Design & Engineering

Purlins and Girts in Steel Buildings

May 16, 2026

Introduction

Purlins and girts form the structural layer between the building envelope and the primary frame.

Roof and wall cladding do not normally transfer their loads directly to the main columns and rafters. Instead, purlins, girts, eaves members, and related secondary steelwork collect these loads and transfer them to the primary structural system.

However, their role extends beyond supporting cladding. Secondary members can also provide restraint to rafters and columns, help maintain the geometry of the building envelope, and influence fabrication and erection efficiency.

In portal-frame buildings, the relationship between secondary steelwork and primary-frame stability is particularly important. When the structural arrangement, connections, and restraint assumptions are designed accordingly, purlins and side rails can provide lateral and, where appropriate, torsional restraint to primary members.

For this reason, purlins and girts should not be treated as simple lightweight attachments. Their section type, spacing, continuity, connection arrangement, and restraint system all need to work together as part of the complete structural and construction system.

1. What Are Purlins and Girts?

Purlins and girts are secondary structural components that sit between the primary frame and the building envelope.

Purlins are roof secondary members. They support roof cladding and transfer roof loads to the rafters.
Girts, also called side rails in many steel-building systems, are wall secondary members. They support wall cladding and transfer wind pressure and suction to the primary frame.
Eaves struts are located at the roof-to-wall transition and can perform a different structural role from ordinary purlins or girts, including longitudinal force transfer and positional restraint, depending on the structural arrangement.

Their functions can be grouped into three categories:

Function	Typical Components
Support the building envelope	Purlins, girts
Transfer loads to the primary frame	Purlins, girts, eaves members
Provide restraint to primary or secondary members	Purlins, girts, anti-sag systems, flange braces, rafter stays

Not every secondary member performs every function. The exact arrangement varies between building types, structural systems, cladding systems, and design assumptions.

2. Purlin Section Types

There is no universally best purlin section. The appropriate choice depends on span, spacing, loading, continuity, restraint, cladding system, connection arrangement, fabrication requirements, transportation, erection method, and project cost.

Cold-formed C- and Z-sections are common choices for conventional lightweight roof and wall secondary steelwork. RHS can also be used where its structural characteristics or project requirements justify the additional weight and fabrication.

2.1 C-Section Purlins

C-sections are commonly used where a relatively straightforward secondary framing arrangement is required.

Advantages:

Simple section geometry
Straightforward end connections
Convenient for simple spans
Easy to cut and handle
Familiar fabrication and installation process

Limitations:

Continuity arrangements are generally less straightforward than typical lapped Z-purlin systems
Section efficiency depends strongly on span and loading
Connection detailing must account for the actual section geometry

C-sections can be a practical choice where simplicity is more important than maximizing continuity or material efficiency.

2.2 Z-Section Purlins

Z-sections are widely used in industrial and portal-frame buildings, particularly where purlins are arranged as continuous or lapped members. Their geometry allows adjacent purlins to overlap at supports, creating a continuous or semi-continuous structural arrangement when designed accordingly.

Advantages:

Efficient use of material for many purlin applications
Suitable for lapped systems
Convenient for continuous or semi-continuous arrangements
Can provide efficient load distribution across frame lines
Commonly available as proprietary cold-formed systems

Limitations:

Connection and lap detailing are more involved
Correct orientation is important
Installation requires attention to lap direction and connection sequence
Capacity depends on the restraint assumptions used by the manufacturer or designer

Z-purlins should not be selected simply because they are "stronger" than C-sections. Their advantage often comes from the complete system arrangement, particularly continuity and lapping.

2.3 RHS Purlins

Rectangular hollow sections (RHS) can also be used as secondary members, although they are not normally the first choice for conventional lightweight roof purlins.

Their closed-section geometry provides greater torsional stiffness and can be useful where robustness, connection requirements, architectural appearance, or other project-specific considerations justify the additional material and fabrication.

