Short answer: No, however…
Mobs generally cannot spawn on slabs, but sometimes they actually can under certain conditions. Why? What is required to spawn a mob? Is there still a way to get a mob spawned even if the floor is made out of slabs? Can mobs spawn on slabs?
Now let’s get those questions a proper answer.
What are the requirements for mobs to spawn?
It depends on which mob you are referring to. When we talk about mob spawning we generally think of the hostile mobs that spawn at night, which is an indicator of their first requirement: dark.
There are 15 levels of light in Minecraft. Hostile mobs are able to spawn once the block has a level of light of 7 or less. You can check out the level of light for the block by pressing F3
You can increase the light level of blocks by placing torches around them. The closer they are to the source of light the brighter the block will be. You can also use glowstone or lamps with the same effect.
The second requirement is the surface on which the mob will be spawned. Mobs cannot spawn in the air, they need a solid “full” block on which they can spawn. This is the reason why zombies don’t spawn on slabs. Slabs of any type are not considered as a “full” block unless you have placed 2 slabs on top of one another to form a full block.
Keep in mind that you cannot place slabs anyway to want to create a full block. You need to follow a pattern in which all the other blocks are placed in Minecraft. Here is an example:
Can I manually spawn a mob on a slab?
Yes, you can spawn some mobs such as chickens in case you have eggs to throw at the slabs. They do not care for the type of block they hit. If you are playing on creative mode you can also spawn every mob that has a Spawn Egg.
In survival mode, however, there are much fewer egg types to be found and as such, you can’t spawn that many mobs manually in general. You can only create a dark place and hope for the best, with “full blocks” or correctly placed slabs as the floor.
Can I use slabs for my spawner farm?
Yes, you can use slabs for your spawner farm BUT ONLY if you still follow the placement pattern shown above. Slabs can be very useful in mob farms to designate blocks you DO NOT want your enemies to spawn, especially because you have to be very careful with how you place torches near the spawner if you still want it to create mobs.
Also keep an eye that even though you can place slabs as your floor inside the spawn room, you should still use full blocks for the outer corners of the room. This way you prevent any light from reaching through holes and openings created by the slabs.
Is there a difference in which type of slabs I use?
No. You can use any type of slabs you want and mobs will still get spawned as there are no slabs that are capable to illuminate light.
Keep in mind however that light from torches or daylight CAN pass through slabs so they might be the reason why your farm is still very bright even though you have no sources of light inside it.
Now I know why mobs don’t spawn on slabs. Why don’t they just “fix” them to be a “full” block and let them spawn in the next update?
Well, because slabs have developed themselves as a way of “Minecraft logic” and there are hundreds if not thousands of devices, structures and machines, especially when it comes to mob farms, which include slabs as the essential part of their structure and they need slabs to be just the way they are. They are used to create pathways without any dangers of encountering hostile mobs at the places you don’t want them to be.
What are slabs useful for?
We have already given some examples of how to use slabs in Minecraft.
The first example is as simple as it gets: decoration.
There are 30+ types of different slabs that you can use to decorate your space however you want! Their biggest bonus is their half-block height which makes them perfect for house ceilings so you don’t bump your head every time you jump.
They are also perfect for expensive floors as they allow you to cover 2 times more blocks with the same amount of resources.
Some of the slabs that exist in Minecraft
The second example of how to use slabs is to use them as windows. Their advantage over “normal” windows is that you can actually interact with the world on the other side of the window without breaking the block. What’s even better, if you place your slabs carefully you can make yourself invisible to the mobs on the other side while you can still shoot them from a safe place.
This is most commonly used in spawn farms where you place a slab to cover the upper side of the block. This way you block the enemy’s sight of you which makes them unable to respond to your attacks. There are other ways to achieve this similar effect, but this specific one is usually most recommended when creating skeleton farms as they tend to be the most “problematic” type of mob to farm.
So to sum it all up. What to think of slab blocks? Are they useful, pretty, efficient, and safe, or just annoyingly small and useless? I would say they are one of the most useful types of blocks we have in Minecraft. Even though they don’t let mobs randomly spawn, if they are placed correctly they can still help you farm whichever mobs you want. On the other side, if you ever feel unsure whether or not slabs would be useful for the job you can always just stack two of the same slabs on top of one another to create the usual, normal “full” block.
I believe they deserve to get a shot.