I built a mobfarm above the ocean with stone slabs, but no mob spawned. Then I tryed it with double stone slabs but there it was the same. Finally I built a platform with stone brick blocks next to the other and there the mobs spawned. (Samsung Galaxy 57)
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As of 1.9.0, mobs will spawn on top slabs (and have been for several releases). However, they still do not spawn on double stone slabs or double wooden slabs (double_stone_slab and double_wooden_slab tested).
The attached world is a convenient way to test this. There is a set of command blocks to help at Y=30, near the middle.
This bug is fixed in the latest beta version, and the fix will be included in a future update when that is ready for full release (no specific date can be given). We will leave this ticket open as a point of reference until the fix has been fully released.
migrated
Parttly confirmed in Windows 10 1.2.10.2.
In a flat world with Simulation Distance set to 4 chunks and time set to midnight, I filled an 80x80 block area of the surface (Y=4) with top stone slabs, then replaced the middle 36x36 blocks with grass. Flying to Y=40, I observed that creepers, skeletons, spiders, zombies, zombie villagers, endermen, and witches spawned only on the grassy areas inside or outside the square ring of slabs. Slimes, however, did spawn on the slabs.
After replacing the slabs with double stone slabs, I repeated the observation but this time I flew around horizontally and vertically to cover more area. This time I did see some spawns on the double slabs, but most were on the grass and those that were on slabs were close to the grass. It's plausible that they spawned at a random offset from a chosen spawn point that was on grass. I only saw one slime spawn this time, and it was on grass.