Meta description: Learn what a purlin is in framing, where it sits in a roof, how it supports rafters or roof panels, and how purlins differ from joists and girts.
A purlin is a horizontal structural member used to support a roof. Depending on the building design, it may carry roof panels directly or help support rafters between larger structural supports. Purlins are common in metal buildings, agricultural structures, and some traditionally framed roofs. Their size, spacing, and connections affect how roof loads travel through a building, so they must be selected as part of a complete structural design—not by appearance alone.
What Is a Purlin in Framing?
In roof framing, a purlin is a horizontal member that runs across the roof slope and supports roofing materials or rafters. It generally spans between primary supports, such as rafters, trusses, or rigid frames. The exact arrangement depends on the building’s framing system.
In a typical metal-building system, roof purlins sit across the main frames, often at regular intervals. Metal roof panels are fastened to the purlins, which transfer the roof’s weight and environmental loads to the frames. In conventional wood construction, purlins may instead support rafters at an intermediate point, reducing the distance those rafters span unsupported.
The word “purlin” is used most often for roof framing. Related horizontal members on exterior walls of metal buildings are usually called girts. Both can be part of a secondary framing system, but they serve different locations and support different building surfaces.
Where Purlins Fit in the Roof Load Path
Roof components work together to carry loads safely to the ground. A common load path in a metal building is:
- Roof panels receive their own weight, snow, wind pressure, and other applicable loads.
- Purlins support the panels and carry those loads to the primary frames.
- Frames, trusses, or rafters transfer the forces to columns or bearing walls.
- Columns, walls, and foundations carry the loads into the ground.
In a wood roof with purlin-supported rafters, the purlin can provide an intermediate support point. The purlin itself then needs support—often from posts, struts, or bearing walls—so loads continue safely down to the foundation. A purlin that is not supported or connected as designed cannot reliably perform its intended role.
Roof loads are not limited to the downward force of gravity. Wind can create uplift, and snow may accumulate unevenly. The structural system and its connections must account for the loads required by the building code and local conditions.
Common Types of Purlins
C-Purlins
C-purlins have a cross-section shaped roughly like the letter C, often with small lips at the edges. They are commonly made from cold-formed steel and used in metal-building roofs and walls. Their shape provides useful strength for their weight, but performance depends on the purlin’s dimensions, steel thickness, span, bracing, and connection details.
Z-Purlins
Z-purlins have a cross-section shaped roughly like the letter Z. Their geometry can allow adjacent members to overlap at supports, creating a continuous arrangement in some systems. This can help distribute forces across spans, but the benefit depends on the engineered design and correct installation. A Z shape alone does not guarantee a stronger or more suitable roof member than a C shape.
Wood Purlins
Wood purlins are used in some conventional roofs, timber structures, barns, and other buildings. They may support rafters between larger supports or form part of a roof system with exposed framing. Species, grade, moisture, treatment, dimensions, span, and connection details all influence capacity.
In older or regional building practices, the term “purlin” may describe a wood beam placed beneath rafters and supported by struts. Terminology and arrangements vary, so plans and local building practices should be checked rather than assuming every purlin looks or functions the same way.
Purlins Compared With Rafters, Joists, and Girts
These framing terms describe different members, although their roles can overlap in some designs. The building plans should identify which members are primary supports and which are secondary.
| Member | Typical location | Common function |
|---|---|---|
| Purlin | Roof | Supports roof panels or rafters and transfers loads to primary framing |
| Rafter | Roof | Runs from the ridge or upper support toward the eaves; supports roof sheathing or other roof components |
| Roof joist | Roof or ceiling framing | Often spans between walls or other supports and may support a ceiling or roof system, depending on the design |
| Girt | Exterior wall | Supports wall panels and transfers their loads to columns or frames |
Names alone do not establish a member’s capacity or exact job. For example, a purlin supporting roof panels is not interchangeable with a rafter sized for a different span and load. Members must be evaluated as part of the complete framing system.
How Purlins Are Used in Different Roof Systems
Metal-Building Roofs
In many pre-engineered metal buildings, rigid frames carry the main building loads. Purlins span between those frames and support roof panels. They may also help brace the frames, depending on the system. Purlin spacing is coordinated with the roof-panel profile, panel capacity, design loads, and the manufacturer’s requirements.
