The roof framing may be interrupted for openings such as a chimney or skylight. Chimneys are typically built with a water diverter known as a cricket or saddle above the chimney. Flashing is used to seal the gap between the chimney and roofing material.
A more recent design is the installation of a roof deck with foil-backed foam along with a second deck that is air-gapped away from the foil-backed foam to allow air to flow vertically to a ventilation outlet at the peak of the roof—it is a double deck design with an air gap. This design improves efficiency.
When the warm, moist air from below rises into the cold roof space; condensation begins as the air temperature drops to the 'dew point' or as the warm air comes into contact with any of the cold surfaces in the roof.
With continual improvements in steel girders, these became the major structural support for large roofs, and eventually for ordinary houses as well. Another form of girder is the reinforced concrete beam, in which metal rods are encased in concrete, giving it greater strength under tension.