Common C Purlin Sizes (MBCI Cee-Only Product Line)

Values shown are drawn directly from MBCI’s published Cee-only product table. Metal Sales publishes a comparable but not identical range, with web heights of 4, 6, 8, 9, 10, and 12 in., flange widths of 2.5, 3, 3.5, and 4 in., and 12, 14, and 16 gauge product families.

C Purlin Thickness Quick Chart

Base steel thickness is the primary engineering value. Gauge is a secondary, market-facing label and should always be paired with the manufacturer’s actual published thickness.

A C purlin, or Cee purlin, is a cold-formed, thin-walled steel secondary framing member with one web, two flanges, usually edge lips or stiffeners, and an overall C-shaped cross-section. Manufacturer literature describes Cee purlins as secondary framing used in metal building systems that distribute loads from the roof or wall surfaces to the building’s primary framing, such as rigid frames, rafters, or trusses.

This page owns Cee geometry and C-specific use cases. For unequal-flange nesting, lap length, and continuous lapped-line behavior, see the dedicated Z Purlin Size Chart.

The 2024 IBC directs cold-formed carbon and low-alloy steel structural members to AISI S100, which is a fundamentally different design specification from the hot-rolled AISC provisions used for C and MC channels. For hot-rolled channel dimensions and section properties, see the Steel Channel Size Chart.

A designation such as 8 x 4 Cee typically means an approximately 8-in. web and a 4-in. flange, with thickness specified separately as a gauge or decimal value. There is no single national naming rule that guarantees every “8×4 Cee” from every manufacturer has identical lip, bend radius, thickness, steel strength, or section properties. MBCI’s current tables show multiple nominal Cee families, such as 6×3, 6×3 1/2, 6×4, 8×4, 10×4, 11×3, and 12×4, with different weights by gauge for sections that share a similar nominal depth label.

Metal Sales currently lists Cee web heights of 4, 6, 8, 9, 10, and 12 in. MBCI’s Cee-only table publishes a related but not identical set, including an 11-in. class not listed by Metal Sales.

Metal Sales currently lists Cee flange widths from 2.5 to 4 in. across its product families. MBCI’s published Cee-only combinations show a related but manufacturer-specific range of flange and gauge pairings at each depth.

The lip helps stiffen the flange edge, improves local stability, influences distortional buckling behavior, and factors directly into section-property calculations. Lip geometry should not be omitted from a professional Cee-section evaluation whenever the manufacturer publishes that dimension, since it materially affects the section’s effective properties under load.

Current product examples commonly use 16, 14, and 12 gauge, with some manufacturer product families including 13 gauge as well. MBCI’s general Cee/Zee tables include 12, 13, 14, and 16 gauge, though specific Cee-only combinations vary by size.

Gauge is a market and product label, while base thickness is the actual engineering dimension used in design calculations. Coated thickness equals base steel thickness plus the metallic coating, and different manufacturer conventions can create confusion if only a gauge number is shown without the corresponding base thickness in inches.

These are MBCI product values, not universal C-purlin weights. Metal Sales and other manufacturers publish their own weight tables for their specific Cee product lines, which may differ even at the same nominal depth and gauge label.

For general steel weight calculations beyond cold-formed sections, see the Structural Steel Weight Chart, or use the Steel Weight Calculator for custom takeoffs.

A complete structural evaluation of a C-purlin section includes gross cross-sectional area, centroid location, moment of inertia about both axes, section modulus, radius of gyration, torsional constant, warping constant, and effective section properties where local buckling reduces the usable cross-section. Cold-formed thin-wall sections can lose effective width due to local buckling, so gross section properties alone do not determine allowable structural capacity, which is one major distinction from a basic steel-size table.

A conventional lipped Cee has symmetry about only one axis, and its shear center is offset from its centroid. Because loads are typically applied through the roof or wall panel rather than through the shear center, this offset can introduce torsion into a member that might otherwise be assumed to bend in simple, uniaxial fashion. This affects the purlin’s tendency to twist, its lateral stability, its behavior on sloped roofs, and the bracing requirements needed to control that twist. AISI’s cold-formed steel design provisions specifically define the distance from the shear center to the web midplane for C-sections and provide design criteria for singly symmetric members subject to combined bending and torsional loading, which reflects how significant this behavior is to a proper Cee design, not merely a theoretical detail.

Span is the distance between primary supports along the purlin. Spacing is the distance between parallel purlins across the roof or wall. For uniform roof pressure q in psf and purlin spacing S in feet, the line load carried by the purlin is w = q multiplied by S, where w is expressed in pounds per linear foot.

Spacing can be governed by roof-panel span, purlin capacity, snow, uplift, insulation or roof-system buildup, or architectural geometry. Manufacturer-specific systems may cite spacing values of several feet on center, but these are system values, not national rules. For general roof-panel span guidance, see the Metal Roof Panel Span Chart, and for a dedicated purlin-spacing walkthrough, see the Purlin Spacing Chart.

This page intentionally does not publish a universal C-depth-to-span table. Instead, the sections below explain how to correctly use a manufacturer’s engineered C-purlin load table for the exact section and project conditions involved.

