Are there any blocks that spiders cannot climb?

I'm building a spider spawner exp farm, and I have everything planned out.

Now the only thing is to build it. But what material should I use?

Are there any blocks that a spider (either type) just cannot hold on to the sides of?

My farm should function if there are none, but it would be a huge boost to convenience and efficiency if there are any such blocks.

Keep in mind that this question is not the same as this question which just asks how to make your house safe from spiders.

6 Answers

Cover the walls with signs. Spiders can't climb a wall if it's covered in signs. I just tested it.

Spiders can climb:

Any solid block, that is, one that obstructs the players movement. This includes 'solid' blocks such as stone, wood, dirt etc. as well as 'transparent' blocks such as fence, glass panes and iron bars.
Ladders and vines, exactly as the player can.

Spiders cannot climb:

Blocks which do not impede the player, such as grass, sugar cane, fire or flowers.
Water or lava, but will behave as other mobs (swim/drown, burn).
Nether portal blocks (they can climb the surrounding obsidian though).

They can climb any solid block. If you want to stop them from escaping, make an overhang inside the trap like so:

XX
X
X
X

X=solid block.

Spiders cannot climb Ice, Packed Ice, or Blue Ice.

You could build something like this B=Brick

BBBBBBBBBBBBBBB
B. B
B. B
BB. BB
B. B
BBBBBBBBBBBBBBB

Spiders can not climb over overhangs so just put a brick on the 2nd block to keep em down. Hope this helps.

While spiders can climb cactus blocks, they will eventually be killed while doing so.