I had this problem with my garage. When I moved in the whole structure was rotten and disintegrating, my neighbour (75) told me that she remembered it being built when she moved in to her house 50 years ago and it hadn't been touched since. Over time, she had a path built on her side (she is slightly up hill) and the concrete slab made for this abutted the wooden sides of the garage which exacerbated the rot over the last 20 or so years!
Anyway, this summer I stripped the garage of its old vertical t&g cladding. The frame was still sound and strong so I re cladded with ship lap and built up one side of the garage with concrete blocks half way up with a 4 inch cavity from the boundary. After I finished this we had a major downpour which lasted quite a few hours.
No problem for my new garage, its completely weather proof (so I thought!)
the next day, I opened the doors to the same problem as before, it was as if I hadn't done any work at all. the roof, while needing replacing (can't afford this year) was only leaking in one place and it was a slow drip for which a bucket was doing its job. the main bulk was still coming in via next doors concrete slab work, soaking through the conrete blocks.
I went outside and had one of those Eureka moments "GUTTERING". So put guttering in on that side (I was experimenting at this point!) and waited for the next rain. 3 weeks later it came, the floor was dry at this point so I could see if the solution worked.
It did, no more leaking!
Bottom line, fit some guttering, you will probably be surprised!
