A purlin is a horizontal steel beam that runs across a straight wall metal building’s rafters or walls, supporting the roof and adding structural rigidity to the whole frame. They’re one of the most important — and most misunderstood — components in a straight wall steel building, and only found in that structural style, not in arch or Quonset-style buildings.
Why Steel Purlins Beat Wood
Unlike wood, steel purlins are lightweight, dimensionally consistent, and free of the natural imperfections wood carries. They expand and contract predictably with temperature changes, and Toro’s purlins are made from galvanized steel with a protective coating for corrosion resistance. Steel also resists rot and pests — a wood purlin can crack from dryness or sag from moisture; a steel purlin holds its shape and strength for the life of the building.
How Purlins Are Load-Tested
During design, purlins are engineered against three load types: dead load (the weight of the accessories and sheeting materials attached to the structure), live load (added weight from repairs, maintenance, or foot traffic on the roof), and environmental load (snow, wind, and seismic forces specific to the building’s location). This ensures each purlin is strong enough to carry every load it will see over the building’s life without sagging or failing.
Purlin Span Types
- Single Span: Supported by bolting the purlin’s web to a cleat or other solid structure at each end.
- Double Span: Supported at both ends and the center — either one continuous purlin or two lapped at the central support.
- Continuous Span: Supported at both ends and a series of equally spaced intermediate points, typically overlapped at each support with a 15% lap length.
Cleats, Laps & Bridging
A few technical details that matter for long-term structural performance: lap length should typically run 15% of the span — if span lengths are unequal, each purlin should have 7.5% of each adjacent span added instead, and laps under 10% (or 5% at any single side) may not hold up structurally over time. Single cleats are standard for most lapped Z-purlin configurations; double cleats are used where successive unlapped purlins butt together, or where reaction loads are high. Bridging prevents purlin rotation during installation and sheeting — maximum recommended bridging spacing is 20 times the purlin depth, capped at 4,000mm, with at least one row used in every purlin span for safety.
Cold-formed C and Z sections are the standard choice for purlins because they’re light, stiff, and strong, while still being easy to transport, fabricate, and assemble. Spacing follows the nodal pattern of the supporting trusses — purlins sit on the truss nodes, not the members themselves, to avoid secondary bending.
Installation Notes
Roof purlins are installed horizontally beneath the roof panels and over the rafters, typically with a felt underlayment or vapor barrier on top. Hot cutting or welding purlins isn’t recommended — the heat affects the material properties of the cold-formed, high-yield steel and can create stress concentrations even with quality welding, while also removing the protective coating that keeps the purlin from corroding. Purlins should stay dry during storage, and assembly requires a proper safety plan regardless of whether it’s a DIY build or handled by a local contractor.
Permits, Codes & Engineering
Purlins are cold-formed steel structural members, which puts them squarely under the specifications published jointly by the American Iron and Steel Institute and CSA Group for cold-formed steel design — the same framework referenced across the whole North American metal building industry, including guidance published by the Metal Building Manufacturers Association. Load calculations for the environmental component (wind, snow, seismic) trace back to the ASCE 7 standard from the American Society of Civil Engineers, referenced by the International Code Council‘s International Building Code, or in Canada, the National Building Code of Canada. Toro’s in-house engineering team sizes and spaces every purlin to these standards for the building’s specific location before it ships.
A Small Component With a Big Structural Job
Purlins don’t get much attention in a building’s marketing photos, but they’re doing real structural work every day — carrying the dead, live, and environmental loads that keep the roof where it belongs. Getting purlin spacing, spans, and lap lengths right isn’t a cosmetic detail; it’s the difference between a roof that performs exactly as engineered and one that develops problems the first time it faces real snow or wind load.
Beyond Purlins: A Complete Building Package
Purlins are one structural component among many that make up a complete straight wall building package. Toro also supplies a full range of accessories — insulation, man doors, turbine vents, sliding doors, trim flashings, and roof accessories — to fully customize a prefabricated steel building to its intended use, all engineered to work together as one system rather than assembled from mismatched parts.
Related Building Types
Straight Wall Buildings | PEMB: The Complete Guide | Arch vs. Straight Wall
Build a Properly Engineered Steel Building
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