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.
Roof space ventilation is needed to combat condensation within the roof space, leading to interstitial condensation within the roof fabric; this can lead to serious structural damage, wet or dry rot, as well as ruining the insulation in the roof spec.
The material of a roof may range from banana leaves, wheaten straw or seagrass to laminated glass, copper (see: copper roofing), aluminium sheeting and pre-cast concrete. In many parts of the world ceramic tiles have been the predominant roofing material for centuries, if not millennia. Other roofing materials include asphalt, coal tar pitch, EPDM rubber, Hypalon, polyurethane foam, PVC, slate, Teflon fabric, TPO, and wood shakes and shingles.
In the UK, a concrete tiled roof would normally have rafters at 600mm centers, roof battens at 300mm centers and ceiling joists at 400mm centers. The United States still uses imperial units of measurement and framing members are typically spaced sixteen or twenty-four inches apart.