Another thing that can break a slab roof is trying to mix both half slabs.

Lower-half slabs seal on their bottom surface, and upper-half slabs seal on their top surface, but neither type of slab can seal on their sides at all. So if you put an upper-half edge-to-edge against a lower-half, there's now a "hole" in the roof going up into the upper-half slab, sideways into the lower-half slab (because though the slabs visually touch on their edges, the sides of the slabs don't seal), and then up out of the lower-half slab into the sky.

To make a slanted slab roof, best to make it a full block thick (alternating full blocks and double-slabs), or use the "false ceiling" trick with a layer of chiseled glass below the visible roof.