Slab

Slabs are blocks that allow the player to change elevation without jumping. They are not as compact as stairs, requiring twice the horizontal space for an equivalent change in elevation. However, since the Beta 1.8 sprinting feature, stairs stops your sprinting unlike slabs, which makes vertical elevation equally quick and horizontal speed twice as fast on stairs on "sprinting state". Stone slabs were introduced to the game on October 24, 2009, in Survival Test 0.27, whereas sandstone, wooden, and cobblestone slabs were added in the Beta 1.3 update on February 22, 2011 and stone brick and brick slabs were added in the Beta 1.8 update on September 15, 2011. The ability to place Slabs upside-down was added in the 1.2 Preview (February 29th, 2012).

All six types of slab have the same data values; like wood and colored wool, they are differentiated by their damage values: stone slabs have a damage value of 0, sandstone slabs are 1, wooden slabs are 2, cobblestone slabs are 3, bricks are 4, and stone bricks are 5.

Occurrence
Stone slabs can be found naturally in NPC Villages lining the roof of a blacksmith shop and inside some of the buildings where they form counters. They are also found in Strongholds where they are used in some of the stairs, ledges and torch pillars. As of Snapshot 12w04a, sandstone slabs can be found naturally in randomly generated desert wells.

Crafting
Slabs can be made from three wooden planks or three stone, cobblestone, sandstone, brick, or stone brick blocks, producing six slabs per crafting operation. There also is a stone slab which only uses the top texture of the normal stone slab, but it can't be crafted. The damage value is 6.

Behavior
Like other partial blocks, slabs are treated as a whole block with other blocks, such as dirt, stone, and glass, and liquids. Two slabs of the same type (e.g. two stone slabs) can be placed one on top of the other to make a single full-size block, but different slab types cannot be mixed in this way.

Sandstone, wooden, and cobblestone double-slabs look exactly the same as their full block counter-parts, but take a longer time to break and drop two slabs.



Wooden slabs can currently only be collected with a pickaxe because they are the same block as other slabs, with a different texture. Wooden slabs also are not affected by fire and have a stronger blast resistance than wooden planks, making them a useful building material but at a 2:1 ratio. This is likely due to the fact that the slab types are differentiated by their damage values instead of by different data values, similar to wood, coal or charcoal, and coloured wool.

An unusual property of slabs is that they are non-solid to redstone. This allows one to hide the redstone wiring in a slab covered channel, while still being able to connect to the wire on the side of the slabs.



Mobs cannot spawn on top of single slabs, but they can spawn on double slabs.

Due to the way blast rays propagate from an explosion, slabs provide extremely effective absorption to explosions taking place directly on top of them. Specifically, this is because explosive entities will be lower in elevation when they explode on top of slabs than they would otherwise be on an ordinary block. Although the few slab(s) directly under the explosion will absorb the full force of the blast (with a resistance of 30) as usual, the propagation of damage to the sides will be greatly reduced. If source of the explosion is elevated for any reason at the time of the blast, this protective quirk is lost.

There is also a glitch where you can place a slab on top of a single slab while standing on it and if there is nothing below the lower slab you will fall through the slab you are standing on.

History
Stone slabs were first introduced in Classic mode. In Classic and Survival Test, stone slabs could be obtained by mining a coal block because of the lack of inventory and crafting.

Stone slab blocks were called stair blocks before the current stairs were added; after this, they were known as steps (the two forms being single steps and double steps) before all items got official names upon the Beta release.

Before the additional slabs were added, a double Stone Slab would only yield one slab when broken. Since the Beta 1.3 update, all double slabs yield 2 of their respective single slabs when broken. Destroying double slabs with TNT, however, still only yields single slabs (when the slabs aren't simply destroyed by the explosion).

Before Beta 1.3 came out, stone slabs were made with cobblestone instead of stone, but that update introduced cobblestone slabs to the game and changed the recipes for Pressure Plates and stone slabs so that there wouldn't be any conflicting recipes.

As of the 1.2 Preview, all types of Slabs can be placed upside down and under blocks.

Bugs
In 1.8.1 wooden slabs break faster, (making it seem as if the slab was just a stone one that had the wood texture, wouldn't burn unlike it's standard wood counterpart.) and drop the wooden slab only when using a Pickaxe instead of an Axe. (tested for vanilla release 1.0.0: pickaxe breaks wood slabs faster than axe but both produce drops)

Using slabs you can place torches on some blocks which open after right-clicking, such as crafting tables or furnaces, by placing the block on a 3x3 slab square under it, and attempting to place a torch on an edge slab.

Dirt slab
Before Stone Slabs were added to 0.26, Notch ran a test of Dirt Slabs. They were never added to the actual game and were mentioned for the first and last time in this post. They replaced all dirt blocks and did not grow Grass on top.

Trivia

 * Despite the fact that sneaking lowers the player's eye level of 1.5 blocks, doing so does not allow the player to walk over a single slab with one block of air above it because of the player's true height.
 * A player can not walk from a block of Soul Sand to a slab without jumping.
 * Wooden slabs are not flammable and have to be destroyed by a pickaxe to drop something, possibly due to the fact that they share the same data value as other slabs. This can be used in SMP to help prevent griefing.
 * Single slabs have the tendency to let through arrows shot from above.
 * Single slabs will destroy gravel and sand blocks that fall onto them, the same as torches.
 * Single slabs are treated as transparent by the game. Because of this, they do not cause suffocation, you cannot place torches or other fixtures on them and chests with single slabs above them can still be opened.
 * The exception to this is that when two slabs are placed side-by-side in water, a ladder can be placed on them. However, any ladder placed in this way is invisible. (NOT reproducible in vanilla release 1.0.0 for wood and cobblestone slabs at depths of 1 and 2 - ladders are not placeable)
 * Because of this, slabs behave like an empty block (air) in redstone circuitry. A wire placed besides a slab which has a redstone torch under it, will run 'into' the block and not take power from the torch. (In comparison, a wire placed besides a normal block which will look like a cross section and will take power from the torch.)
 * Since the Beta light update, slabs now let a small amount of light pass through their edges. This light is only visible with Smooth Lighting turned on, and does not affect mob spawning or other light-dependent processes.
 * The exception of this is that any light directed through a slab does not affect any block's light values north of the source.
 * When a slab is placed on top of ice, the slab has the same "slippery" contents as the ice below it. (confirmed for cobblestone and wood slabs in vanilla release 1.0.0)
 * If slabs are placed below TNT, it will significantly reduce their damage. By making a floor of slabs, the TNT will lower its explosive power to two blocks instead of six.
 * Stone slabs are the only slab which, when stacked, do not mimic their block in texture.
 * You can sneak walk off slabs at ground level.
 * By using an Inventory Editor, you can obtain the old stone slab. Its damage value is 6. Previously, trying to place it would create a wood slab and if you put your mouse over it in the inventory, it would freeze and then crash the game. The 'seamless' stone slab can also be pseudo-legitimately obtained with the redstone 'block transmuter' bug; the Minecraft fourms thread can be found  here.
 * Minecarts on power rails will not be repelled from a slab. But, they will be repelled by a slab with a minecart on top.
 * When using an inventory editor to hack in any double slab, placing it will yeild a double stone slab.