Coming from a place that gets lots of snow, I think of roofs as being slanted and inaccessible. For me, the word 'roof' does not conjure an image of usable space. However, from the looks of it, the top of Japanese buildings (ie roofs) are largely accessible and put to good use.
Most dictionaries define 屋上 as "roof" or "rooftop". Does the term 屋上 refer specifically to the human-usable space on top of a building, or more broadly to the top of a building, whether or not it is accessible?