Building problems and solutions

we are in a detached house but our garage is attached to the neighbours,the previous occupiers have left the house in need of much repair,however our neighbours have had a pitched roof put on there garage (before we moved in ) and ours is still a flat roof,there finished edging and guttering seems to overlap ours and we our experiencing problems with everything spilling over onto and down the side of our garage from theirs, should there have been rules in place that said they can not overlap ours and what do we do know,thanks .