Design & Engineering

Purlins and Girts in Steel Buildings

May 16, 2026

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Purlins and girts form the secondary framing between a steel building’s envelope and its primary structure. Purlins normally support roof cladding between rafters, while girts support wall cladding between columns.

Their role is not limited to supporting panels. Depending on the structural arrangement, secondary members may also restrain rafters or columns, maintain envelope alignment, and transfer defined forces into other parts of the structure.

For that reason, purlins and girts should not be selected from section weight or spacing alone. Their sections, continuity, connections, cladding, restraint assumptions, fabrication, and erection sequence must work as one coordinated system.

At a Glance

Purlins support roof cladding, while girts or side rails support wall cladding and transfer envelope loads to the primary frame.
C- and Z-sections are widely used for lightweight secondary framing, but their performance depends on the complete span, continuity, connection, and restraint arrangement.
Purlin and girt spacing must satisfy both structural-member requirements and the spanning, fastening, deflection, and weathertightness requirements of the cladding.
Secondary steelwork can restrain primary members only when its stiffness, continuity, connections, and load path support that design assumption.
Construction-stage conditions require separate consideration because restraint from the completed cladding may not yet be available.

1. What Are Purlins and Girts?

In a typical metal-clad steel building, purlins and girts transfer loads from the building envelope to the primary framing.

Member	Typical location	Primary envelope function	Possible additional role
Purlin	Roof, spanning between rafters or roof beams	Supports roof cladding and transfers roof loads	May restrain the connected rafter flange or support a rafter stay
Girt or side rail	Wall, spanning between columns	Supports wall cladding and transfers wind pressure and suction	May restrain columns when continuity, stiffness, and connections are adequate
Eaves beam	Roof-to-wall junction	Supports adjacent roof-edge and wall cladding and may support gutters	Coordinates the roof and wall envelope
Eaves strut	Along the eaves line	Performs a system-specific structural function	May transfer longitudinal forces or provide positional restraint

Not every steel building uses conventional purlins and girts. Some deep-profiled decks, liner trays, or other engineered envelope systems can span directly between primary members. Their ability to do so must be established by the design of the complete system.

Purlins and girts also do not automatically perform every possible secondary function. A member intended to restrain the primary frame or transfer longitudinal forces must have suitable stiffness, continuity, connections, and anchorage.

Terminology note: Girt, side rail, cladding rail, eaves beam, and eaves strut are not used identically in every country or proprietary building system. Drawings and specifications should define the member’s physical position and structural function instead of relying on its name alone.

2. Common Purlin and Girt Section Types

Cold-formed lipped C- and Z-sections are common in lightweight roof and wall secondary framing. Rectangular hollow sections may also be used when their closed geometry, robustness, connections, or architectural appearance provide a project-specific advantage.

There is no universally best section type. Selection depends on span, loading, continuity, restraint, cladding, connections, availability, fabrication, transportation, and erection.

2.1 C-Section

C-sections are widely used in simple-span and sleeved systems, particularly where straightforward geometry and connections are important.

Typical characteristics include:

Familiar section geometry
Convenient cleat and end connections
Straightforward cutting, punching, handling, and installation
Suitability for many simple-span applications
Availability as proprietary cold-formed systems

Continuity is possible through sleeves or other designed connections, but it should not be assumed from section shape alone.

2.2 Z-Section

Z-sections are frequently used in multi-bay buildings where adjacent members overlap at frame lines to form a designed lapped or semi-continuous system.

Potential advantages include:

Compatibility with lapped arrangements
Increased capacity near supports when laps are properly designed
Efficient material use in many repetitive multi-bay systems
Broad availability from proprietary purlin manufacturers

Their benefit comes from the complete system arrangement, including orientation, lap length, fastener position, restraint, and installation sequence.

A Z-section should not be described as inherently stronger than a C-section. Products with similar nominal depths or thicknesses may have different geometry, material properties, test data, and design capacities.

