Minecraft wiki entry for Roof_decorations, associated text: Whenever a roof's volume is large enough, it will tend to be used for additional rooms, perhaps even additional floors, but that in turn requires dormers, skylights, and other adjustments rather than a large expanse of blank sloping roof area. Extending the parapet vertically is often useful on larger constructions, as it allows the roof size to be reduced. This helps bring it back into scale with the rest of the building.