I am working on a Wither Skeleton grinder by wattles, and I have this long tunnel which is 33 blocks wide. It has nether brick slabs as its roof (for decor) but there are Zombie Pigmen always spawning on top!

How can I prevent them from spawning?

2 Answers 2


Slabs do not prevent mob spawning just by their existence. It depends on their positioning.

See this fine piece of art:

enter image description here

If you use top slabs (yellow) or double slabs it will count as normal block and allows spawning. Using slabs at the bottom half (grey) will prevent mob spawning as it is now considered a transparent block.

  • This ^. Also, it's a known bug that mobs are spawning where they shouldn't at the moment, and it's being worked on right now. The plan is to have a fix to release with the Nether update. Apr 6, 2020 at 9:20

Use cobblestone instead. Theoretically they will not spawn on that.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .