I have a large over world mob farm that's broken in 3 ways, 1 redstone (never going to be changed) 2 mobs barely spawn in PE (not the point) and 3 Its bloody daylight in there! Slabs (just bottom half?) don't block any light, meaning its only useful at night, even then you won't get anything to spawn..

See picture.

If you need any more pics or video just ask