Why don't slabs stop the spawning of mobs?

[ajgaming1396]

Title says it all. If there is a block that stops mob spawning I'll delete this, or if there is a reason that this mechanic has been turned off. I was just confused when mobs were spawning where I didn't want them to, even after trying to stop them with slabs and carpets.

 

[TexasTuberTimYT]

What specifically are you trying to do? Are you just trying to stop the mobs temporarily or are they spawning where you don't want them to.

Maybe you are trying to make a mob grinder and they are "escaping"?

 

[ajgaming1396]

What specifically are you trying to do? Are you just trying to stop the mobs temporarily or are they spawning where you don't want them to.

Maybe you are trying to make a mob grinder and they are "escaping"?

My grinder has no holes in it, it is a box with a staircase made of bottom slabs. Mobs shouldn't spawn on bottom slabs due to vanilla minecraft mechanics. I can attach a picture of it if that would help?