Basic Problem: There is a somewhat badly maintained car garage at street level above an area of my basement property. The garage has a concrete floor; the (very small) utility room below has a solid concrete ceiling.
In heavy rain, water leaks very badly down from the garage into my flat, through my concrete ceiling. This is in sufficient volume that it is effectively raining within my utility room; this is not a slow trickle leak / damp.
Rough diagnosis: Having had a look in the garage, it appears that the water runs to edges of the garage concrete floor, and down a crack/gap between the brick wall and concrete floor slab.
I don't believe that any water was going down through the concrete slab itself (but please do correct me if this is a real possibility), although by the time it reached me below it was coming through various points in the ceiling (some obviously where there were old screw holes; some less obviously holes). So I am reasonably sure it's only going down the edges but have only had a brief look as I do not have access to the garage without begging the owner for help with the leak.
Questions: So - firstly does this seem feasible that water would be getting down the wall<->floor gap in reasonably large volume? And secondly what is the solution? Should I (ask the owner to) caulk the gap somehow? Is there a particular type of sealant I should use? Or is mortar/concrete more appropriate?
[ Explanation of property layout: The property I live in was converted (from a period house into flats) in a slightly strange way, with my property being 'lower ground floor' down a full flight of stairs from the street; essentially a basement. The 'ground floor' is up half a flight of stairs from street level to the entrance door. This is not hugely strange except between these two, is a garage! (owned by someone who owns a flat in the property, but not me) - so the key point to the layout is that the garage is above my flat. ]
Alternatively/additionally, should I do something to waterproof my ceiling below? i.e. so that whatever leakage occurs quite simply does not reach me. Would this have a negative effect of allowing water into the concrete and never escaping thus damaging the structural integrity? Am I better off allowing the leak to 'escape' i.e. rain indoors on me?
Side-note: This problem is massively exacerbated by the drainage channel in front of the garage door often blocking with leaves, so rainwater overflows and runs into the garage, but even if I/the garage owner remembered to clean it out frequently, the gap between the concrete slab and the brick wall would still be an issue. I see this is a something of a side-problem that would alleviate rather than solve the main issue. Also open to any suggestions here.