What are Purlins

A purlin is a longitudinal, horizontal, structural member which provides secondary framing support to loadbearing elements in the construction of a building’s roof and/or wall elements.

The term ‘purlin’ is mostly used in reference to roofing applications where purlins run between the roof’s main rafters to provide support for the roof cladding.

When used as secondary support for walling/cladding construction the purlin is referred to as a side rail.

What materials are purlins manufactured from?

Strictly speaking, purlins can be manufactured from wood or steel. However, the most common material for the manufacture of purlins is steel. In fact, the term ‘purlin’ is generally associated most readily with steel frame construction where the loadbearing roof beams or wall columns of the structure are bridged by the purlins or side rails which provide support for the roofing and cladding.

In both roof and wall construction, the purlin needs to provide sufficient support to which roofing and cladding materials can be fixed without imposing unnecessary additional loads on the structure and foundations, assuring a lightweight and cost-effective solution.

Steel makes the ideal material from which to manufacture purlins due to its high strength to weight ratio. It can be engineered into different sections, which deliver greater strength than is inherent in the raw steel material itself.

The prime example of such sections comes in cold roll formed purlins, which are manufactured in C and Z sections, so named due to the shape of the section in which they are formed. Mezzanine floor sections are also manufactured under similar principles.

At voestalpine Metsec, purlins are manufactured by feeding coils of light gauge galvanised steel through a succession of rollers arranged in a long line. The steel does not have to be heated to become malleable, hence the process being termed ‘cold rolling’. As the steel sheet passes through each set of rollers, it is bent very slightly. Its final C or Z section profile being achieved as it completes its pass through the roller line.

The light gauge of steel used together with the relatively low-energy production methods of cold roll forming create a product which is light in weight, strong, sustainable and economic, whilst the galvanising process assures resistance to corrosion to assure a longer service life.

What type of purlins are there?

voestalpine Metsec Purlins Division designs and manufactures a variety of products under the banner of ‘purlins’. These include Z and C section purlins, side rails and mezzanine floor sections together with a wide variety of accessories, such as panel joint rails (PJR) and horizontal cladding supports (HCS), to facilitate installation and fixing for the majority of cladding types and designs.

Z-section purlins are used in roof construction where they provide the greater stability required for the fixing and support of roof covering materials. They can also be used as side rails in wall construction.

C-section purlins are used as side rails to provide a fixing for elevational coverings, usually cladding. They can also be used as mezzanine floor joists to provide a fixing for flooring, which is usually some form of timber boarding.

The sections are available in sizes ranging from 142mm to 402mm in depth and can be manufactured from different thicknesses, or gauges, of steel.

Purlin specification, design and manufacture are dictated by the requirements of the ultimate application.

The different thickness & depth of sections is to enable products to be used on a wide range of spans & load cases, whilst minimising the amount of material required, to provide a cost-effective design. The varying thicknesses & depths are also useful when providing both internal & external fixing faces using a common member. MetSPEC design software can be utilised to design purlin, side rail & mezzanine floor systems. Alternatively, structural engineers sometimes refer to the load tables found in our technical handbook to design their members.

Z-section purlins

As previously mentioned, Z-section purlins are so called because of the shape to which their section/profile is formed. They provide a high strength, lightweight support in both roof and wall constructions.

Z-section purlins for roofing

Z-section purlins are suitable for a number of jointing arrangements in the construction of roofing structures, where they are used to support the roof covering, whether this be cladding, roof tiles or some other form of covering.

The main jointing arrangements are detailed below:

Butt purlin system for roofing

This system can cater for odd bays and can be used in both inset and oversail applications. The butt system is suitable for buildings with single bays or more, with spans up to 12 metres a roof pitch up to and including 25 degrees.

The butt system offers a simple cleat connection and is intended for smaller buildings, short or uneven spans, or light loading conditions.

The butt system can be used in conjunction with the other jointing arrangements below or as a system in its own right.

Sleeved purlin system for roofing

The sleeved purlin system optimises the use of steel by incorporating sleeves at the penultimate rafter with a staggered sleeving arrangement on internal bays. It is suitable for single or double span lengths in buildings with 2 bays or more, where a heavy end bay system is precluded. It can be used in structures with rafter spacings up to 15m and a roof pitch up to and including 25 degrees.

End bay purlins are single span, with double span purlins on internal bays. The maximum span between rafters is 8 metres, therefore maximum double span length is 16 metres. A sleeve must always occur at a joint position.

The sleeved system caters for spans up to 16m depending on the load to be applied and where the cladding or liner tray offers adequate lateral support by virtue of its screw fixings at 600mm maximum spacings to the purlins. To enable the purlin and sleeve to nest together the sleeve is inverted.

