Common Cold-Formed Steel Purlin Size Reference

Z-Purlin vs C-Purlin Quick Chart

MBMA specifically treats both Z- and C-sections and notes important differences in their lateral and torsional behavior. Section dimension ranges above reflect commonly seen manufacturer product families, not a national standard.

In metal buildings, purlins commonly support metal roof panels, transfer gravity loads to primary framing, resist wind uplift, and participate in roof diaphragm and bracing behavior. MBMA’s current guide describes cold-formed steel purlins as integral parts of roof-panel, diaphragm, and anchorage systems, not isolated beams.

Cold-Formed Steel Purlins

The main subject of this chart. Used extensively in metal buildings, warehouses, industrial buildings, agricultural structures, and commercial metal roofs.

Wood Purlins

In conventional wood roof framing, “purlin” can mean a member used to reduce rafter span. The IRC’s wood-rafter purlin provision requires the purlin to be not smaller than the rafters it supports, with prescribed bracing requirements. For wood roof framing reference, see the Roof Rafter Size Chart and Roof Rafter Span Chart.

Z-Purlins

Advantages in many metal-building systems include easy overlapping or lapping at supports, continuous multi-span systems, and efficient material use.

C-Purlins

Common for simple-span conditions, end conditions, and selected structural layouts.

MBMA explains that Z- and C-sections respond differently to lateral and torsional effects because Z-sections have oblique principal axes while C-sections have their principal axes aligned with the geometric axes and have an offset shear center.

Manufacturer designations may communicate nominal depth, flange width, steel thickness, product series, and section type.

Shallow Z-Purlins

Medium Z-Purlins

Deep Z-Purlins

The 8ZS2.25×059 and 8ZS2.25×070 designations shown above are from an actual published AISI/CFSEI structural design example for a four-span metal building Z-purlin line supporting a standing-seam roof. Other depths shown follow the same manufacturer designation logic but should be verified against the specific product line before use; do not mix dimensions from different manufacturers into one fictional standardized profile.

These are classification ranges only; exact dimensions and thickness options for a specific C-purlin product must be taken from the manufacturer’s current catalog or AISI-based load table.

Variables include flange width, lip length, bend geometry, and steel thickness. These influence local buckling, distortional buckling, torsional stiffness, and roof-panel attachment. This is an important reason not to compare purlins by depth alone.

Base steel thickness first; gauge is only a manufacturer or market reference and should not be treated as the primary engineering field.

Gauge label, design or base metal thickness, and coated thickness are three separate concepts. Industry standards specify that minimum base steel thickness represents 95% of the design thickness, and that this minimum acceptable thickness is the value delivered to the jobsite.

Weight is useful for comparing alternatives and estimating material and shipping costs, but do not use weight as a proxy for structural capacity. For weight calculations on a specific section, see the steel weight calculator or metal weight calculator.

AISI cold-formed design examples explicitly use properties such as Ix, Iy, Ixy, Cw, and J when analyzing Z-purlins, since Z-sections are not doubly symmetric and their principal axes are rotated relative to the geometric x and y axes. This page does not turn these properties into a full cold-formed steel mathematics article.

ASCE 7-22 covers dead load, roof live load, snow, rain, wind, tornado where applicable, seismic effects, and load combinations.

Gravity Load

Downward load from dead, live, snow, and rain sources.

Wind Uplift

Upward roof pressure that reverses the bending direction.

Snow, Unbalanced Snow, and Drift

ASCE 7-22 specifically includes sloped-roof snow, partial loading, unbalanced snow, drifts, sliding snow, and rain-on-snow. This makes a simple “psf to purlin size” chart inherently project-specific.

These specific spacing values are from one manufacturer’s system documentation, labeled here as a real example, not a universal purlin-spacing rule. Some roof panel approvals separately restrict maximum spacing to about 5 ft regardless of the purlin’s own structural capacity. Roof-panel allowable span can control purlin spacing independently of purlin structural capacity.

Typical manufacturer load-table columns include section designation, span, number of spans, gravity allowable load, uplift allowable load, deflection-limited load, and bracing condition.

This workflow is much safer and more technically useful than an oversimplified size chart.

MBMA’s 2024 guide devotes an entire chapter to continuous purlin design, confirming that continuity and lapping are a core part of roof-purlin design.

Z-purlins are commonly overlapped at interior frames, bolted through webs or flanges depending on the system, and designed as continuous lines.

Lap Length

Project and manufacturer specific.

Double Section at Lap

Changes stiffness and strength locally where the two purlins overlap.

End Span vs Interior Span

Can require different sections, as shown in the AISI four-span example above, where exterior spans used a heavier 070 mil section and interior spans used a lighter 059 mil section within the same purlin line.

Purlins often rely on roof-panel restraint, discrete bridging, anti-roll devices, flange braces, and anchorage. The 2024 IBC specifically identifies purlin and girt restraint, bridging, and bracing as items subject to special inspection in metal building systems.

MBMA explains that roof panels can provide lateral support to purlins, but effectiveness depends on panel profile, the panel-to-purlin connection, fixed versus sliding clips, insulation, and diaphragm stiffness. Standing-seam systems can provide less reliable lateral restraint than direct through-fastened systems because the clip permits movement.

MBMA specifically discusses fixed and sliding standing-seam clips and how those details affect purlin lateral support.

Purlins must often be checked for vertical deflection, roof-panel serviceability, ponding, appearance, roof drainage, and brittle finishes if present.

Under gravity load, the top flange is often on the compression side. Under uplift, the force direction reverses, a different flange may be in compression, and restraint conditions can change dramatically.

On a sloped roof, load components parallel and perpendicular to the roof surface act relative to the purlin and web geometry differently than on a flat roof. MBMA’s current guide includes purlin behavior relative to roof slope and torsional or lateral restraint. Do not use one flat-roof load table for all slopes unless the table specifically permits it.

Relevant factors include yield strength, galvanized or coated steel, base metal thickness, and corrosion exposure. Do not assume every Z-purlin is the same grade just because its geometry matches another manufacturer’s section.

Metal Warehouses

Common cold-formed purlin application.

Agricultural Buildings

Frequently use lightweight metal-building roof systems.

Industrial Buildings

Often use heavier or deeper purlin sections for larger spans.

Commercial Metal Roofs

Common in retail and office metal-building construction.

Canopies

Often use shallower or lighter purlin sections depending on span.

Solar-Support Roofs

Roof loading can change when solar equipment is added, requiring separate structural evaluation.

Some metal building manufacturers explicitly treat purlins and girts as cold-formed C or Z secondary members and eave struts as C-type members within the same secondary framing system.

2024 IBC

Section 2204 requires cold-formed carbon and low-alloy structural members not covered elsewhere in the chapter to be designed in accordance with AISI S100.

AISI S100

The core cold-formed structural design framework. The edition currently referenced across the 2024 model codes is AISI S100-16 (2020) with Supplement 2 (S2-20), which includes the modern cold-formed steel design provisions and examples used for purlin analysis.

The 2024 MBMA guide, now in its 2nd edition, covers an introduction to roof framing, purlin design methods, continuous purlin design, diaphragm requirements, system anchorage requirements, and miscellaneous roof-framing topics. This should be the main system-behavior reference alongside AISI for cold-formed steel purlin roof framing.