@thirdpotato If there are mods installed the first explanation is that they interfere, and so the first troubleshooting step would be to remove them and test in that save; and then test in a new save that has never had mods or cc. Have you done that?
You need to go up a floor in build mode so the grid shows on top of the walls where you want to place the roof. Going by your screenshot the game thinks you want to place the roof on the floor of that room, rather than on top.
@thirdpotato Was it that easy? Did you not know that you need to go up a level? Does it work when you do?
