In Plain English
Quick Answer
A horizontal roof member that spans between trusses or rafters to directly support the roof covering.
Definition
Definition
A horizontal structural member in a roof frame that spans between trusses, rafters, or other primary roof framing to support roof decking or cladding. Purlins run parallel to the ridge of the roof and are supported at their ends by the primary structural members. They reduce the span of the roof covering and allow greater spacing of the primary framing.
Context
Why It Matters in Bidding
Purlins are a major line item in pre-engineered metal buildings, so their size, gauge, and spacing drive both the steel takeoff tonnage and the roofing labor estimate. Underestimating purlin spacing or specifying the wrong section can throw off material pricing and trigger costly engineering revisions, making accurate purlin quantification central to a competitive structural bid.
Example
Example
The steel estimator counted 84 cold-formed Z-purlins at 5-foot spacing across the warehouse roof, adding roughly 11 tons to the framing takeoff that drove the structural bid.
See Also
Related Terms
FAQ
Questions Contractors Ask
What is the difference between a purlin and a rafter?
Rafters are primary sloped members running from ridge to eave that carry the main roof load. Purlins are secondary members running horizontally, perpendicular to rafters or trusses, supporting the decking or panels between them. In metal buildings, purlins let designers space the heavier primary frames farther apart, reducing steel cost.
What materials are purlins made from?
In commercial and agricultural construction, cold-formed steel C and Z sections dominate because they are light, consistent, and easy to bolt. Wood purlins appear in timber and residential roofs, while heavy structural steel purlins serve long-span industrial roofs. The material choice directly affects takeoff units, weight, and pricing.
How does purlin spacing affect a construction estimate?
Tighter spacing increases the number of purlins, raising steel tonnage and connection labor, but it lets thinner, cheaper roof panels span the gaps. Wider spacing reduces purlin count yet demands heavier decking. Estimators must price the whole roof assembly together, since changing spacing shifts cost between framing and cladding.
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