The Rotten Log is a type of decoration block that were added in Public Beta 21.
Occurrence
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They can be found strewn across the landscapes of moist biomes such as Far Harad and the woodlands of many biomes. If you want to collect lots of rotten logs, forests that have been damaged by evil, such as Ithilien and Fangorn Wasteland are good places to look.
Naturally spawned rotten logs.
Use
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These logs can be broken to obtain a block of rotten log. This block of Rotten Log can then be turned into Rotten Wood Planks, which can be further crafted into stairs and slabs.
As of Beta 25, Ruined Houses may consist of these planks and logs.

Rotten Planks, Log, Stairs, Slab, and Fence
Vanilla:
Acacia •
Birch •
Dark Oak •
Jungle •
Oak •
Spruce |