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 some timber frame construction.

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.

In traditional timber truss construction purlins rest on the principal rafters of the truss.

In all metal or mixed building roof systems, purlin members are frequently constructed from cold-formed steel, (or roll formed) Cee or Zed sections. The Zed sections can be lapped and nested at the supports which creates a continuous beam configuration between the bays.

When Cee and Zed sections are used in wall construction is normal to call them girts.

Gallery Of Sketches

Note! The sketches in this section reference terminology commonly used in the UK and Australia.^[1]

A section though lightweight timber frame construction showing the position of under purlins.	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.	metal purlins or roof battens screwed to roof gang nail type trusses.	hardwood purlins fixed to steel supports on a Skillion roof and main roof. House under construction, tropical North Australia.
Cee and Zed purlins in all steel construction.	The ability of Zed purlins to rotate 180 degrees and fit together.	Portal rafters made from Cee section material in turn supporting Zed roof purlins.	A traditionally framed timber queen post roof showing the placement of timber purlins.

References

1. ^ Glossary of Australian Building Terms - Third Edition.(NCRB)

See also

• Joist
• Roof construction
• Timber roof trusses

References

• This article incorporates text from the Encyclopædia Britannica, Eleventh Edition, a publication now in the public domain.

External links

• An Illustrated glossary of roofs and roofing terms.