What I expected to happen was...:
Place wooden slabs on the roof, so they would be placed on the underside of the block above them.

What actually happened was...:
The desired effect, but when I placed multiple slabs next to each other, the lighting on any surrounding slabs would appear as if there was no light source anywhere near them.

Steps to Reproduce:
1. Place multiple wooden/stone slabs (only tested with wood and stone half slabs) on the underside of any block.