In Minecraft, some monsters will only spawn within a specific range of block height (Y coordinates). For example, slimes will spawn on level 51 through 69.

Using the slimes as an example, if I place a block at level 69, will slimes be able to spawn on it? Or does the block need to be at level 68 for slimes to be able to spawn on it?

2 Answers 2


From minecraft wiki:

Slime spawning in swamps can occur between Y=51 and Y=69 "inclusive"

this means you have to leave the 69 itself empty (aka place the block on 68) for the spawn to happen.
Wiki link for additional information

  • Don't slimes in swamps also have other special conditions. If I remember correctly a full moon was one of them, unless this was updated/patched since then.
    – Slippo
    Feb 2, 2017 at 2:07

In order for mobs to spawn there must be air. therefor placing a block in y=69 will get rid of that air and make it unable to spawn

If you place a block on y=69 the ground will be at y=70 therefor to high

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