You need to build a wall under it. However, this will make the rooms very small.

i cant even select basic floor. cant select most thing actually. things too bugged to be early access even...

Seems that the was not repaired for full release, i just bought today this crap and since 1,5 hours trying to figure it how the logic of roof floor, i cannot put random parts, i have no words how crap it is, and sadly i can not refund it because ive been playing more than 2 hours to try this piece of garbage. Any ideas?