Yeah, using linked walls results in no roof. If you want to build a custom building with a roof, you have to use the ‘grow walls’ tool, after selecting a floor.
I have no idea why it doesn’t work, honestly It’s just something I’ve come across during gameplay. If you want to work with the custom walls, you can still make a ‘roof’ out of slabs, though! I’ve done that bunches of times, if I want a building that abuts the side of a mountain, or oddly spaced openings.
first you must make a big flooring in the shape you like then use the growth tool and then you can use the pillars to make innerrooms ^^ its a bit tricky but it works xD
i think they have used two systems: one for only walls (for the stronghold) and one for the housebuilding ^^ lets look what ponder oder sdee says ^^