Roof affecting furniture placement...

I have a room which has a roof over the top made up of a cone shaped roof merged with a hipped roof. At the circumference of the cone shaped roof, I can't place anything UNDERNEATH in the room below?! At first I thought I just couldn't place anything too high up otherwise it might interfere with a very slight overlap from the ceiling, but I can't even place a rug on the floor!

This is really frustrating me, is there anything I can do without taking everything apart? It took me hours...
I have tried deleting the cone-shaped part of the roof and adding the furniture, but when I put the roof back the furniture gets deleted.

I'm playing base game with no mods, cheats or CC.