This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
There are two classes of roofs - flat and pitched. Flat roofs (so-called) have only the slightest inclination, merely sufficient to allow rain-water to flow to the outlets. They are usually of wood covered with lead, copper, or zinc, or of concrete and other materials covered with asphalt. Pitched roofs have usually a framework of wood or iron or other materials covered with felt, galvanized iron, lead, zinc, copper, glass, slates, or tiles. Pitched roofs are almost invariably adopted for houses in these islands, as they throw off the rain and snow more quickly, and are considered more beautiful. Small "flats", however, frequently occur over bay-windows, porches, and out-buildings. In Fig. 73 the smallest inclinations which may be given to different roofing materials are shown.