do, in the end i made a mesh from cheap gaffa tape to hold it in place.
Next move is easier if you have the building empty. I didn;t cos I like to make things bloody awkward for myself, it takes up more of the day that way and the Mrs stays away when the anger rises.
Using 3mm thick 8foot by 4foot sheets of hardboard cut to size and shape you screw this to the framework you already have in place.
The building is warmer, and with a heater in there and switched on it warms up a hell of a lot quicker and stays warmer for longer. Everything is dry and the extra bonus is I cannot hear the neighbours shouting at me anymore to turn that sodding motorbike off even at midnight.
Yes I have a home build exhaust extraction system.