Deleting Floor Tile

I'm trying to build my house and I can't seem to delete a single piece of floor tile. I've been at it for days. All because I dont want my whole second floor to have flooring. I want an open ceiling up to the roof right above my front foor, you know? HELP PLEASE.

I've tried making the part of the floor I'd like to delete a seperate room and than deleting the floors and than the walls but it makes the roof come into my house for some reason. very fustrating. I'd like to know also if there is just a legit way to execute this; not some other weird creative way around it.