View our Sleeved Purlin Roof System here.

Heavy end bay sleeved purlin system for roofing

The heavy end bay sleeved purlin system uses heavier gauge purlins in the end bays and lighter gauge purlins for the inner bays. It provides the most economic solution by utilising the benefits of the sleeved system but further maximising these by the use of lighter gauge purlins on internal bays.

The heavy end bay purlin system can be used in single or double span lengths in buildings with 5 bays or more and rafter spacings up to 16 metres. Heavier gauge purlins are fitted in the end bays with lighter purlins fitted on internal bays.

Penultimate rafter joints are fully sleeved in the same gauge material as the end bay purlins and are longer than the standard sleeve. Internal bays are sleeved at every joint with standard sleeves of the same gauge material as the inner bay purlins.

In double span situations, the purlins are in maximum lengths of 16 metres.

Z-section purlins as side rails in wall construction

The Z-section purlin can be used as a side rail system for wall construction.

Z-section Butt Side Rail System

The Z-section Butt Side Rail System is suitable for use in smaller buildings comprising one or more bays with short or uneven spans or light loading conditions. It does not use sleeves in the connections.

The system is useful where rails/purlins are inset between the hot rolled supporting member and sleeves cannot be utilised, although this system can also be use on an oversail arrangement and can be utilised on buildings with unequal bays. The system can only cater for single spans.

The Butt System caters for spans up to 16 metres depending on the load to be applied and where the cladding or liner tray offers adequate lateral support by virtue of its fixing: i.e. fixed according to the cladding manufacturer’s instructions.

Z-section Sleeved Side Rail System

The Z-section Sleeved Side Rail System optimises the use of steel by incorporating sleeves to the connections.

The Sleeved Side Rail System caters for spans up to 15 metres depending on the load to be applied and where the cladding or liner tray offers adequate lateral support by virtue of its fixing: i.e. fixed according to the cladding manufacturer’s instructions.

The system can be used in single or double span situations. The maximum double span rail is 15 metres. Therefore, the maximum span between columns is 8m.

For single span systems sleeves are fitted to all the connections on the penultimate rafters and the staggered on internal bays. For double span systems, again sleeves are fitted to all the connections on the penultimate rafters, but as the joints are staggered on a double span system, sleeves are required at every connection.

C-section purlins

As previously mentioned, C-section purlins are so called because of the shape to which their section/profile is formed. They provide a high strength, lightweight support for cladding in wall constructions and can also be used as joists in mezzanine floor arrangements.

C-section Butt Side Rail System

The C-section Butt Side Rail System is suitable for use in smaller buildings comprising one or more bays with short or uneven spans or light loading conditions.

The Butt System caters for spans up to 15 metres depending on the load to be applied and where the cladding or liner tray offers adequate lateral support by virtue of its fixing: i.e. fixed according to the cladding manufacturer’s instructions.

C-section Sleeved Side Rail System

The C-section Sleeved Side Rail System optimises the use of steel by incorporating sleeves at the penultimate rafter with a staggered sleeving arrangement on internal bays.

The Sleeved Side Rail System caters for spans up to 16 metres depending on the load to be applied and where the cladding or liner tray offers adequate lateral support by virtue of its fixing: i.e. fixed according to the cladding manufacturer’s instructions.

The system can be used in single or double span situations. The maximum double span rail is 16 metres. Therefore, the maximum span between columns is 8m.

How are purlins installed?

Purlins, side rails & mezzanine floor joists are generally fixed back to the structural hot rolled steel frame of the building using cleats. These can be welded or bolted to the hot rolled steel section. The cold rolled purlins are then bolted to the cleats using M16 bolts.

The fixings for the wall cladding, roofing, flooring boards, etc. are generally screwed directly into the purlin section. Slot holes in the purlins (and other fixings) are generally used solely for tolerance on site or to accommodate fire wall fixings. With firewalls, the slots are in the cleats (which are supplied by others) & not the section.

All voestalpine Metsec purlins & cleats are supplied complete with pre-punched holes to facilitate fixing.

Where can I get more specific data about Metsec purlins?

For more than 90 years voestalpine Metsec has been helping customers maintain their competitive edge by designing the best steel manufacturing and construction solutions on the market. Our products and services are renowned for delivering high quality, added value solutions with a reputation for short lead times and industry-leading technical and sales support.

We are passionate about ensuring that we provide our customers with the very best in product design, performance and quality to assure complete satisfaction and the highest sustainability credentials.

Further details on voestalpine Metsec Purlins can be found here – https://www.metsec.com/products/purlins-side-rails/purlins/