Minecraft wiki entry for Roof_types, associated text: A monitor is a raised structure running along the ridge of a double-pitched roof, with its own roof running parallel to the main roof. The long sides of monitors often contain louvers or clerestory windows to help light or ventilate the building. A monitor that runs the full length of the building is more commonly found on larger structures such as barns, warehouses and factories, and is rarely seen on domestic buildings. However, a monitor does not necessarily have to run the entire length of a roof.