A rafter is one of a series of sloped structural members such as steel beams that extend from the ridge or hip to the

wall plate A plate or wall plate is a horizontal, structural, load-bearing member in wooden building framing. Timber framing A plate in timber framing is "A piece of Timber upon which some considerable weight is framed...Hence Ground-Plate...Window-plat ...

, downslope perimeter or

eave The eaves are the edges of the roof which overhang the face of a wall and, normally, project beyond the side of a building. The eaves form an overhang to throw water clear of the walls and may be highly decorated as part of an architectural sty ...

, and that are designed to support the roof

shingles Shingles, also known as herpes zoster or zona, is a viral disease characterized by a painful skin rash with blisters in a localized area. Typically the rash occurs in a single, wide mark either on the left or right side of the body or face. T ...

, roof deck, roof covering and its associated loads. A pair of rafters is called a ''couple''. In home construction, rafters are normally made of

wood Wood is a structural tissue/material found as xylem in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulosic fibers that are strong in tension and embedded in a matrix of lignin t ...

. Exposed rafters are a feature of some traditional roof styles.

Applications

In recent buildings there is a preference for

truss A truss is an assembly of ''members'' such as Beam (structure), beams, connected by ''nodes'', that creates a rigid structure. In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so ...

ed 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 A strut is a structural component commonly found in engineering, aeronautics, architecture and anatomy. Struts generally work by resisting longitudinal compression, but they may also serve in tension. A stay is sometimes used as a synonym for ...

, principal

purlin A purlin (or historically purline, purloyne, purling, perling) is a longitudinal, horizontal, structural member in a roof. In traditional timber framing there are three basic types of purlin: purlin plate, principal purlin, and common purlin. P ...

collar beam A collar beam or collar is a horizontal member between two rafters and is very common in domestic roof construction. Often a collar is structural but they may be used simply to frame a ceiling. A collar beam is often called a collar tie but this ...

, or, rarely, an auxiliary rafter (see below). Rafter types include: * Principal rafter (major rafter, rarely a chief rafter): A larger rafter. Usually land directly on a tie beam. Usually the purpose of having a larger rafter is to carry a

which supports the rafters in each bay. Sometimes the top cord (uppermost member) of a

looks like a principal rafter. Principal rafters are sometimes simply called "principals". * Common rafter (minor rafter): being smaller than a principal rafter. A "principal/common rafter roof" or "double roof" has both principals and commons. (Also called major/minor, primary/secondary). * Auxiliary (cushon, compound, secondary, sub-) rafter: A secondary rafter below and supporting a principal rafter. A rare type of rafter. * Compass rafter: A rafter curved or bowed on the top (the top surface of a rafter is called its "back") or both the top and bottom surfaces. * Curb rafter: The upper rafters in a curb (kerb,

gambrel A gambrel or gambrel roof is a usually symmetrical two-sided roof with two slopes on each side. The upper slope is positioned at a shallow angle, while the lower slope is steep. This design provides the advantages of a sloped roof while maxim ...

Mansard roof A mansard or mansard roof (also called French roof or curb roof) is a multi-sided gambrel-style hip roof characterised by two slopes on each of its sides, with the lower slope at a steeper angle than the upper, and often punctured by dormer wi ...

) roof. * Hip rafter (angle rafter): The rafter in the corners of a

hip roof A hip roof, hip-roof or hipped roof, is a type of roof where all sides slope downward to the walls, usually with a fairly gentle slope, with variants including Tented roof, tented roofs and others. Thus, a hipped roof has no gables or other ve ...

. The foot of a hip rafter lands on a dragon beam. * King rafter: the longest rafter on the side of a hip roof in line with the ridge. * Valley rafter (historically also called a sleeper): A rafter forming a valley (look for illustration showing a valley). * Intermediate rafter: "one between principal or common rafters to strengthen a given place" (rare). * Jack rafter, cripple rafter, cripple-jack rafter: A shortened rafter such as landing on a hip rafter or interrupted by a

dormer A dormer is a roofed structure, often containing a window, that projects vertically beyond the plane of a Roof pitch, pitched roof. A dormer window (also called ''dormer'') is a form of roof window. Dormers are commonly used to increase the ...

. * Arched rafter: Of segmental form in an arched roof. *

Knee In humans and other primates, the knee joins the thigh with the leg and consists of two joints: one between the femur and tibia (tibiofemoral joint), and one between the femur and patella (patellofemoral joint). It is the largest joint in the hu ...

(crook, kneeling, cranked) rafter: A rafter with a bend typically a few feet from the foot used to gain attic space like adding a kneewall. Rare in America. * Barge rafter: The outermost rafter on a gable end, sometimes forming a roof overhang. * Butt rafter: A smaller rafter interrupted by and joined to a butt

. Common rafters pass over and are supported by a principal purlin, if present. * A "binding rafter" is not a rafter but an obsolete name for a purlin or support. * Part of a

cruck A cruck or crook frame is a curved timber, one of a pair, which support the roof of a building, historically used in England and Wales. This type of timber framing consists of long, generally naturally curved, timber members that lean inwards and ...

frame may function as a rafter but they are called a cruck blade. Rafters are usually made of

pine A pine is any conifer tree or shrub in the genus ''Pinus'' () of the family Pinaceae. ''Pinus'' is the sole genus in the subfamily Pinoideae. ''World Flora Online'' accepts 134 species-rank taxa (119 species and 15 nothospecies) of pines as cu ...

or cedar. For longer span rafters, building materials manufacturers have created

laminated veneer lumber Laminated veneer lumber (LVL) is an engineered wood product that uses multiple layers of thin wood assembled with adhesives. It is typically used for headers, beams, rimboard, and edge-forming material. LVL offers several advantages over typi ...

(LVL) rafters that can be 2–5 times longer than typical wood rafter. In the US, most wood rafters have maximum length of . If a longer rafter is needed, LVL is an alternative.

References

{{Wood products Roofs Building engineering Structural system Carpentry

Applications

Types in traditional timber framing

See also

References