Thanks for the replies everyone! Second question, how do I make a roof?

Tried manually placing it but that was really hard, none seemed to fit.
The quick and dirty way is to copy/paste the walls, move the copied walls on top of the existing walls, and then change the root template on the sidebar to one of the roof root templates.
It's easier to do this before making doors on the lower level, or delete the doors before changing the root template.