2.3 Rectangular Hollow Sections

Rectangular hollow sections, or RHS, can be used as purlins, girts, or other secondary members when a closed section provides a meaningful project benefit.

Possible reasons for selecting RHS include:

Greater torsional stiffness than many comparable open thin-walled sections
Increased robustness
Architectural exposure
Particular connection or support requirements
Loading conditions not efficiently addressed by a conventional proprietary system

Potential trade-offs include:

Greater self-weight in many conventional applications
Higher material cost
More drilling, welding, and end-plate work
More extensive surface treatment
Heavier handling and installation requirements

For ordinary lightweight roof and wall framing, cold-formed C- or Z-sections are often more economical. RHS becomes appropriate when its additional characteristics improve the complete structural or construction system.

For Eurocode-based projects, the European Commission Joint Research Centre record for an ECCS cold-formed steel design guide provides technical background on sections, sheeting, members, and connections under the EN 1993 framework, particularly EN 1993-1-3.

2.4 General Comparison

Consideration	C-section	Z-section	RHS
Typical arrangement	Simple-span or sleeved system	Lapped or semi-continuous system	Project-specific secondary framing
Continuity	Requires sleeves or designed connections	Commonly developed through designed laps	Requires specific connection detailing
Connection access	Usually straightforward	Lap position and orientation require coordination	End plates, brackets, or welded details may be required
Weight efficiency	Often efficient in simple arrangements	Often efficient in repetitive multi-bay systems	Commonly heavier for conventional lightweight applications
Torsional behavior	Open-section behavior must be considered	Open-section behavior must be considered	Closed geometry generally provides greater torsional stiffness
Fabrication	Repetitive cutting and punching	Repetitive cutting, punching, and lap preparation	May require additional drilling, welding, and finishing
Typical reason for selection	Simplicity and familiar detailing	Designed continuity and repetitive bay efficiency	Robustness, closed geometry, special detailing, or appearance

This table describes typical system characteristics, not fixed performance rankings. Final selection requires structural verification using the applicable standard and product-specific data.

3. Span, Continuity, and Connection Arrangements

A purlin or girt cannot be assessed only by its cross-section. Its structural behavior also depends on how it spans and connects across the primary frame.

Common arrangements include:

Simple-span systems: Each member spans between two primary frame lines without relying on continuity over the supports.
Sleeved systems: Individual members are connected by sleeves intended to develop a defined degree of continuity.
Lapped systems: Adjacent Z-sections overlap near supports and act as a designed semi-continuous or continuous system.
Continuous members: One member passes across several supports, subject to transport length, handling, erection, and connection constraints.

Continuity changes bending distribution, support reactions, deflection, local forces, and connection demand. The resistance of a lapped or sleeved system therefore cannot be determined from the isolated section properties alone.

Cleats, bolts, screws, sleeves, and laps must correspond to the structural model and product data. Connection eccentricity, bearing, local section behavior, fastener resistance, and installation access may all influence the final detail.

Products from different manufacturers should not be treated as interchangeable merely because they share a nominal depth and thickness. Proprietary capacity tables may depend on specific section geometry, material strength, laps, fasteners, and restraint assumptions.

4. Purlin Spacing: A Design Variable

Purlin spacing affects both structural performance and construction cost.

Closer spacing can reduce the load carried by each purlin, allow smaller purlin sections, and support lighter cladding systems. But it also increases the number of purlins, connections, installation time, and fabrication and handling effort.

Wider spacing can reduce the number of purlins but may require larger sections and stronger cladding.

Purlin spacing may also vary within the same building. In portal-frame structures, additional restraint or higher structural demand near regions such as the eaves haunch can influence the selected arrangement.

The design loading should account for the relevant actions, including permanent and imposed loads as applicable. For background on the distinction between permanent and imposed loads, see Dead Load vs Live Load in Steel Buildings.

