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.
Purlins also appear in steel frame construction. Steel purlins may be painted or greased for protection from the environment.
Information on the origin of the term "purlin" is scant. The Oxford Dictionary suggests a French origin, with the earliest quote using a variation of purlin in 1447, though the accuracy of this claim has been disputed.
A purlin plate in wood construction is also called an "arcade plate" in European English, [1] "under purlin", and "principal purlin". The term plate means a major, horizontal, supporting timber. Purlin plates are beams which support the mid-span of rafters and are supported by posts. By supporting the rafters they allow longer spans than the rafters alone could span, thus allowing a wider building. Purlin plates are very commonly found in large old barns in North America. A crown plate has similarities to a purlin plate but supports collar beams in the middle of a timber-framed building.
Principal purlins in wood construction, also called "major purlins" and "side purlins," are supported by principal rafters and support common rafters in what is known as a "double roof" (a roof framed with a layer of principal rafters and a layer of common rafters). Principal purlins are further classified by how they connect to the principal rafters: "through purlins" pass over the top; "butt purlins" tenon into the sides of the principal rafters; and "clasped purlins," of which only one historic U.S. example is known,[ citation needed ]) are captured by a collar beam. Through purlins are further categorized as “trenched,” “back,” or “clasped;” butt purlins are classified as “threaded,” “tenoned,” and/or[ clarification needed ] “staggered.” [2]
Common purlins in wood construction, also called a "major-rafter minor-purlin system". [3] Common purlins are typically "trenched through" the top sides (backs) of principal rafters and carry vertical roof sheathing (the key to identifying this type of roof system). Common purlin roofs in North America are found in areas settled by the English and may have been a new invention in the Massachusetts Bay Colony. No examples of framed buildings with common purlin roofs have been reported in England, however some stone barns in England have vertically boarded, common purlin roofs. Historically, these roofs are found in New England, the highest concentration in Maine, and isolated parts of New York and along the St. Lawrence River in Canada. One of the oldest surviving examples is in the Coffin House in Newbury, Massachusetts, from 1678. The purpose of a common purlin roof may be they allow a board roof, that is a roof of nothing but vertically laid boards with seams covered with battens or another layer of boards. [4]
In steel construction, the term purlin typically refers to roof framing members that span parallel to the building eave, and support the roof decking or sheeting. The purlins are in turn supported by rafters or walls. Purlins are most commonly used in Metal Building Systems, where Z-shapes are utilized in a manner that allows flexural continuity between spans.
Steel industry practice assigns structural shapes representative designations for convenient shorthand description on drawings and documentation: Channel sections, with or without flange stiffeners, are usually referenced as C shapes; Channel sections without flange stiffeners are also referenced as U shapes; Point symmetric sections that are shaped similar to the letter Z are referenced as Z shapes. Section designations can be regional and even specific to a manufacturer. In steel building construction, secondary members such as purlins (roof) and girts (wall) are frequently cold-formed steel C, Z or U sections, (or mill rolled) C sections.
Cold formed members can be efficient on a weight basis relative to mill rolled sections for secondary member applications. Additionally, Z sections can be nested for transportation bundling and, on the building, lapped at the supports to develop a structurally efficient continuous beam across multiple supports.
Note: The sketches in this section reference terminology commonly used in the UK and Australia. [5]
A truss is an assembly of members such as beams, connected by nodes, that creates a rigid structure.
Timber framing 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 pegs. If the structural frame of load-bearing timber is left exposed on the exterior of the building it may be referred to as half-timbered, and in many cases the infill between timbers will be used for decorative effect. The country most known for this kind of architecture is Germany, where timber-framed houses are spread all over the country.
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 subfloor sheathing, allowing it to function as a horizontal diaphragm. Joists are often doubled or tripled, placed side by side, where conditions warrant, such as where wall partitions require support.
A hammerbeam roof is a decorative, open timber roof truss typical of English Gothic architecture and has been called "...the most spectacular endeavour of the English Medieval carpenter". They are traditionally timber framed, using short beams projecting from the wall on which the rafters land, essentially a tie beam which has the middle cut out. These short beams are called hammer-beams and give this truss its name. A hammerbeam roof can have a single, double or false hammerbeam truss.