Potential advantages:

Greater torsional stiffness
Closed-section geometry
Robustness
Predictable behavior under certain connection arrangements
Suitability for applications where architectural appearance matters

Trade-offs:

Higher section weight in many conventional applications
Potentially higher material cost
More fabrication work
More involved connections
Different handling and installation requirements

RHS can be particularly practical for wall girts or other secondary members where a closed section is beneficial or where the member must satisfy requirements that are not efficiently addressed by conventional cold-formed C- or Z-sections.

For conventional roof purlins with typical spans and loading conditions, cold-formed sections are often more economical because they provide efficient lightweight solutions for these applications.

The selection should therefore be based on the complete structural and construction system rather than on section shape alone.

For further background on the design of cold-formed steel structures within the Eurocode framework, see the European Commission Joint Research Centre guidance on cold-formed steel structures.

3. Choosing Between C, Z, and RHS

The decision should be based on the complete structural and construction system.

Factor	C-Section	Z-Section	RHS / Box
Simple single-span arrangement	Excellent	Good	Good
Lapped continuous system	Limited	Excellent	Possible
Material efficiency	Good	Often very good	Depends on design
Torsional stiffness	Moderate	Moderate	High
Connection simplicity	High	Moderate	Moderate to low
Lightweight construction	Excellent	Excellent	Usually lower
Fabrication convenience	High	High	Moderate
Installation familiarity	High	High	Moderate
Robustness	Moderate	Moderate	High

This table is a general engineering comparison rather than a universal ranking. Final section selection should be based on structural calculations, applicable design standards, manufacturer or system data, and the actual fabrication and erection conditions.

In practice, section selection is often driven by the interaction between span, loading, restraint, connection detailing, and fabrication method. A section that appears efficient in isolation may not produce the most economical building system once laps, cleats, restraint components, handling, and erection are considered.

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. Bypass vs Flush Girt Arrangement

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.

6. Sag Rods and Anti-Sag Systems

Purlins are relatively slender members. Their behavior can be affected by lateral movement, torsional rotation, wind uplift, construction-stage instability, and the slope component of roof loading.

Anti-sag systems can be introduced between purlins to:

Maintain purlin alignment
Provide intermediate restraint
Improve stability under uplift
Support construction-stage stability before cladding is installed
Help resist the downslope component of roof loading

The exact arrangement may use round rods, angles, flat bars, sleeves, or proprietary restraint systems.

The need for anti-sag systems depends on factors such as purlin section, spacing, span, loading, and the restraint assumptions used in the design. They should therefore not be added automatically to every purlin system.

In some cases, a heavier purlin section may be selected instead of providing intermediate restraint. The most economical solution depends on the balance between steel weight, components, fabrication, and installation.

For additional technical guidance on purlin restraint, anti-sag systems, and construction-stage stability in portal-frame buildings, see the SCI Engineering Guide to Single-Storey Portal Framed Buildings.

7. Purlins and Primary Member Restraint

This is one of the most important relationships in a portal-frame building.

A purlin connected to a rafter can provide lateral restraint to the flange to which it is attached. However, the critical compression flange can change with the governing load case.

For example:

Under gravity loading, one flange may be in compression.
Under wind uplift, the opposite flange may become critical.
Near the eaves haunch, the stress state can differ significantly from the mid-rafter region.

Purlins and side rails therefore cannot automatically be assumed to restrain both flanges of a primary member. Additional flange braces, rafter stays, or other restraint systems may be required where the compression flange is not adequately restrained.

The actual restraint arrangement must be considered by the structural designer together with the member stability assumptions, connection details, cladding restraint, and applicable design standard.

The restraint assumed in design should also be consistent with the actual connection and construction details provided on site.

For a practical overview of portal-frame restraint arrangements and their relationship with secondary steelwork, see the SCI guidance on portal-frame design and member restraint.

For more information on how these restraint systems interact with the building's longitudinal stability system, see Bracing Systems in Steel Buildings.

8. Girts, Side Rails, and Eaves Struts

Girts, also commonly referred to as side rails in European steel construction, are horizontal secondary members that support wall cladding between the primary columns.

