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According to the wiki, mobs do not spawn in water or lava, or in a block that has higher light level than the limit for the respective type of a mob. However, it appears that mobs also cannot spawn in pressure plates, and that seems like a an exception to me, since pressure plates aren't solid.

Are there any other non-solid blocks that prevent mobs from spawning in them (not on them)?

  • I suspect anything with an interfering collision box, like fences and fence gates, would prevent spawning. Pressure plates, rails, and redstone(?) sometimes prevent spawning, but sometimes don't, depending on the version. Will do some tests on the latest version to see the mechanics – phflack Feb 13 '18 at 18:49
  • @phflack Thank you; an answer for 1.12 is sufficient. – IllidanS4 Feb 13 '18 at 19:04
  • I'm pretty sure they only spawn in air. Xisuma has a series/category of Mythbusting called "will it spawn", but most videos are outdated. – Fabian Röling Feb 13 '18 at 19:32
  • Related – Ben Feb 14 '18 at 3:18
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Yes, all the blocks that are non-full and have a hitbox that touches the bounding box of the mobs will prevent mobs from spawning in them. This includes Carpets, Slabs, Stairs, Daylight Sensors, Redstone Repeaters, etc. As an exception (as you said) Pressure Plates also prevent mobs from spawning, maybe in order to make mob spawning harder, but this was not the case before the Minecraft 1.9 version.
They will spawn in most non-solid blocks, regardless of being full or not. This include Grass, Flowers, Sugar Cane, etc.

  • I count carpets, slabs etc. as solid. So rails or redstone, as one comment suggested, do not prevent mobs from spawning? – IllidanS4 Feb 13 '18 at 19:43
  • Rails obstruct mobs from spawning, as of 1.9, since they have a collision box. I don't think that this is the case for Redstone, since it doesn't have a collision box. @IllidanS4 – John Feb 13 '18 at 19:47

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