FREE QUOTEIf you are planning a new commercial or industrial building, or extending an existing one, roof sheeting is probably not the first thing on your mind. Most of the early conversations focus on floor space, structural steel, and budget. But the relationship between purlin spacing and sheeting span quietly determines how your roof performs for the next few decades, and it is far cheaper to get right at the design stage than to correct after the frame is up. This article explains the basics in plain terms so you can ask the right questions before construction begins.
The Relationship Between Purlins and Sheeting
Purlins are the horizontal structural members that run across the roof frame, and the roof sheeting is fixed directly to them. The distance between purlins, known as purlin spacing, determines how far each sheet has to span without additional support. A wider purlin spacing reduces the amount of steel needed in the frame, which can lower construction costs, but it also demands a thicker or stronger sheet to span that distance safely. Get the balance wrong and you either end up with an over-engineered frame that cost more than it needed to, or sheeting that is under-specified for the span it has to cover.
Why This Decision Cannot Be Made in Isolation
Span and purlin spacing are not decided by the roofing contractor alone. They depend on several factors working together, including:
- The wind region and exposure category of the site
- The pitch of the roof, since flatter roofs behave differently under load
- Expected foot traffic for maintenance access, such as servicing rooftop plant or solar arrays
- The sheeting thickness being specified, which we covered in our guide to Colorbond gauge and thickness for commercial and industrial roofs
- Manufacturer span tables, which set the maximum recommended distance for a given thickness and wind load
This is why the roof should be part of the structural design conversation from the beginning, not an afterthought once the frame is already fabricated.
What Happens When Spans Are Pushed Too Far
Occasionally, a builder will specify wider purlin spacing than the sheeting can properly support, usually in an effort to save on structural steel costs. The result can include visible sheet deflection, a rippled appearance across the roof known as oil canning, and in more serious cases, reduced wind uplift resistance. None of these issues tend to show up immediately, which is part of why they catch building owners off guard months or years after completion, once the building is already in use.
Questions Worth Raising With Your Builder and Roofer Early
Before construction locks in the roof frame, it is worth asking directly what purlin spacing has been designed around and whether that spacing has been checked against the sheeting thickness and profile you intend to use. It is also worth asking whether the roof design accounts for future additions, such as solar panels or rooftop plant, since added point loads can change what span is appropriate. These conversations are far easier to have before steel is ordered than after.
Where Roof Design Fits Into the Bigger Picture
Span and purlin spacing also interact with pitch, since the two work together to determine how effectively a roof sheds water and handles wind loading. If you are still deciding between a flat or pitched design for your new building, our article on flat roofs vs pitched roofs is a useful starting point before finalising your structural drawings.
Getting the Roofing Voice Into the Room Early
The building owners who avoid problems here are usually the ones who bring an experienced roofing contractor into the conversation early, alongside their builder and structural engineer, rather than treating the roof as a line item to be finalised last. Our team has years of experience across commercial roofing projects throughout Melbourne and can review span and purlin specifications before construction locks them in. If you are planning a new build or extension and want the roofing side of the design checked properly, contact us early, and we can advise on the right Colorbond roofing specification for your structure.