Rafter
A rafter is one of a series of sloped structural members such as wooden beams that extend from the ridge or hip to the wall plate, downslope perimeter or eave, and that are designed to support the roof shingles, roof deck and its associated loads. A pair of rafters is called a "couple". In home construction, rafters are normally made of wood. Exposed rafters are a feature of some traditional roof styles. Applications. In recent buildings there is a preference for trussed rafters on the grounds of cost, economy of materials, off-site manufacture, and ease of construction, as  well as design considerations including span limitations and roof loads (weight from above). Types in traditional timber framing. There are many names for rafters depending on their location, shape, or size (see below). The earliest surviving roofs in Europe are of common rafters on a tie beam; this assembly is known as a "closed couple". Later, principal rafters and common rafters were mixed, which is called a major/minor or primary/secondary roof system. Historically many rafters, including hip rafters, often tapered in height 1/5 to 1/6 of their width, with the larger end at the foot. Architect George Woodward  discusses the purpose of this in 1860: "The same amount of strength can be had with a less amount of lumber. There is an additional labor in sawing such rafters, as well as a different calculation to be made in using up a log to the best advantage. It is necessary always to order this special bill of rafters direct from the mill, and the result will be that the extra cost will, nine times out of ten, overbalance the amount saved." John Muller also discussed a one-sixth taper for rafters. A piece added at the foot to create  an overhang or change the roof pitch is called a sprocket, or "coyau" in French. The projecting piece on the gable of a building forming an overhang is called a lookout. A rafter can be reinforced with a strut, principal purlin, collar beam, or, rarely, an auxiliary rafter (see below). Rafter types include: