Impossible to do. Found a house that had long been abandoned so decided to set up shop in it. Then I found out WHY it had been abandoned.
The building consisted of two foundations side by side with walls all around and a doorway with no door on it, a roof over one of the foundations but not the other. So I built myself a door, placed it down and proceeded to build two campfires to double my smelting speed. Lastly I built a roof to put over the second foundation in order to prevent people from building a ramp to jump in, when I noticed that there was a fifth pillar on the second foundation sitting squarely in the middle. So I finished building the roof, went to place it yet no matter where I looked, no matter what angle I managed to look at that gaping hole where the ceiling should be, I was unable to place the roof up there.
Is this a bug due to there being a fifth pillar there? Does it count that as an obstruction thus preventing you from placing down?
If you ask me, a fifth pillar in the centre of the room makes it feel more homely and it looks austhetically pleasing.