Steel Purlin Types, Sizes and Uses

A steel purlin is a cold-formed secondary member that supports roof and wall cladding across the main frames of a steel building. Z purlins are the usual choice for roof spans because they lap and suit continuous multi-span roofs; C purlins are common for shorter spans and as wall girts. The size, spacing and steel grade always depend on the span, design loads, roof slope and local code — there is no single universal purlin size.

This guide explains the types, sizes and uses so you can specify purlins correctly on your next project.

What a steel purlin is and where it is used

A purlin is a secondary structural member. It spans between the primary frames (portal frames, trusses or rafters) and carries the roof sheeting or wall cladding, transferring wind and snow loads back into the main frame.

Purlins appear in almost every type of steel building: workshops, warehouses, agricultural buildings, cold stores, and industrial plants. They are usually cold-formed from galvanised steel strip, which gives a good strength-to-weight ratio and a clean, repeatable profile.

Z purlin vs C purlin: the core difference

The two most common purlin profiles are the Z section and the C section. Both are cold-formed, but they behave differently:

Aspect	Z purlin	C purlin
Typical role	Roof purlins on longer, multi-span roofs	Shorter roof spans and wall girts
Lapping	Can be lapped at supports for continuity	Usually not lapped in the same way
Bending efficiency	Higher in its strong axis when used in continuous spans	Simpler to detail and connect
Connection	Bearing and lapped at supports, often with cleats	Bearing at supports with cleats or brackets
Suitability	Longer spans, low to medium roof slopes	Short spans, simple frames, wall applications

The choice is not “one is better than the other” — it depends on the span arrangement, roof slope, load combination and connection design of the project.

What Z and C purlins are used for

Z purlins are most commonly used as roof purlins in portal frame and truss buildings. Their shape allows adjacent purlins to lap over the frame, creating a continuous member across two or more bays, which improves load efficiency on longer spans.

C purlins are frequently used where spans are shorter or where a simple section is easier to detail — for example at ridges, eaves, openings, and as wall girts supporting cladding. C sections are also used for secondary framing such as window and door surrounds.

How purlin sizes are described

Purlin sizes are usually given as a nominal section designation such as C100x50x20x2.0 or Z150x65x20x2.5. Reading from left to right, these figures describe the depth, flange width, lip length and steel thickness.

• Depth (the vertical dimension) is the main driver of bending capacity.
• Flange width and lip length control local buckling behaviour and fixability.
• Steel thickness (typically 1.5–3.0 mm in cold-formed purlins) affects capacity and weight.

The actual size for a project is determined by calculation from span, spacing, roof slope, cladding weight and wind or snow loads — never guessed. A purlin that works for a 6 m bay in a light industrial shed may be completely different from what a heavy industrial building needs.

Spacing and span: what drives the numbers

Purlin spacing usually follows the cladding system. Common spacing values in the industry range roughly from 1.2 m to 2.0 m depending on the roof sheet span capability, but the governing factors are:

• the cladding’s own spanning capacity (sheet profile and gauge),
• the roof slope and wind uplift,
• snow or maintenance loads,
• the purlin section and steel grade,
• the deflection limit required for the roof finish.

The purlin span is normally the frame spacing. Where frames are spaced wider, deeper purlin sections or stronger profiles are needed. This is why purlins cannot be sized from a single generic table — the design loads of the specific site always apply.

Steel grade and coating

Most cold-formed purlins are supplied in structural steel grades equivalent to Q235 or Q345, or international equivalents, depending on the project standard. For outdoor or corrosive environments, the galvanised coating matters: a typical specification such as Z275 describes the zinc coating mass per square metre. The right grade and coating depend on the local code, the environment and the project specification.

How purlins are connected and fixed

Purlins are normally fixed to the primary frame with cleats or brackets welded or bolted to the rafter, then bolted or screwed through the purlin. Roof sheeting is then screwed to the purlin flange. Connection details — cleat spacing, bolt sizes, whether laps are bolted — come from the structural drawings, not from general habit.

ZhongSai practical perspective

When you send a project enquiry for a steel building with purlins, ZhongSai coordinates the purlin design with the structural engineer and includes purlins as part of the roof and wall cladding package, or supplies them separately with the structural steel. Because our manufacturing bases fabricate and pack purlins with the main steelwork, the sections, marking and container loading are coordinated in one shipment, which reduces missing-part issues on site.

What buyers should prepare

• Roof and wall layout drawings or frame spacings (purlin span and spacing)
• Design loads: wind zone, snow load, roof dead load, any hanging loads
• Roof slope and cladding type (sheet profile and gauge)
• Preferred standard: GB, EN, ASTM or the local project code
• Environment class if corrosion protection matters (coastal, chemical, industrial)
• Whether purlins are to be supplied with the steel package or with the cladding package

Responsibility boundary

ZhongSai provides: engineering coordination, purlin detailing, fabrication, galvanising coordination, marking, packing and container loading, export coordination, and installation technical guidance for purlin fixing.

Local parties normally handle: local permits, registered engineer approval of the design, foundations and civil works, physical erection, and authority submissions. The structural design and connection checks are always confirmed against the project drawings and local code.

FAQ

Can I use Z purlins on a flat roof? Z purlins work on low to medium slopes; on a flat or very low slope, drainage and connection design become the deciding factors rather than the profile itself.

Are Z purlins stronger than C purlins? Not inherently. A Z purlin is often more efficient in continuous multi-span roof arrangements, but capacity depends on the section, thickness and how it is connected.

What spacing should purlins be? Spacing follows the cladding span capacity and the design loads. Typical values are often around 1.2–2.0 m, but the project calculation always governs.

Do I need purlins if I have a truss roof? Yes. Trusses carry the main loads, but purlins are still the members that support the roof sheeting between trusses.

Can C purlins be used as wall girts? Yes, C purlins are commonly used as wall girts and around openings.

Are purlins included in a steel building package? Most steel building suppliers include purlins in the structural steel package. Confirm whether they are coordinated with the cladding system for fixing and spacing.

Do purlins need to be galvanised? In most outdoor buildings, yes — galvanised purlins resist corrosion from condensation and weather. The coating grade should match the environment.

What does a purlin designation like Z150x65x20x2.5 mean? Depth 150 mm, flange width 65 mm, lip 20 mm, steel thickness 2.5 mm, in a Z-shaped cold-formed section.

Project Inquiry

Every building is different, and purlin selection depends on real spans, loads and local codes. Send your project drawings and requirements and our engineering team will coordinate the purlin specification with the structural design.

Send Project Drawings to get a coordinated purlin and steelwork package for your project.