Choosing the Correct Purlin for Roof Framing

When designing any new roof or renovating an existing one, it is crucial to check all technical parameters to achieve perfection. A well-designed roof is the ideal solution for protecting us from extreme weather and various natural disasters.

The roof is the most important component of any industrial building and needs to be kept stable and properly maintained. Currently, most metal buildings use gently sloping roof designs for a safe and economical construction solution.

When the roof slope is gentle, a negative pressure is generated on the roof, attempting to lift the roof panels upwards.

The long-term performance of a roof framing system depends not only on rafters and trusses. Purlins are often overlooked but crucial components. Roof stability can be achieved by selecting the appropriate purlin spacing and roof profile.

This article will explain what purlins are, their main functions, common types, and practical considerations for selecting the right purlins for your roof framing.

What Are Purlins?

Purlins are secondary structural members that form the framework of a metal building. They are longitudinal horizontal members that connect the longitudinal rafters of a building.

The structural support of every building consists of purlins. Their primary function is to provide intermediate support for the roof panels or roof slabs and to transfer loads to the main structural frame.

In metal and steel structures, purlins are typically made of cold-formed steel. Purlins are parallel to the ridge line and arranged at regular intervals according to the roof design, material type, and expected loads.

Because purlins directly support the roof material, their selection must take into account environmental conditions, roof geometry, and structural requirements.

What Are The Main Functions of Purlins?

Purlins perform several interconnected functions within a roof framing system.

What are the different types of Purlins?

Purlins are available in several shapes and materials, each suited to specific structural and environmental conditions.

Tips For Choosing the Best Purlin For Roof Framing

When designing any structural member, we need to determine the loads acting on it. To determine the loads, the complete parameters of the building should be considered, such as width, length, height, roof slope, and wind speed.

Purlin For Roof Framing

1. Loads on Purlins

The dead load of the roof system includes the self-weight of the roof panels, solar panels, roof terraces, ventilation equipment, walkways, etc.

2. Bracing Type

The second important factor is the bracing type, i.e., whether single-span bracing is used or continuous span bracing is achieved through overlapping purlins.

CEE members are typically used as simply supported purlins, while ZEE or ZED purlins are used as continuous span bracing because we can use overlapping purlins.

3. Purlin Span

The third important factor is the purlin span, i.e., the span distance or rafter spacing. The strength and deflection of the purlins will be determined based on the span.

The design of purlins depends on the roofing system. For example, if a screw-fixed system is used at the top of the purlins, the screw profiles form a partition, providing support strength for the purlins.

In the case of a vertical joint roof where the profiles are fixed to the top of the purlins, in addition to moment and deflection checks, lateral torsional buckling of the purlins also needs to be checked.

Cold-formed purlins must be designed according to cold-formed design specifications.

4. Purlin Type Selection and Design Optimisation

Compared to hot-rolled ISMC channel steel and other hot-rolled tube purlins, cold-formed Z-shaped purlins are currently the most economical solution. Hot-rolled purlins are heavier.

Furthermore, because they cannot overlap, they must be designed as simply supported sections, which limits optimization.

Cold-formed Z-shaped steel purlins, on the other hand, are thin, ranging in thickness from 1.2mm to 3.15mm, and can be overlapped to different lengths, allowing for the design of optimal purlin sections. On average, using cold-formed Z-shaped steel purlins can reduce weight by 30% to 50%.

5. Purlin Surface Treatment

Cold-formed purlins are rolled from pre-galvanized steel coils, with a minimum zinc coating thickness of 120 g/m² and a maximum of 275 g/m².

If the environment is controlled and far from coastal and chemical industries, such as warehouses, small manufacturing plants, or factories that do not generate chemicals, a 120 g/m² coating is sufficient.

For other chemical and coastal environments, at least 275 g/m² of coating is required. For highly corrosive environments, the zinc coating thickness can be further increased, or an anti-corrosion coating can be applied to the purlins. Additionally, ceiling materials can be used to seal the purlins.

Conclusion

Purlins are a fundamental component of roof framing systems, providing support, stability, and load distribution for various building types.

By considering factors such as load requirements, span, environmental exposure, roofing materials, and installation methods, purlins can be selected to ensure long-term structural performance and construction efficiency.

A carefully chosen purlin system helps create a roof structure that remains reliable in daily use and under constantly changing environmental conditions.