1. If you are not sure if the roof is flat or warped, you could create an in-place mass using the edges of the structure as reference. Then, create a roof by face from the mass.
2. If you are sure the roof is flat, you could create a regular roof by sketching the boundary, then draw the slope arrow at each edge to match the slope from structure.
By the way, if a roof is flat, which I think yours is, then it technically slopes in one direction. You see different slopes on different edges because they are not parallel to each other.