What is a purlin in construction?

Purlin roof framing: where a roof purlin sits in the roof structure

In conventional roof framing, purlins can cross over principal rafters or roof trusses instead of spanning between rigid metal frames. The roof panel, sheathing, or deck bears on the purlins; the purlins deliver those reactions into the rafters or trusses; and those primary members transfer the load to bearing walls or columns. A purlin plate is a different conventional arrangement: it supports rafters near midspan, so the roof section and bearing details must be read before assigning the member's supports.

In a pre-engineered metal building, the load path is roof panel or deck to cold formed purlin to rigid-frame rafter. The purlins typically run parallel to the eave and span from frame to frame, while the roof panels run across the purlin rows. That direction change lets an estimator distinguish the secondary roof members from the primary rafters and trace which span controls each takeoff length.

The conventional arrangement is illustrated by Wikipedia's purlin overview, including purlins supported by rafters and trusses. For the steel case, Norsteel's steel purlin guide describes how roof loads pass through purlins to the primary frame. In commercial roof assemblies, the supported surface may be metal deck.

Purlin types and sizes: C purlin vs Z purlin

Cold formed C purlins have a channel-shaped web with both flanges pointing in the same direction; Z purlins have opposing flanges. That geometry changes the support condition. Adjacent Z sections can be rotated, nested, and bolted together over a frame to form a lapped continuous run, while C sections are normally butted and used as simple spans between supports. For takeoff, the Z system needs lap footage at frame lines, while the C system follows the scheduled support-to-support member lengths and connections.

Metal purlins are offered in 12, 14, and 16 gauge, and in two finishes: red oxide, often called red iron, and galvanized. Both variables move the price before any dimension does, so a takeoff that carries only linear feet is not yet a buyable quantity. Depth, flange, gauge, finish, and lap condition all come off the secondary-framing schedule for the job rather than from a catalog default, which is why the schedule is the document to price from.

For the C and Z profiles, gauge range, and common depths, see Western States Metal Roofing's metal purlin guide. Wheeler Metals' Cee purlin catalog shows how depth and flange dimensions appear in stocked C sections. The corresponding wall member is a girt; Butler Manufacturing Parts' purlin and girt guide explains the roof-versus-wall distinction.

Z purlins in metal building construction

In a pre-engineered metal building, the purlin question is really a Z purlin question: the manufacturer designs and supplies cold formed Z sections as part of the building package, called out like 8 x 2.5 Z in a specified gauge, and they show up on the manufacturer's roof framing plan and erection drawings rather than on the engineer of record's typical S sheets. The same Z and C stock frames the walls as girts, so the roof framing plan, not the section shape, is what tells you which member you are counting.

Two Z purlin behaviors drive the takeoff. Continuous lapped Z systems overlap a few feet at each frame line, so the supplied length runs past the plan length and the lf quantity carries a lap allowance; simple span C systems do not lap. And the nominal 5 ft spacing on the typical section is not the whole story: end bays and wind uplift edge zones often tighten it, so the row count comes off the framing plan. The lf base then converts to weight through the section's lb per lf, because metal building suppliers price secondary members by the pound.