A purlin (or historically purline, purloyne, purling, perling) is a longitudinal, horizontal, structural member in a

roof A roof ( : roofs or rooves) is the top covering of a building, including all materials and constructions necessary to support it on the walls of the building or on uprights, providing protection against rain, snow, sunlight, extremes of t ...

. In traditional

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 woode ...

there are three basic types of purlin: purlin plate, principal purlin, and common purlin. Purlins also appear in

steel frame Steel frame is a building technique with a "skeleton frame" of vertical steel columns and horizontal I-beams, constructed in a rectangular grid to support the floors, roof and walls of a building which are all attached to the frame. The developm ...

construction. Steel purlins may be painted or greased for protection from the environment.

Etymology

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.

In wood construction

Purlin plate

A purlin plate in wood 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 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 ...

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.

Principle purlin

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,) 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 “staggered.”

Common purlin

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 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.

In steel construction

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.

Gallery

Note: The sketches in this section reference terminology commonly used in the UK and Australia.Glossary of Australian Building Terms - Third Edition.(NCRB) File:Rf-trad-section.gif, A section through lightweight timber-frame construction showing the position of under purlins Image:Metal-purlins.jpg, Roll-formed metal roof purlins, also called roof battens. They are cropped to the angle of the purlin top cuts and can be lapped for joining. Image:Fixed-metal-purlins.jpg, Metal purlins or roof battens screwed to roof gang-nail-type trusses Image:Hw-purlins.jpg, Hardwood purlins fixed to steel supports on a skillion roof and main roof. House under construction, tropical North Australia. File:Cee-zed-purlins.svg, C and Z purlins in all-steel construction File:Zed-purlin.png, The ability of Z purlins to rotate 180 degrees and fit together File:Cee-portal.jpg, Portal rafters made from C section material supporting Z roof purlins File:Trad-queen-post.png, A traditionally framed timber queen post roof showing the placement of ''principal purlins'' or ''purlin plates'' supporting common rafters File:Inside_wboylston_old_stone_church.jpg, Timber roof truss with purlins

References

External links

An Illustrated glossary of roofs and roofing terms.
Roofs Building engineering Structural system Timber framing{{angle bracket}