I figured it out! If I remember correctly, a ceiling is only there as long as there is flooring on the above level. I'll try to break it down for a single story house.
1. Go to the roof level and press END until you can't see it.
2. Select the room you would like to add a ceiling to
a yellow ring should show up around the upper perimeter of the room
3. Click said ring and select Build Ceiling
Taaa-Daaaa
Now if someone can figure out how to colour the ceiling, that'd be cool.