Except if a stray spark simmers on the floor in your living room floor for a bit before you catch it, it more than likely won't cause you issues every time it rains. I suppose the likely hood of a spark burning a hole all the way through that tar is pretty low, but still something of which I'd be wary. As far as the slope, I believe most flat roofs have a minor slope (like the way a gutter is sloped), but not much. Certainly less than 1/12 pitch.