Wind actions and, where applicable, snow actions can also govern purlin design. See Wind and Snow Loads in Steel Structure Design.

Seismic actions may also need to be considered depending on the building location and applicable design requirements. See Seismic Load Considerations for Steel Structures.

These actions must then be considered in the applicable load combinations. See Load Combination in Steel Structure Design.

Purlin spacing is therefore a design variable that balances structural capacity, cladding requirements, restraint, fabrication, and installation. It is not a fixed number that can be applied to every project.

5. Wall Girt Arrangements and Eaves Coordination

The position of wall girts relative to the primary column can significantly affect the building envelope, connection detailing, and installation.

Terminology varies between manufacturers, designers, and regions. Terms such as bypass girt and flush girt are commonly used in steel-building practice, but their exact definitions should always be checked against the particular framing system or manufacturer's details.

Terminology note: In general usage, a bypass girt is arranged outside the primary column flange so that the girt passes or "bypasses" the column line, while a flush girt is arranged more closely within the plane of the primary framing. Some systems and manufacturers may use different terminology or define the arrangements slightly differently. The physical relationship between the girt, column flange, and cladding should therefore take precedence over the name alone.

5.1 Bypass Girt Arrangement

With a bypass arrangement, the wall girt is positioned outside the primary column line and the wall cladding is correspondingly further away from the main frame.

Potential advantages:

Easier access for installers
More working space around connections
Greater tolerance accommodation
Convenient installation of wall cladding
Potentially simpler coordination between the primary frame and building envelope

The arrangement can be particularly useful where fabrication, transportation, foundation, and erection tolerances need to be accommodated.

5.2 Flush Girt Arrangement

With a flush arrangement, the wall girt is positioned more closely within the plane of the primary frame.

Potential advantages:

More compact wall build-up
Reduced projection of secondary steelwork
A closer relationship between cladding and primary frame

Potential limitations:

Less installation clearance in some details
Greater sensitivity to column and girt tolerances
More difficult access for fixing in certain arrangements
Greater coordination requirements around connections

The question is not simply which arrangement is stronger. The appropriate arrangement depends on envelope geometry, structural requirements, connection access, tolerances, insulation and cladding build-up, and installation efficiency.

5.3 Eaves Members

The roof-to-wall junction requires separate coordination.

An eaves beam commonly supports roof-edge or wall cladding and may support gutters. An eaves strut may instead form part of a defined longitudinal load path or provide positional restraint.

One member can perform more than one function, but only when its section, connections, and supporting load path have been designed for those functions.

6. Anti-Sag Systems and Secondary-Member Stability

Cold-formed purlins and girts are slender members whose behavior can be influenced by lateral movement, twist, weak-axis deflection, wind uplift, construction-stage loading, and the downslope component of roof loads.

Anti-sag systems may use rods, angles, straps, sleeves, tension wires, or proprietary components. Depending on the design, they can:

Maintain alignment during installation
Provide intermediate restraint at defined points
Reduce weak-axis movement or sag
Assist with the downslope component of roof loading
Support stability before cladding is installed
Contribute to restraint under wind uplift or wall suction

Their presence alone does not prove that effective structural restraint exists. The components must have suitable stiffness, connections, anchorage, and a complete load path.

The number and position of anti-sag lines depend on the section, span, spacing, roof slope, loading, cladding, and manufacturer’s design model. In some cases, a heavier purlin without intermediate anti-sag components may be more economical after fabrication and installation are considered.

The SteelConstruction.info technical guidance on building envelopes discusses purlins, side rails, anti-sag systems, cladding restraint assumptions, and construction-stage behavior in single-storey steel buildings.

7. How Secondary Steelwork Can Restrain the Primary Frame

Purlins and girts may contribute to the out-of-plane stability of portal-frame rafters and columns.

A purlin connected to a rafter can provide lateral restraint to the connected flange when the purlin, connection, and supporting system have sufficient strength and stiffness. A continuous side rail may similarly restrain a column when its position and connections correspond to the structural design.

