Purlin
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In architecture or structural engineering or building, a purlin (or purline) is a horizontal structural member in a roof. Purlins support the loads from the roof deck or sheathing and are supported by the principal rafters and/or the building walls, steel beams etc. The use of purlins, as opposed to closely spaced rafters, is common in pre-engineered metal building systems and both the ancient post-and-beam and newer pole building timber frame construction methodologies.
In lightweight timber roof construction under purlins were used to support rafters over longer spans than the rafters alone could span. Under purlins were typically propped off internal walls. For example, an 8 x 4 under purlin would support the center of a row of 6 x 2 rafters that in turn would support 3 x 2 roof purlins to which the roof cladding was fixed.[clarification needed]
In traditional timber truss construction[clarification needed], purlins rest on the principal rafters of the truss.
It is the practice in the steel industry that structural shapes are assigned representative designations for convenient short-hand 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.
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[edit] Etymology
Information on the origin of the term "Purlin" is scant at best. According to Websters it comes from the 15th century English. According to the Oxford Concise Dictionary, it states "Middle English, perhaps of French origin".
[edit] Gallery
Note: The sketches in this section reference terminology commonly used in the UK and Australia.[1]
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A section though lightweight timber frame construction showing the position of under purlins.
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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.
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metal purlins or roof battens screwed to roof gang nail type trusses.
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hardwood purlins fixed to steel supports on a Skillion roof and main roof. House under construction, tropical North Australia.
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C and Z purlins in all steel construction.
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The ability of Z purlins to rotate 180 degrees and fit together.
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Portal rafters made from C section material in turn supporting Z roof purlins.
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A traditionally framed timber queen post roof showing the placement of timber purlins.
[edit] See also
[edit] References
- ^ Glossary of Australian Building Terms - Third Edition.(NCRB)
This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press.
