I think you have a poorly worded contract. Surface means like it sounds, the top surface of the roof. You have to read it together with the other part with is roof deck. These deal with separate areas. The roof deck is more easily definable as the plywood sheeting on the roof rafters/framing. Reading those two together tells me that the surface would be the shingles and tar paper.
An analogy would be to think of the surface of your kitchen table or your kitchen cabinets. Surface here would mean to me to be the laminate and not the table or countertop itself.
Another analogy, the surface of an interior wall is what you paint (basically a paper surface) but would not be the entire piece of drywall wall itself.
In your application, if wind blew some shingles off, then the first floor owner with exclusive rights would be assessed. If a falling object, hail, strong wind, leaking water, rain, ice damage, etc. also damaged the roof sheeting, chimney, vent piping, insulation, and the inside ceiling then everyone would share in that portion of the costs. I don't really understand why the contract is written to assess the first floor owner instead of sharing with a second floor owner but there may be other language that explains that. If the garage is used by all then the costs should be borne by all.