However, the critical compression flange can change with:

Member location
Bending-moment distribution
Gravity loading
Wind uplift
Load combination
Frame geometry and haunch arrangement

A purlin attached to one flange should not automatically be assumed to restrain the opposite flange. Where the inner or unconnected flange requires restraint, a rafter stay, column stay, flange brace, or another designed arrangement may be necessary.

Openings also affect continuity. A side rail interrupted by an industrial door or large window cannot provide the same restraint as an uninterrupted member unless an alternative load path is introduced.

Engineering note: Secondary-member restraint and overall building bracing are related but different functions. A purlin or girt used to restrain a rafter or column does not automatically replace the roof and wall bracing required to transfer longitudinal actions through the building.

Most importantly, the restraint assumed in structural calculations must match the members, connections, cladding, fasteners, and details installed on site.

8. Fabrication, Installation, and Construction-Stage Conditions

Cold-formed purlin and girt systems are well suited to repetitive prefabrication. Standardized lengths, punched holes, galvanized finishes, and repeated cleat details can reduce workshop and site work.

RHS systems may require more cutting, drilling, welding, end plates, and surface treatment. These operations do not make RHS unsuitable, but their cost and production time should be included when comparing systems.

Before fabrication, coordinate:

Member marks and orientation
Span arrangement
Lap direction and lap length
Hole positions
Cleat geometry
End-bay requirements
Openings and interrupted girts
Anti-sag attachment points
Rafter and column stay locations
Cladding and fastener requirements
Transport and lifting limits

Erection tolerances accumulate from foundations, anchor bolts, primary-frame fabrication, column and rafter alignment, cleats, and secondary members. A structurally efficient arrangement can still be difficult to install if it provides inadequate access or tolerance accommodation.

Practical note: Manufacturer load/span data may assume restraint from the completed cladding system. Before that cladding is fixed, purlins, girts, and primary members may require temporary bracing, controlled erection sequencing, or additional restraint.

Construction-stage stability must therefore be assessed separately from the completed-building condition.

9. Common Purlin and Girt Design Mistakes

Several mistakes occur when secondary steelwork is treated as a collection of simple components.

Mistake 1: Choosing a section only by weight.
The lightest section is not automatically the most economical system when fabrication, connections, restraint, and installation are considered.

Mistake 2: Treating C and Z sections as interchangeable.
Their continuity, orientation, lap arrangements, and connection requirements can be different.

Mistake 3: Assuming purlins automatically restrain the entire rafter.
Restraint depends on the actual structural arrangement, flange being restrained, cladding assumptions, and load combination.

Mistake 4: Adding anti-sag rods without understanding their function.
Their arrangement should correspond to the structural design, purlin behavior, and restraint requirements.

Mistake 5: Ignoring construction-stage stability.
A building that is stable after completion may not yet be stable during construction, before the cladding system provides its intended restraint.

Mistake 6: Selecting a girt arrangement only from the section drawing.
Bypass and flush arrangements can have different connection access, tolerance, cladding, and installation implications. The complete envelope and framing system should be considered.

Conclusion

Purlins and girts form the structural interface between a steel building’s envelope and its primary frame. They support cladding and transfer loads, but they can also influence primary-member restraint, envelope alignment, fabrication, and erection.

Their design cannot be reduced to selecting a section from a weight table. Section shape, span, spacing, continuity, connections, cladding behavior, restraint assumptions, tolerances, local loads, and construction sequence must operate as one coordinated system.

Cold-formed C- and Z-sections are efficient solutions for many conventional buildings. RHS can be appropriate when closed-section behavior, robustness, connection requirements, or architectural considerations provide a clear project benefit.

The most economical solution is therefore not necessarily the lightest individual member. It is the secondary framing system that satisfies structural, envelope, manufacturing, transportation, and erection requirements with the fewest avoidable complications.

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