Cold-formed steel purlins can be sensitive to how they are restrained. Bridging, sag rods, panel attachment, and connections may be needed to control twisting or lateral movement. The specified arrangement should be installed as shown in the approved drawings.
Conventional Wood Roofs
In some wood-framed roofs, a purlin runs parallel to the ridge and supports rafters at an intermediate location. Posts or struts may transfer the purlin’s load to a bearing wall or other designed support. This arrangement can reduce the unsupported span of rafters, but it creates additional loads at the purlin supports.
That distinction matters in remodeling. Adding a purlin without verifying where its supports land can concentrate weight on a ceiling joist, nonbearing partition, or other member that was not designed to carry it.
What Determines Purlin Size and Spacing?
There is no universal purlin size or spacing that works for every roof. A designer evaluates the member within the full structural system, using the applicable building code, load criteria, and product information. Common considerations include:
- Span: The distance between supports strongly affects bending and deflection.
- Loads: Roof dead load, snow, wind pressure and uplift, and any applicable maintenance or equipment loads matter.
- Material and section: Steel thickness and shape, or wood species and grade, affect strength and stiffness.
- Spacing: Purlin spacing influences the area of roof carried by each member and the support conditions for roof panels.
- Bracing and continuity: Bridging, overlaps, restraints, and end conditions can affect how a member performs.
- Connections: Fasteners and supports must transfer forces, including uplift, without failing or slipping.
- Roofing system: Panels, sheathing, insulation, and attachment methods may affect support requirements.
Designers also check serviceability, such as deflection, because excessive movement can affect roof-panel performance, drainage, finishes, or connections even when a member does not break. Load combinations and local requirements can vary by location and building use.
Installation and Inspection Considerations
Purlins should be installed to the layout and details in the structural plans or approved manufacturer drawings. Field changes—such as shifting a member, omitting bridging, cutting a flange, or substituting a different thickness—can alter the system’s behavior. Any proposed change should be reviewed by the responsible design professional or manufacturer before work proceeds.
During inspection, attention commonly goes to member alignment, bearing, spacing, connections, bracing, and visible damage. For metal systems, the specified fasteners and their locations are important; for wood, condition, grade markings where required, bearing, and connectors matter. Inspection requirements depend on the project and local authority.
Corrosion, water damage, splitting, crushing at supports, loose fasteners, or unusual sagging may warrant prompt evaluation. A visual check cannot establish a member’s design capacity, and covering a damaged member can make a problem harder to assess.
Common Purlin Problems and Misconceptions
Assuming All Purlins Are the Same
Purlins vary by material, profile, dimensions, span, and framing arrangement. A C-purlin in one engineered building is not automatically suitable for another building with different loads or support spacing.
Confusing Purlins With Girts
Purlins are generally roof members; girts are generally wall members. They may look similar, especially in metal buildings, but they support different surfaces and may be designed for different forces.
Adding a Purlin Without Designing Its Supports
A new purlin may reduce the span of rafters, but it also collects and redirects loads. Its posts, struts, bearings, connections, and supporting structure need to be checked as part of the modification.
Ignoring Uplift and Bracing
A roof member must be connected and restrained for the forces in the design, not merely held in place against gravity. Wind uplift and member twisting are among the reasons specified fasteners and bracing should not be omitted.
How to Find the Right Purlin Information for a Project
For new construction, the structural drawings and project specifications are the starting point. In a pre-engineered building, the manufacturer’s approved drawings identify the intended purlin layout, member designations, connections, and bracing. For a repair or alteration, the existing member’s label or appearance is not enough to confirm suitability.
Building codes are adopted and amended by state and local jurisdictions, so the applicable requirements depend on the project’s location. The International Code Council publishes model building codes, while the American Wood Council provides design resources for wood construction. These references help explain the design framework, but they do not replace project-specific engineering or local code review.
In practical terms, a purlin is a roof-support member that helps carry loads from roofing or rafters to the building’s primary structure. Its role can differ between metal-building and wood-framing systems, so correct sizing, spacing, bracing, and connections must follow the plans and the conditions at the site.
Sources and Further Reading
- International Code Council: I-Codes — model codes adopted and amended by jurisdictions.
- American Wood Council: Codes and Standards — wood-design standards and technical resources.
- Metal Building Manufacturers Association — resources on metal-building systems and industry practices.