In the build of alpha 10 we have some upgrades to the slab tool. but i saw some flaws to it yet when you work with multiple slabs per building.
If you make a tower with only 1 block they will build it without problem, after the first slab, they will build scaffolding going up to the top of the tower.
But if you make a 3x3 base and a tower in the middle, they will only build the base and the first block of the tower, the same happened with a 5x5 base.
If you build the base first, then after it is completed you build the tower they will make it without any problems
As you can see i was messing around trying to test the new things the slab can make. After some tests, right now the white building i was trying to make would only be possible to finish if a build each part alone.