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In architecture, structural engineering or building, a purlin (or historically purline, purloyne, purling, perling) is any longitudinal, horizontal, structural member in a roof except a type of framing with what is called a crown plate. In traditional Timber framing there are three basic types of purlin: purlin plate, principal purlin and common purlin.
A purlin plate in wooden construction is also called an "arcade plate" in European English, "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 purlin", "side purlin". Principal 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 purling 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, "clasped purlins" are captured by a collar beam. (Only one known historic example of this type in the U.S.). Through purlins are further identified as trenched, back, clasped and butt purlins as threaded, tenoned, and staggered.
Common purlins in wood construction, also called a "major-rafter minor-purlin system". 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 Englishmen 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 extant (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.
It is the practice in the steel industry that structural shapes are assigned 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.
Information on the origin of the term "Purlin" is scant at best. According to Websters it comes from the 20th century English. According to the Oxford Dictionary suggests a French origin with the earliest quote using a variation of purlin in 1447.
Note: The sketches in this section reference terminology commonly used in the UK and Australia.
hardwood purlins fixed to steel supports on a Skillion roof and main roof. House under construction, tropical North Australia.
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