There are a few things about the new builder that seem to not work; despite how much I feel it should:
you can’t use the hole tool on roofs (would be nice to be able to add, sky lights or Dormer windows)
you can’t have roofs that slant only one way, you can remove the other slants, but that just creates flat sections of roof (would be nice to let the slant reach to the other side, would also look even nicer if you could adjust the slant angle.)
removing the walls from the roof causes it to float at least one block above the walls, requiring you to raise them (easiest fix it to just remove the restriction of the roof base height not being allowed to be lowered)