Thanks for the reply. Yes, the roof has been leaking. The roof has felt and galvanised corrugated tin sheets, however it looks like they were never sealed properly at front or back. The gutter is at the back of the garage . My intention is to- 1. replace all wall plates and joists but create a fall from front to back by staggering the joist height slightly 2. use OSB3 t&g roof decking on the top. 3. re felt properly, sealing it all round. I'm not sure how to do this at the sides, whether I should raise the facia boards to channel the water towards the back rather than encourage it to