Construction glossary · Steel and structural
What is purlin in construction?
A purlin is a secondary roof framing member that spans between the primary structural frames and supports the roof deck or roof panels. In pre-engineered metal buildings, purlins are typically cold formed Z sections, such as an 8 inch Z purlin, spaced about 5 ft on center. Estimators take them off in linear feet from the roof framing plan and price them per lf or by weight.
Updated June 2026 · Reviewed by the Ruh construction team
How roof purlins are spaced
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Book a walkthroughPurlins are the secondary members that carry the roof skin between the primary frames. In a pre-engineered metal building (PEMB) they are almost always cold formed Z sections, called out like 8 x 2.5 Z in a specified gauge, and they appear on the metal building manufacturer's roof framing plan and erection drawings rather than on the engineer of record's typical S sheets. The manufacturer designs and supplies them as part of the building package, the steel erector sets them, and the GC's estimator quantifies them when the package is bought loose or when a reroof, addition, or canopy is bid without a manufacturer quote in hand. New estimators trip on three things: confusing purlins (roof) with girts (walls), which use the same Z and C stock; missing the lap lengths where continuous Z purlins overlap a few feet at each frame line; and assuming a uniform 5 ft spacing when the framing plan tightens spacing in end bays and edge zones for wind uplift. Always take the spacing off the framing plan, not off the typical section.
Purlins are taken off in linear feet (lf) from the roof framing plan, then often converted to weight because metal building suppliers price cold formed secondary members by the pound. The quick method: divide the building length by the purlin spacing to get the number of purlin rows, then multiply rows by the building width to get total lf. Verify the spacing in end bays before trusting a uniform number, since uplift zones often tighten it. Add a lap allowance for continuous Z purlin systems, commonly a few percent of total lf, and remember purlins ship cut to bay lengths, so rounding follows bay logic rather than stock lengths. The quantity lands in the estimate as an lf line with a material unit price, or rolled into secondary steel weight on a tonnage based PEMB line.
Worked example
Take a PEMB warehouse roof 150 ft long and 60 ft wide with 8 inch Z purlins at 5 ft on center. Step 1, purlin rows: 150 ft / 5 ft spacing = 30 rows. Step 2, base length: 30 rows x 60 ft = 1,800 lf. Step 3, lap allowance: continuous Z purlins lap at each frame line, so carry 5 percent, 1,800 lf x 1.05 = 1,890 lf. Step 4, convert to weight for a supplier check: an 8 inch Z purlin in a common gauge runs about 3.0 lb per lf (typical, verify against the manufacturer's section data), so 1,890 lf x 3.0 lb = 5,670 lb, and 5,670 lb / 2,000 = 2.835 tons, carried as 2.8 tons of secondary steel. Step 5, price it: at an illustrative $2.40 per lf for material, 1,890 lf x $2.40 = $4,536 before erection labor, clips, bridging, and sag rods.
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How Ruh handles purlin
Ruh reads the roof framing plan in the contractor's drawing set, counts the purlin rows, and runs the linear foot takeoff including lap allowances. It then prices those quantities against the contractor's own price book, so the unit costs are the contractor's real numbers rather than database averages. The estimator reviews the row counts, spacing calls, and pricing, then signs off before the line goes into the bid.
See construction estimating softwarePurlin: frequently asked questions
What is the difference between a purlin and a girt?+
Both are cold formed secondary members, usually the same Z or C stock, and the difference is orientation: purlins frame the roof, spanning between primary frames to carry the roof panels, while girts frame the walls to carry the wall panels. On a takeoff they are separate line items because spacing, lengths, and attachment hardware differ, and lumping them together makes the buyout and the erection labor pricing harder to check.
Are purlins priced per linear foot or by weight?+
Both conventions are live in the market. Metal building manufacturers typically price secondary members by weight as part of the building package, while loose purchases from a cold formed supplier and erection labor are usually quoted per linear foot. The takeoff should carry the lf quantity as the base number, since lf converts cleanly to weight with the section's lb per lf but weight does not convert back without knowing the gauge.
Do purlin laps add to the takeoff quantity?+
Yes. Continuous lapped Z purlin systems overlap at each frame line, commonly a couple of feet on each side of the frame, which means the supplied length exceeds the plan length. Carrying a lap allowance of a few percent on total lf, or taking off actual lap lengths from the framing plan on lap critical jobs, keeps the material quantity honest. Simple span C purlin systems do not lap, so check the system type first.
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Figures on this page are illustrative. Construction estimates depend on project-specific conditions, source documents, market pricing, and professional judgment. Ruh's AI assists the estimator and does not replace professional review: your team reviews, validates, and approves every estimate, bid, and pricing decision.