Specifically when trying to place an upper slab it will not place, it will not stack easily on top of bottom slabs either. Instead placing beside or behind first. The only way I was able to get upper slabs to place (specifically copper have not tried other slabs) was by building a bottom slab with a full block behind it. Then placing on the upper half of the full block a top slab would be place (not on the half slab but one block in front of it. This makes roofs and other items where. Upper half slabs might be needed nearly impossible.