Minecraft wiki entry for Fire, associated text: Fire spreads over flammable surfaces and can climb up walls, across floors and ceilings, and over small gaps. More precisely, a fire block can turn any air block that is adjacent to a flammable block into a fire block. This can happen at a distance of up to one block downward, one block sideways (including diagonals), and four blocks upward of the original fire block (not the block the fire is on/next to). Therefore, if the player is using fire to build a fireplace, caution is needed. Blocks in the way do not prevent fire from igniting blocks above it—so even if the player protects a wooden roof with cobblestone between it and the fire, the fire ignores that cobblestone.