Girts perform a similar structural role in the wall system to that of purlins in the roof system. They support wall cladding, transfer wind pressure and suction to the primary frame, help maintain wall alignment, and may contribute to column restraint.

C- and Z-shaped cold-formed sections are common choices for wall girts, although other structural sections, including RHS, can be used where the required structural performance or connection arrangement justifies them.

The major loading difference is that wall girts are strongly influenced by wind pressure and suction, while roof purlins are influenced by a combination of permanent, imposed, snow, maintenance, and wind actions, depending on the building and applicable design requirements.

Their design is therefore not simply a smaller version of roof purlin design.

At the eaves, an eaves strut may be used to connect the roof and wall secondary systems while also contributing to longitudinal force transfer and positional restraint, depending on the structural arrangement.

An eaves strut should not automatically be treated as simply another purlin or girt. Its exact function depends on its location, connections, and role within the building's longitudinal stability system.

9. Fabrication and Installation Considerations

Secondary steelwork is often lightweight and repetitive, which makes it highly suitable for prefabrication. However, fabrication efficiency depends heavily on the selected system.

C/Z purlins and girts usually benefit from repetitive production, standardized lengths, punched holes, galvanized finishes, and simple cleat connections.

RHS systems may require cutting, welding, end plates, drilling, and additional surface treatment. These requirements can increase fabrication effort compared with proprietary cold-formed systems.

During erection, tolerances accumulate from foundation position, anchor bolts, column fabrication, frame erection, rafter alignment, and secondary-member fabrication. A system with very tight geometric requirements may become difficult to install even if it is structurally efficient.

This is one reason why connection access and tolerance accommodation should be considered during design. A bypass wall-girt arrangement, for example, can provide more installation clearance than a tightly arranged flush system.

The best secondary system is not necessarily the one with the smallest steel weight. It is often the one that provides a good balance between structural efficiency, fabrication efficiency, and installation efficiency.

Construction-stage stability also needs to be considered. Purlins are often designed with restraint from the completed cladding system, but that restraint may not be available during erection. The construction sequence and temporary stability requirements therefore need to be assessed separately from the final completed condition.

10. 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 are not simply lightweight components attached to a steel building. They form an important intermediate structural system between the building envelope and the primary frame.

Their selection should consider more than structural capacity. Section efficiency, continuity, restraint, connections, fabrication, installation, tolerance, cladding requirements, and construction sequence all influence the performance of the complete system.

Cold-formed C- and Z-sections remain common solutions for conventional lightweight secondary steelwork, while RHS can be appropriate where its torsional stiffness, robustness, connection characteristics, or architectural requirements justify the additional material and fabrication.

For practical steel building projects, the best purlin and girt system is the one that works as a complete structural and construction system, rather than the one that simply uses the least amount of steel.

Key Takeaways

Purlins and girts connect the building envelope to the primary frame and transfer roof and wall loads.

C-sections suit many simple secondary framing arrangements, while Z-sections are particularly effective for lapped or semi-continuous systems.

RHS can be used as secondary members, especially where greater torsional stiffness, robustness, connection requirements, or architectural considerations justify its additional weight and fabrication.

Purlin spacing and restraint arrangements must be developed together with structural loading, cladding requirements, fabrication, installation, and construction-stage stability.

The most economical secondary system is not necessarily the lightest one. Structural efficiency, connections, fabrication, transportation, erection, and the complete load path should be considered together.

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Design & Engineering

Purlins and Girts in Steel Buildings

Introduction

1. What Are Purlins and Girts?

2. Purlin Section Types

2.1 C-Section Purlins

2.2 Z-Section Purlins

2.3 RHS Purlins

3. Choosing Between C, Z, and RHS

4. Purlin Spacing: A Design Variable

5. Bypass vs Flush Girt Arrangement

5.1 Bypass Girt Arrangement

5.2 Flush Girt Arrangement

6. Sag Rods and Anti-Sag Systems

7. Purlins and Primary Member Restraint

8. Girts, Side Rails, and Eaves Struts

9. Fabrication and Installation Considerations

10. Common Purlin and Girt Design Mistakes

Conclusion

Key Takeaways

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