Minecraft wiki entry for Adding_beauty_to_constructions, associated text: If your ceiling is 2 blocks high, people can't jump. Adjust it so that it measures 3 to 4 blocks high, so your home will have a more spacious feel. Remember that endermen are three blocks tall, so keep your house well lit to prevent them from spawning; note that this will not prevent endermen that have spawned elsewhere from teleporting into your home. Or you could just make a 2.5 tall room by having a slab ceiling. A top slab floor will prevent any mobs from spawning.