A plate girder bridge is a bridge supported by two or more plate girders.
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, downslope perimeter or eave, and that are designed to support the roof shingles, 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. Exposed rafters are a feature of some traditional roof styles.
Framing, in construction, is the fitting together of pieces to give a structure support and shape. Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.
A plate or wall plate is a horizontal, structural, load-bearing member in wooden building framing.
Portal frame is a construction technique where vertical supports are connected to horizontal beams or trusses via fixed joints with designed-in moment-resisting capacity. The result is wide spans and open floors.
Domestic roof construction is the framing and roof covering which is found on most detached houses in cold and temperate climates. Such roofs are built with mostly timber, take a number of different shapes, and are covered with a variety of materials.
A king post is a central vertical post used in architectural or bridge designs, working in tension to support a beam below from a truss apex above.
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 is rarely correct. A tie in building construction is an element in tension rather than compression and most collar beams are designed to work in compression to keep the rafters from sagging. A collar near the bottom of the rafters may replace a tie beam and be designed to keep the rafters from spreading, thus are in tension: these are correctly called a collar tie.
Pole framing or post-frame construction is a simplified building technique that is an alternative to the labor-intensive traditional timber framing technique. It uses large poles or posts buried in the ground or on a foundation to provide the vertical structural support, along with girts to provide horizontal support. The method was developed and matured during the 1930s as agricultural practices changed, including the shift toward engine-powered farm equipment and the demand for cheaper, larger barns and storage areas.
A bent in American English is a transverse rigid frame. Historically, bents were a common way of making a timber frame; they are still often used for such, and are also seen in small steel-frame buildings, where the term portal frame is more commonly used. The term is also used for the cross-ways support structures in a trestle. In British English this assembly is called a "cross frame". The term bent is probably an archaic past tense of the verb to bind, referring to the way the timbers of a bent are joined together. The Dutch word is bint, the West Frisian is bynt, and the German is bind. Compare this with the term bend for a class of knots.
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 truss is known as a bay.
The Parker Training Academy Dutch Barn is located at that institution on Turkey Hill Road in the town of Red Hook, New York, United States. It is a wooden structure built in two stages between 1790 and 1810. In 2007 it was listed on the National Register of Historic Places.
A post is a main vertical or leaning support in a structure similar to a column or pillar, the term post generally refers to a timber but may be metal or stone. A stud in wooden or metal building construction is similar but lighter duty than a post and a strut may be similar to a stud or act as a brace. In the U.K. a strut may be very similar to a post but not carry a beam. In wood construction posts normally land on a sill, but in rare types of buildings the post may continue through to the foundation called an interrupted sill or into the ground called earthfast, post in ground, or posthole construction. A post is also a fundamental element in a fence. The terms "jack" and "cripple" are used with shortened studs and rafters but not posts, except in the specialized vocabulary of shoring.
American historic carpentry is the historic methods with which wooden buildings were built in what is now the United States since European settlement. A number of methods were used to form the wooden walls and the types of structural carpentry are often defined by the wall, floor, and roof construction such as log, timber framed, balloon framed, or stacked plank. Some types of historic houses are called plank houses but plank house has several meanings which are discussed below. Roofs were almost always framed with wood, sometimes with timber roof trusses. Stone and brick buildings also have some wood framing for floors, interior walls and roofs.
This glossary of structural engineering terms pertains specifically to structural engineering and its sub-disciplines. Please see glossary of engineering for a broad overview of the major concepts of engineering.
A Gothic-arched roof barn or Gothic-arch barn or Gothic barn or rainbow arch is a barn whose profile is in the ogival shape of a Gothic arch. These became economically feasible when arch members could be formed by a lamination process. The distinctive roofline features a center peak as in a gable roof, but with symmetrical curved rafters instead of straight ones. The roof could extend to the ground making the roof and walls a complete arch, or be built as an arched roof on top of traditionally framed walls.
This article incorporates text from a publication now in the public domain : Chisholm, Hugh, ed. (1911). "Purlin". Encyclopædia Britannica . Vol. 22 (11th ed.). Cambridge University Press. p. 665.