Purlins and Girts

Role of Purlins in PEB Buildings

The Role of Purlins in PEB Buildings goes well beyond just holding roof sheets in place — they’re an active part of how loads move through the building, and treating them as a minor detail is a common cause of avoidable roof problems. 

1.   Supporting Roof Panels — Purlins give extra support to metal roofing panels in between the rafters. For example, with rafters spaced 6 metres apart, roofing sheets left to span that whole distance unsupported would bend far more than is acceptable — purlins break that distance up into shorter stretches the panel is actually rated to handle.
    
2.   Transferring Roof Loads —Roof loads include the roofing materials themselves, insulation, the weight of maintenance work, wind effects, and project-specific loads such as rooftop equipment. Purlins gather all these loads and pass them on toward the main structural frames, which is why their capacity has to be checked against the real combination of loads expected — not just their own weight.
3.   Maintaining Roof Geometry — Properly placed purlins hold the intended roof shape in position; without them, roofing panels have no consistent line to follow, and visible waviness in a finished roof can often be traced back to purlins that weren’t aligned consistently during assembly.
4.   Supporting Roofing Accessories — Depending on the design, purlins can give fixing points for skylights, ventilators, and other roof-mounted parts — which means their spacing sometimes needs to be adjusted locally to line up with where these accessories are, not just follow the usual structural spacing rules.
5.   Contributing to Structural Stability — Purlins work together with bracing and other structural parts to help keep the whole roof system stable, especially in resisting a sideways-twisting type of buckling in the rafters they’re attached to.

Function of Girts in Steel Structures

The Function of Girts in Steel Structures is mainly about supporting the building’s wall shell — but because walls face wind pressure from both directions (pushing in and pulling out), girt design has a detail that roof purlin design usually doesn’t need to deal with.

• Supporting wall panels and keeping them aligned across the full height of the wall.
• Passing wind pressure and suction from the wall cladding to the main structural frame — a cladding panel on the side facing the wind feels inward pressure, while one on the sheltered or side face can feel outward suction, and girts need to be checked for both directions, not just one.
• Keeping the wall consistently aligned so the cladding goes on flush and straight.
• Supporting openings and accessories such as doors, windows, and louvers, where extra framing is needed around the opening.

Here’s a practical example: a warehouse wall facing storm winds on a coastal site will usually need closer girt spacing or a heavier piece than an identical wall on the sheltered side of the same building — treating all four sides of the building the same way ignores a real, calculable difference in wind load.

Purlins and Girts as PEB Secondary Structural Members

Purlins and girts are classed as PEB Secondary Structural Members because they connect the building’s outer shell to the main structural framework, rather than carrying the load straight down to the foundation themselves. The simplified path the load takes looks like this: 

•     Roofing → Purlins → Primary Frames → Columns → Foundation
•     Wall Panels → Girts → Primary Frames → Columns → Foundation

Secondary framing systems commonly use Cee and Zee-shaped steel members to span between primary frames and support roof and wall panels. Metal Sales provides additional information on secondary framing and purlin and girt applications.

Types of Purlins Used in PEB Structures

Different purlin shapes are used depending on the structural requirements and what the manufacturer offers — the choice isn’t just about looks, it affects how much load the piece can carry, how it connects, and the cost.

Z Purlins

Z-shaped, cold-formed pieces are the most widely used shape in PEB projects. Their shape lets them overlap at supports (called “lapping”), which improves how the load carries continuously across spans and increases load capacity compared to a simple, single-span C-shaped piece of the same size — this is a big part of why Z-shaped pieces are so commonly used for longer roof spans.

For additional technical information on Zed and Cee steel purlin profiles and their span applications, see LYSAGHT steel purlins and girts.

C Purlins

C-shaped pieces are used for particular framing situations and smaller support needs — commonly at the ends of the building, at the eaves, or over shorter spans where the overlapping advantage of Z-shaped pieces isn’t needed.

Built-Up Purlins

Built-up or specially fabricated pieces are used where the required strength or shape needs something beyond a standard cold-formed profile — for instance, at heavier point loads from hanging equipment, where a standard Z or C shaped piece wouldn’t have enough capacity.

The choice of purlin type should always be based on engineering calculations and the specific needs of the project — not simply on whichever shape a particular manufacturer happens to have in stock.

Types of Girts

Common girt options include Z-shaped girts, C-shaped girts, built-up steel pieces, and project-specific cold-formed pieces. The right choice depends on wall height, frame spacing, the type of cladding, wind loads, connection requirements, and the applicable design standards — the same basic engineering logic as choosing purlins, just applied to wall loads instead of roof loads. 

Purlin Spacing and Design Guidelines

Purlin Spacing and Design Guidelines matter because the spacing directly affects how well the roof system performs — and getting this number right involves a real trade-off, not one single “correct” answer that works everywhere.

Purlin spacing is worked out through structural design, taking into account the roof panel’s specifications, roof loading, wind loads, snow loads where relevant, the building’s span, frame spacing, the purlin piece’s capacity, how much bending is allowed, roofing requirements, connection details, and the applicable codes and standards.

Purlin spacing and secondary-framing design should therefore be determined from the applicable structural requirements rather than from a fixed spacing rule. For further technical background, see GlobalSpec’s reference on secondary framing, girts and purlins.

