I want to be able to build walls adn floors and usual building ontop of the roofs of buildings and inside buildings instead of just using barricades is there a way I can get this to work?

One way would be to place the Floor type structure next to the building or somewhere on higher natural ground. Then after, you build towards the building itself using roofs/more floors while salvaging anything in between.
It would look the same as if you build it straight on the building if you do it right.
(Summary: Floor structure can only be placed on the map ground, so have to get creative to get structures where you want them to be at.)

Pretty good answer by this guy

Originally posted by Ongjming:

One way would be to place the Floor type structure next to the building or somewhere on higher natural ground. Then after, you build towards the building itself using roofs/more floors while salvaging anything in between.
It would look the same as if you build it straight on the building if you do it right.
(Summary: Floor structure can only be placed on the map ground, so have to get creative to get structures where you want them to be at.)