Minecraft wiki entry for Roof_construction_guidelines, associated text: For example, a building 20m wide with a single 45° gable roof would have a roof with a height of 10–11 blocks. Assuming 4–5 block stories, the roof would be 2–2½ stories high. At that size, it would be advisable either to put an extra floor inside the roof volume, or else split the roof into two or three smaller sections so that the rise per section is around 5–7 blocks. Alternatively, by adding a 1 block high parapet and a 1 block wide walkway around the roof, it is reduced to an apparent height of 7 blocks, which falls roughly within the rule of thumb for roof heights.