What it is called, by market
One concept, four markets. Where a name means something else locally, the card says so rather than leaving you to find out on site.
United Kingdom
Same word, different thingpurlin
also under-purlin, side purlin, Z-purlin
In a cut timber roof it supports the rafters at mid-span; in a steel portal frame the same word names the cold-formed Z or C section spanning between frames and carrying the cladding.
Australia
Same word, different thingpurlin
also top hat purlin, C-purlin, batten
As the UK, and in Australian sheet-roofing practice the light top-hat section carrying the sheeting is often called a batten rather than a purlin, with the distinction resting on size rather than on position.
United Statesyours
Same word, different thingpurlin
also roof purlin, Z-purlin, girt
Most often the metal-building sense. A girt is the same kind of member on a WALL rather than a roof, and the two are frequently listed together on a schedule.
Canada
Same word, different thingpurlin
also purline, Z-purlin, girt
As the United States, with purline appearing as an older spelling in timber-framing documents.
The governing standard, by market
United States
AISI S100 / MBMA Metal Building Systems Manual
Design of cold-formed steel members, including purlins and their bracing.
United Kingdom
BS EN 1993-1-3
Design of cold-formed thin-gauge steel members and sheeting, including purlin systems.
Calculators for this
Each works in either measurement system, and the terminology on the page follows whichever market you have selected.
Frequently asked questions
- Is a purlin the same thing in a timber roof and a steel building?
- The word is the same and the position in the load path is not, which is why purlin spacing means two different calculations. In a traditional cut roof the rafters run from eaves to ridge and would be too slender to make the trip alone, so a purlin runs along the roof under them and halves the span each rafter has to manage — the purlin supports rafters, and the rafters support the covering. In a portal-frame or metal building there are no rafters: the purlins span between the frames and the sheeting is fixed straight onto them, so purlin spacing is set by what the SHEET can span, not by what the rafter needs. Read a purlin spacing figure out of the wrong system and it is not conservative or generous, it is answering a different question.
- What decides purlin spacing?
- Whichever member is being rescued by the purlin. Where purlins support rafters, the spacing comes from the rafter: its section, species and grade give an allowable span under the roof load, and the purlins are placed to keep every rafter segment inside it — so a heavier rafter can accept purlins further apart. Where the sheeting sits directly on the purlins, the spacing comes from the manufacturer's load-span table for that profile and gauge, checked at the wind uplift for the site, and tightened in the perimeter zones where suction is highest. In both cases the purlin itself then has to span between its own supports, which is a separate check people frequently skip: getting the spacing right does not make the purlin adequate.
- What is a girt, and why does it appear next to purlins?
- It is the same kind of member turned through ninety degrees onto a wall. In a metal building the cladding needs a line of support at intervals in both places: purlins do it on the roof and girts do it on the walls, and both are typically the same family of cold-formed Z or C sections from the same supplier, differing in spacing and in how they are loaded. The practical difference is the direction of the main load — a purlin carries gravity load from snow and the roof build-up, so its strong axis works with it, while a girt mainly resists wind pressure and suction across its face and carries the cladding's self-weight along its length. They appear together on schedules because they are ordered together, and mixing up which line of a schedule is which is an easy way to get the wrong lengths delivered.