Oh yeah, I guess 'roof' is a sub-section of the wall it serves, huh. It could simply be that the game's available resources don't support flat roofing, and assum all roofs to have the sloping outer rim. The part shown in your image is the inner 'roof tiling', which is just a subsection within even the roof subsection. Can you use the slope roof? The house-building tutorial from Larian goes through that process. If you absolutely need a flat roof, you need to start experimenting I think... I'd try making a second, invisible wall, 2 units bigger on both the X and Y axis, and use the roof tiling for that instead, since it will fit in your visible roof's gap once positioned correctly.
