Minecraft wiki entry for Roof_construction_guidelines, associated text: You will quickly note that when roofs join, they usually do so along a V-shaped line, and roof blocks inside that V will be removed at the final construction stage. Once you understand this behavior, it becomes easier and quicker not to place the unneeded blocks to begin with. But whenever you are dealing with a complex roof structure, it is simpler to build the entire roof and then remove unneeded blocks afterwards.