Re: flat roof/floor

@kreativeangel93 judging by your images, the game does not consider the structure you've built a room.  This can happen if you drew the structure using individual walls; the game may not always automatically create a room when you build in that manner.  Instead, try using the custom room tool for odd shaped rooms.  Using that tool, you can draw all along the exact outline that you already have, and the game should generate it as a room once you have fully connected the area.  When the game considers an area a room, it will automatically place a flooring (not grass) and ceiling (although you have to go up a level to see the ceiling).