Minecraft wiki entry for Curved_roofs, associated text: This example produces a conical roof which shrinks from diameter 8 to 2 while rising 12 meters. It's not uncommon for a sharp spire to rise 3−5 blocks, sometimes more, before its radius reduces. You can also change the rate at which the cone reduces in size to give give it a slightly curved profile. For example, instead of a radius reduction every 5 meters, you might start at 4, then increase to 6, 8, 10 as you work up the spire.