The trade-off works in both directions: if purlins are spaced too far apart, roofing panels have to handle more structural demand and bend more — possibly beyond what the panel maker rates it for, leading to water pooling, visible waviness (called “oil-canning”), or in extreme cases the panel failing under load. If the spacing is unnecessarily close together, steel use and the labour needed for making and installing it goes up without any real benefit in performance. For instance, tightening purlin spacing from 2.0m to 1.5m across a large roof can meaningfully increase the amount of steel used and the number of connections needed — a cost that’s only worth it if the roofing panel or loading conditions actually require it, not something applied everywhere just as an extra safety margin.

Girt Spacing and Design Considerations

Girt spacing is affected by what the wall system needs, including the type of wall panel, wind pressure and suction, wall height, the building’s location, frame spacing, cladding capacity, bending limits, openings, connection design, and applicable standards.

Just like with purlins, girt spacing should be checked against the specific wind zone the project is in — a building in a high-wind coastal zone genuinely needs closer girt spacing or heavier pieces than the same wall height in a low-wind inland location, and using one standard spacing across every site condition either under-designs the exposed building or overspends on the sheltered one.

Purlin and Girt Sizes and Specifications

Purlin and Girt Sizes and Specifications change from project to project — there is no single standard size that works for every PEB, no matter how often generic size charts are treated as a shortcut.

The right size depends on the building’s dimensions, the loading conditions, the roofing and cladding systems, the structural design, and the environmental conditions. Because of this, purlin and girt sizes should be confirmed through engineering calculations specific to the project, rather than picked purely from a generic size chart — a chart can be a useful starting point, but it isn’t a substitute for checking the actual loads on the actual project.

Connections of Purlins and Girts

Purlins and girts are usually connected to the main frame using connection plates, cleats, bolts, or other engineered fittings. Connection design needs to take into account the loads being applied, the piece’s capacity, bolt requirements, connection shape, how it will be installed, structural movement, and the applicable design standards.

Connections deserve more attention than their small size might suggest: a purlin or girt is only as effective as the connection passing its load onward, and an under-designed cleat or bolt pattern can actually become the real point of failure, even when the piece itself was sized correctly.

Importance of Proper Purlin and Girt Design

Incorrectly designed or poorly installed secondary members affect how well the building’s outer shell performs in ways that are usually visible well before they become a serious structural issue — which is exactly why it’s worth getting them right the first time, rather than treating them as an easy place to cut costs.

• Too much bending in the roof or wall, seen as sagging or waviness on the finished surface.
• Panels not lining up properly, leading to poor water drainage and an unfinished look.
• Weak connection performance, which can loosen over time under repeated wind loads.
• Difficulty during installation, since misaligned secondary pieces slow down fitting the cladding.
• More maintenance needed, since poorly performing parts need attention sooner.
• Lower structural efficiency, undoing some of the material savings that were the whole reason for choosing a PEB system in the first place.

Proper design and manufacturing make sure purlins and girts keep working well throughout the building’s whole working life, instead of becoming the first thing that needs fixing after handover.

Installation of Purlins and Girts in PEB Structures

During PEB assembly, secondary pieces are installed following a set order:

1. Positioning the primary frames.
2. Installing purlins and girts according to the assembly drawings.
3. Completing the required bracing and connections.
4. Checking alignment and levels.
5. Installing roofing and wall cladding.
6. Completing accessories and finishing work.

Skipping the alignment check in step 4 is a common shortcut taken when a schedule is tight — and it’s also the step most likely to cause a visibly wavy roof or wall line once cladding goes on top of secondary framing that isn’t properly lined up.

Advantages of Efficient Secondary Framing

Properly designed secondary framing gives benefits that add up across the whole project, rather than just showing up as one single cost saving.

• Efficient steel use through well-chosen piece sizes and spacing, avoiding both under-designing and unnecessary over-designing.
• Faster installation through standardized parts and connections that assembly crews can repeat consistently across each section.
• Better cladding support, which translates directly into a straighter, better-performing outer shell for the building.
• Improved structural performance, since correctly sized secondary pieces help the overall frame’s stability rather than working against it.
• Easier inspection and maintenance, since a well-designed system wears out predictably rather than creating unplanned repair work.

Design Technology for Purlins and Girts, and the Role of Human Judgment

Designing secondary members has increasingly moved toward software-assisted calculation — structural design tools can now size purlins and girts directly from the governing load combinations, check bending limits automatically, and work out the best spacing across an entire roof or wall in a fraction of the time manual calculation would take. More and more, AI-assisted detailing tools are being added on top of this to suggest ideal spacing patterns and flag possible clashes with accessories like skylights or louvers before the fabrication drawings are finalized.

This is a genuine efficiency gain, but it doesn’t remove the need for an engineer to check what the software is actually optimizing for. A spacing-optimization tool will reliably reduce the amount of steel used within the load information it’s given — but it won’t independently notice that a wind-load assumption was pulled from the wrong exposure category, or flag that tighter spacing is needed near a roof-mounted equipment platform the model didn’t fully account for. The projects that get the most value from these tools keep an experienced structural engineer reviewing the load inputs and the final spacing output, rather than treating an automatically generated purlin schedule as final just because it passed a software check.

Purlins vs Girts: Key Differences