Spread

Safe building area around a fire
Fire will spread over flammable surfaces. Fire 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 downwards, one block sideways (including diagonals), and four blocks upwards 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 will not prevent fire from igniting blocks above it—so even if the player protects their wooden roof with cobblestone between it and the fire, the fire will completely ignore that cobblestone.