Minecraft wiki entry for Placement, associated text: Signs and banners can be placed anywhere on a door or a trapdoor and cannot be destroyed unless the door or trapdoor itself is destroyed. Cakes and sea pickles can be placed on the tops of doors and trapdoors and are not destroyed when the door or trapdoor opens and closes. Rails, redstone dust, redstone repeaters, and redstone comparators can be placed only on a closed trapdoor in the top half of a block, but are not destroyed if the trapdoor is opened until the block is updated. All other blocks are destroyed when the door or trapdoor changes states.