![Coyaux](https://upload.wikimedia.org/wikipedia/commons/6/64/Coyaux.png)
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
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 styl ...
, and that are designed to support the roof
shingles
Shingles, also known as zoster or herpes zoster, 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. ...
, roof deck and its associated loads. A pair of rafters is called a ''couple''. In home construction, rafters are normally made of
wood
Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin th ...
. 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 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 that the assembl ...
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
A tie, strap, tie rod, eyebar, guy-wire, suspension cables, or wire ropes, are examples of linear structural components designed to resist tension. It is the opposite of a strut or column, which is designed to resist compression. Ties may be ...
; 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 lookout or look-out is a person in charge of the observation of hazards. The term originally comes from a naval background, where lookouts would watch for other ships, land, and various dangers. The term has now passed into wider parlance.
...
.
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.
Human anatomy
Part of the functionality o ...
, 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.
Pu ...
,
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 thi ...
, 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
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.
Pu ...
which supports the rafters in each bay. Sometimes the top cord (uppermost member) of a
truss
A truss is an assembly of ''members'' such as 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 that the assembl ...
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 usual architectural term in eighteenth-century England and North America was "Dutch roof".) The upper slope is positioned at a shallow angle, w ...
,
Mansard roof
A mansard or mansard roof (also called a French roof or curb roof) is a four-sided gambrel-style hip roof characterised by two slopes on each of its sides, with the lower slope, punctured by dormer windows, at a steeper angle than the upper. The ...
) 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 downwards to the walls, usually with a fairly gentle slope (although a tented roof by definition is a hipped roof with steeply pitched slopes rising to a peak). Thus, ...
. The foot of a hip rafter lands on a
dragon beam
Dragon beam is a horizontal, diagonal beam in the corner(s) of some traditional timber framed buildings. The term is commonly used in both hip roof framing and jettying. Older publications may use the synonyms dragging beam, dragging piece, dragg ...
.
* 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 pitched roof. A dormer window (also called ''dormer'') is a form of roof window.
Dormers are commonly used to increase the usable space ...
.
* 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
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.
Pu ...
. 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. The World Flora Online created by the Royal Botanic Gardens, Kew and Missouri Botanical Garden accep ...
or cedar. For longer span rafters, building materials manufacturers have created LVL (
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 ...
) rafters that can be 2–5 times longer than typical wood rafter.
*In US, most wood rafters have maximum length of 20 feet.
If a longer rafter is needed, LVL is the ideal combination alternative.
See also
*
Birdsmouth joint
*
Chantlate
In architecture, a chantlate is a piece of wood fastened near the ends of the 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 ...
*
Fascia (architecture)
Fascia () is an architectural term for a vertical frieze or band under a roof edge, or which forms the outer surface of a cornice, visible to an observer.
Typically consisting of a wooden board, unplasticized PVC (uPVC), or non-corrosive sheet ...
*
Joist
A joist is a horizontal structural member used in framing to span an open space, often between beams that subsequently transfer loads to vertical members. When incorporated into a floor framing system, joists serve to provide stiffness to the s ...
*
knee wall
A knee wall is a short wall, typically under three feet (one metre) in height, used to support the rafters in timber roof construction. In his book ''A Visual Dictionary of Architecture'', Francis D. K. Ching defines a knee wall as "a short wall ...
*
Lookout (architecture)
A lookout, lookout rafter or roof outlooker is a wooden joist that extends in cantilever out from the exterior wall (or wall plate) of a building, supporting the roof sheathing and providing a nailing surface for the fascia boards. When not expo ...
*
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.
Pu ...
*
Rafter angle square
*
Soffit
A soffit is an exterior or interior architectural feature, generally the horizontal, aloft underside of any construction element. Its archetypal form, sometimes incorporating or implying the projection of beams, is the underside of eaves (to ...
*
Truss
A truss is an assembly of ''members'' such as 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 that the assembl ...
*
Timber framing
Timber framing (german: Holzfachwerk) and "post-and-beam" construction are traditional methods of building with heavy timbers, creating structures using squared-off and carefully fitted and joined timbers with joints secured by large wooden ...
*
Timber roof truss
A timber roof truss is a structural framework of timbers designed to bridge the space above a room and to provide support for a roof. Trusses usually occur at regular intervals, linked by longitudinal timbers such as purlins. The space between each ...
*
Wind brace
In architecture, wind braces are diagonal braces to tie the rafters of a roof together and prevent racking. In medieval roofs they are arched, and run from the principal rafters to catch the purlin
A purlin (or historically purline, purloyne, pur ...
References
{{Wood products
Roofs
Building engineering
Structural system
Carpentry