Z purlins are critical components in steel-framed buildings, providing essential support for both roofs and walls. Choosing the right Z purlin size matters because not all Z purlins are the same. Their size, strength and profile must be carefully matched to the span, load and environmental conditions of your structure to ensure safety, durability and cost-efficiency.
At Cardinal Steels we manufacture a wide range of Z purlins and C sections in various thicknesses and depths to suit roofing and cladding projects of all sizes.
In this guide, we explain what Z purlins are, why selecting the correct size is important, the key factors that influence Z purlin size and spacing, and how the size is calculated.
What Are Z Purlins?
Z purlins are Z shaped steel beams used as horizontal structural supports for roof and wall panels in steel-framed buildings.
Key points:
- They are made from cold-rolled galvanised steel for strength and corrosion resistance.
- They act as secondary structural members, transferring loads to the main frame.
- They support dead and live loads from roofing and wall sheets.
Advantages over other profiles:
- When lapped at supports, Z purlins can achieve longer spans than C sections of a similar size.
- The Z shape allows purlins to be lapped at supports, creating continuous runs across several bays.
They are lightweight but strong, reducing the overall weight of the structure.
Why the Correct Z Purlin Size Matters
Selecting the correct Z purlin size is essential for:
- Safety: Prevents buckling, bending or structural failure.
- Stability: Maintains roof and wall alignment.
- Cost efficiency: Avoids unnecessary steel usage.
- Component fit: Ensures compatibility with beams, trusses and cladding.
Incorrect sizing can cause sagging, excessive deflection or expensive repairs.
What Factors Influence Z Purlin Size?
Z purlins can be made in various sizes, lengths and depths. When selecting the correct size Z purlin, you must consider the following factors.
The Span Length
- The span length is the distance the purlin stretches between supports such as beams or trusses.
- Longer spans require deeper or thicker purlins to resist bending.
- Span is usually the main factor in choosing a purlin depth.
The Load Requirements
When choosing the right Z purlin, you must consider the load it will be carrying.
- Dead load: Roofing sheets, insulation, ceilings and the weight of the purlins themselves.
- Imposed and environmental loads: Snow, wind, temporary equipment and maintenance access.
- On lightweight metal roofs, wind uplift can be greater than the weight of the roof, so purlins and fixings need to be checked for upward load as well as downward load.
The Purlin Spacing
- The distance between two adjacent Z purlins affects how much load each purlin carries.
- Closer spacing reduces the load on each purlin but increases steel usage.
- Every roof sheet has a maximum unsupported span, which limits how far apart the purlins can be. Check the data for the profile you are using, such as our box profile roof sheets or corrugated roof sheets.
There is no single spacing that suits every roof. The right Z purlin spacing depends on the sheet profile and the loads involved, and the span tables in our Z purlin brochure are a good starting point.
Material Properties
- The grade and thickness of the steel influence the strength and stiffness of Z purlins.
- A higher grade steel allows a shallower purlin while still maintaining load capacity.
- Galvanised or coated steel improves corrosion resistance.
At Cardinal Steels, we manufacture our purlins in house at our Bromsgrove facility, which lets us control quality and cut each purlin to the length your job needs.
The Roof Pitch and Shape
The roof angle affects how loads act on the purlins.
- On a steeper roof, more of the load acts along the slope, and snow tends to slide off more readily.
- Low pitch roofs can hold more snow, so they may need stronger Z purlins to resist bending.
Environmental and Site Conditions
- Areas prone to high winds or heavy snowfall may require larger Z purlins or additional bracing.
- Corrosive environments, such as coastal or industrial areas, may require galvanised or coated steel for added longevity.
Z Purlin Sizes Available from Cardinal Steels
We manufacture Z purlins and C sections in the following sizes. All purlins and eaves beams can be cut to length.
| Section depths | CS 121, CS 140, CS 177, CS 200, CS 235, CS 265 |
| Thicknesses | 1.4mm, 1.6mm, 1.8mm, 2.0mm, 2.5mm |
| Standard lengths | Up to 6.100m, cut to length |
| Common stock depths | 177mm and 200mm |
| Stock codes | CS17716 (177mm depth, 1.6mm thickness); CS19020 eaves beam (190mm depth, 2.0mm thickness) |
Our Z purlins are suitable for both insulated and uninsulated roofs and walls, and are widely used in agricultural buildings and industrial buildings.
How Is Z Purlin Size Calculated?
Taking into account the factors above, the calculation process is based on:
1. Determine Loads
You must know how much weight the purlin will carry, calculating both the dead and imposed loads, including wind uplift.
2. Measure Span
Identify the distance between supports to prevent excessive deflection.
3. Calculate Bending Moment
The purlin must resist the load without excessive bending or deforming.
4. Check Stability
The flanges and lips of a Z purlin help prevent twisting or buckling under heavy loads, ensuring the purlin stays straight and strong over time.
Frequently Asked Questions
What size Z purlin do I need?
It depends on the span between frames, the purlin spacing, the roof loads and how exposed the site is. Use the span and load tables in our Z purlin brochure as a starting point, and have the final size confirmed by a structural engineer.
How far apart should Z purlins be spaced?
Spacing depends on the roof sheet profile and the loads the roof must carry. The sheet has a maximum unsupported span, and the purlins must be close enough together to stay within it, so there is no single figure that suits every roof.
How thick should Z purlins be?
Thickness is chosen to suit the load and span. We supply Z purlins from 1.4mm to 2.5mm thick. A thicker section of the same depth will carry more load.
How far can Z purlins span?
Span depends on the depth and thickness of the section, the load it carries and whether it is lapped across supports. Our standard lengths go up to 6.100m, and the brochure sets out spans for each section.
What is the difference between Z purlins and C sections?
Z purlins can be lapped at supports to create continuous runs, which makes them the usual choice for the main roof. C sections do not lap in the same way and are often used at eaves, as side rails or where a plain channel suits the connection better.
Conclusion
Choosing the right size Z purlin is crucial for the safety, stability and longevity of any steel-framed building.
By carefully considering key factors such as span length, load requirements, material strength and environmental conditions, you can achieve a durable, efficient and cost-effective structural solution for your project.
At Cardinal Steels, our detailed Z purlin brochure provides the span and load tables you need to make informed decisions about Z purlins. Once you know the sizes you need, contact our team and we will manufacture and cut your purlins to length. We can also supply the roof fixings and steel roof sheets to complete your roof.
This guide is for general information only and is not a substitute for a structural calculation for your building.