A single-span C purlin runs as one section between two primary supports with reactions at each end, requiring no Z-style nesting. Simple-span behavior still depends on roof-panel restraint and lateral or torsional stability, so it should not be treated as inherently simpler from a design standpoint. Cees can also be used in multi-span systems depending on the manufacturer’s engineered system, though unlike Z purlins, Cees do not naturally nest into each other, so continuity details and splice or connection arrangements differ from a lapped Zee line.

End connections for C purlins commonly use web bolts, splice plates, clips, cleats, or screws through punched holes in the section. The specific connection detail depends on the manufacturer’s engineered system and the forces that must be transferred at that location, so there is no single universal bolt pattern that applies to every Cee product or project.

Gravity loading on a C purlin includes roof or wall sheeting self-weight, insulation, roof live load, snow, and any supported equipment where specifically designed for that purpose. Capacity should not be identified from section modulus alone, since local buckling, distortional buckling, and lateral-torsional behavior can govern before the gross section modulus limit is reached.

ASCE 7’s wind provisions govern roof wind pressures, including component and cladding considerations relevant to secondary framing. Under load reversal, the compression flange changes, roof-panel restraint can change, and torsional behavior can change relative to the gravity-load case. A C purlin that passes downward loading does not automatically pass wind uplift, and each condition should be checked independently against the manufacturer’s load table.

ASCE 7-22 includes specific snow provisions covering sloped roofs, partial loading, unbalanced snow, roof drifts, sliding snow, rain-on-snow, and ponding instability. These effects can significantly increase the required gravity capacity of a roof-framing system in snow-exposed regions.

Roof Slope and C Purlin Behavior

Roof slope can create load components relative to the C-section’s principal axes. Because the Cee’s shear center is offset from its centroid, slope and restraint conditions can contribute to twist, lateral movement, and combined bending beyond a simple vertical gravity case.

Current national roof-framing design guidance for metal building systems specifically covers purlin design, continuous systems, diaphragm requirements, and system anchorage together, rather than treating the purlin as an isolated beam. This system-level view is directly relevant to Cees, since their offset shear center makes the actual restraint provided by roof panels and bridging especially important to real-world performance.

Discrete bridging, anti-roll restraint, sag rods where used, flange braces, roof-panel restraint, and temporary erection bracing all contribute to a C purlin’s real-world stability. Final roof sheeting should not be assumed to provide erection-stage stability, since the panel system is often not fully attached until later in construction, leaving the purlin more dependent on discrete bracing in the interim.

Possible governing controls include vertical deflection, roof-panel performance, roof drainage, ponding risk, ceiling or finish sensitivity, and appearance. This page does not state one universal deflection limit as mandatory, since the governing limit should come from the project specification, the manufacturer’s engineered system, or the applicable code requirement for the specific application.

Current manufacturer specifications identify Cee purlins as structural-quality steel with galvanized options. Metal Sales specifically identifies ASTM A653 structural-quality sheet with G90 coating for its Cee purlin product line, with minimum yield strength and tensile strength values stated in its published specification data. MBCI offers red-oxide and G90-galvanized finish options depending on the product. Not every C purlin nationwide is necessarily the same ASTM grade or coating, so the exact manufacturer and project specification should always govern.

A purlin and a girt may share a similar or identical Cee profile, but load direction, span, spacing, restraint, and connections can differ between the roof and wall applications. Metal Sales explicitly offers both Cee purlin and Cee wall girt product families as distinct offerings. An eave strut should not be treated as simply another C purlin unless the specific building system uses the same product for that purpose.

The 2024 International Building Code states that cold-formed carbon and low-alloy structural members must be designed according to AISI S100. For the 2024 edition I-Codes, AISI identifies AISI S100-16 (2020) with Supplement 2-20 as the adopted North American Specification for cold-formed steel structural members. Cite the standard actually adopted by the governing code and jurisdiction, not merely the newest technical publication available, since a newer AISI S100-24 edition also exists but may not yet be the version legally adopted on a given project.

For purlins specifically, the most relevant ASCE 7-22 provisions address roof and live loading, wind pressures including component and cladding effects, snow and drift, rain and ponding, and roof loads at solar panel installations. The Metal Building Manufacturers Association’s current Roof Framing Design Guide devotes complete coverage to purlin design methods, continuous purlin design, diaphragm requirements, and system anchorage, making it one of the main authority sources for a serious C-purlin reference.

C Purlins with Solar Panels and Added Roof Loads

Added rooftop systems can affect dead load, introduce concentrated attachment forces, change wind effects and load distribution, and affect roof-panel restraint. ASCE 7-22 specifically includes provisions addressing roof loads at solar panels. Added rooftop equipment should never be installed based only on a spare-looking purlin appearance; engineering verification of the purlin, panel, and connection capacity under the new loading is required.

Current Metal Sales Cee offerings span web heights of 4, 6, 8, 9, 10, and 12 in., flange widths of 2.5, 3, 3.5, and 4 in., and 12, 14, and 16 gauge, in red-oxide or galvanized finish. MBCI’s current Cee-only offerings include combinations such as 6×3, 6×3 1/2, 6×4, 8×4, 10×4, 11×3, and 12×4, with weight varying by gauge within each nominal size. This proves that “8-inch C purlin” or “10-inch C purlin” is only a size class, not a complete structural specification, and the exact manufacturer and product line must be identified before design or ordering.