Key takeaways
- The roof sheet sets the spacing. Lysaght's Australian TRIMDEK catalogue publishes 1,100 mm on a single span and 1,900 mm on an internal span at 0.42 mm sheet.
- The section size then follows, because the purlin or batten must span truss to truss within its published capacity.
- A light-gauge steel truss roof uses battens; a mild-steel portal frame uses C or Z purlins. The two are not interchangeable.
- Span tables carry conditions - support thickness, bridging, wind region - and stop applying when one changes.
On this page7 sections
What is the difference between a purlin and a roof batten?
A purlin is the member that spans between portal frames or rafters and carries the roof above it. A roof batten is the lighter top-hat or channel section fixed across truss top chords to give the roof sheet something to screw into. A mild-steel portal frame roof uses purlins; a light-gauge steel truss roof uses battens.
Both are secondary steelwork, but they are sized by different people. A purlin spans between the primary frames and is sized by the project engineer from the frame layout; a batten comes with the truss system and is sized by the truss designer from the truss centres. Asking for purlins when the roof is a light-gauge truss system usually produces a quotation for the wrong thing.
| Question | Purlin | Roof batten |
|---|---|---|
| Sits on | Portal frames or rafters | Truss top chords |
| Usual section | C or Z section; Lysaght's MAX-SPAN range runs 152-254 mm deep | Top-hat or small channel, much lighter than a purlin |
| Typical roof | Factory, warehouse, mild-steel portal frame | Housing and commercial light-gauge steel truss roof |
Ducero quotes sections, battens and trusses from your drawings, not from a section name. Send the roof layout through the Ducero contact page to have them checked against your sheet. Which system a building needs at all is covered in our guide to light-gauge steel and mild-steel frames.
What decides how far apart purlins and battens are spaced?
The roof sheet decides it. Sheet manufacturers publish a maximum support spacing for each profile, derived from load testing, and it changes with the sheet's base metal thickness and with whether a given support is a single, end or internal span. Wind uplift at the site, calculated in Malaysia to MS 1553:2002, then decides which of those spacings is actually safe.
The sheet manufacturer prints the figure. Lysaght's TRIMDEK Design & Installation Guide, an Australian catalogue, publishes one for each thickness and span position:
| Type of span | 0.42 mm BMT | 0.48 mm BMT |
|---|---|---|
| Single span | 1,100 mm | 1,600 mm |
| End span | 1,300 mm | 1,850 mm |
| Internal span | 1,900 mm | 2,600 mm |
| Unstiffened overhang | 150 mm | 200 mm |
| Stiffened overhang | 300 mm | 350 mm |
Conditions travel with a table like that one. The guide states these spacings "are based on testing in accordance with AS 1562.1, AS 4040.1 and AS 4040.2", that "roof spans consider both resistance to wind pressure and light roof traffic (traffic arising from incidental maintenance)", and that "support must not be less than 1mm BMT". Its pressures also assume "buildings up to 10m high in Region B, Terrain Category 3" - an Australian wind region, not a Malaysian one.
Capacity also falls away quickly as supports move apart: Lysaght's published TRIMDEK serviceability capacities at 0.42 mm drop from 4.98 kPa at 600 mm centres to 0.53 kPa at 2,400 mm on a single span, with the strength capacities falling from 10.25 kPa to 2.75 kPa over the same range. Widening centres costs capacity far faster than it saves money.
A Malaysian roof takes its wind actions from MS 1553:2002, which sets out "procedures for determining wind speeds and resulting wind actions to be used in the structural design". The sheet's table and the site's wind actions are two documents, and both have to be satisfied. Choosing the sheet is covered in our guide to choosing metal deck roofing.
What decides the size and thickness of the section?
The span between supports and the load the section carries. Once spacing is fixed by the sheet, the purlin or batten has to span from truss to truss, or frame to frame, within its published capacity. That capacity comes from a published table, and which code the table was calculated to is worth knowing before you rely on it.
A manufacturer's table may have been calculated to its own country's code, so check which one it names. Lysaght's MAX-SPAN manual says its capacities were worked out "in accordance with the provisions of AS/NZS 4600". The Malaysian adoption for this class of member is MS EN 1993 Part 1-3:2019, adopted as a Malaysian Standard on the recommendation of the Working Group on Cold-formed Steel Structure. Have the project engineer confirm which code applies to your roof.
How the section is joined over its supports matters as much as its size. Lysaght's MAX-SPAN purlins and girts design manual states that its Zed sections "may be used over single spans, lapped continuous and unlapped continuous spans in multi-bay buildings", that "lapped continuous spans result in a considerable capacity increase in the system", and that its Cee sections "cannot be lapped". That is why multi-bay factory roofs tend to use Z purlins.
A capacity table is not a property of the steel alone. The same manual records that its capacities were calculated "using approved Esy-Lok bridging systems, bolting and other accessories", so a section bought without that bridging sits outside the table it came from.
What thickness and grade do purlins and battens come in?
Thickness is published per section, not per depth. Lysaght lists its MAX-SPAN Zed and Cee purlins from 1.2 mm to 2.4 mm at web depths of 152, 203 and 254 mm, roll-formed from GALVASPAN steel "complying with AS 1397", and publishes TRIMDEK sheet at 0.42 and 0.48 mm.
A light-gauge truss member is different: its thickness and grade come out of the truss system's engineered design, not off a catalogue size, which is why Ducero publishes neither per profile and confirms both from your drawings. CIS 12:2009 covers the product class, and coating class is the question worth asking for a Malaysian site.
Battens follow the same logic from the other direction. A batten's spacing is the sheet's published maximum support spacing, and its size comes from the truss centres and the truss designer, so there is no universal millimetre figure to quote. Take the spacing from your own supplier's Malaysian catalogue rather than an overseas one.
What should you send a supplier to get the right sections quoted?
Six things decide whether a quotation is usable: the sheet profile and its thickness, the truss or frame spacing and span type, the location and height for wind, the roof layout, the distance to the coast, and whether you want supply only or supply and install. Without them a supplier can price a section by name but cannot confirm it suits the roof.
- The roof sheet profile and base metal thickness, with its published support spacing.
- The spacing between trusses or portal frames, and the span type at each support - single, end or internal.
- The building's location, height and surrounding terrain, for wind actions under MS 1553:2002.
- The roof layout: spans, overhangs, valleys, penetrations, and any plant or solar array.
- The distance to the coast or to heavy industry, which decides the coating class.
- Whether you need supply only, or supply and install.
A supplier who quotes without any of this is pricing a length of steel, not a roof. This page is about the sections and their spacing; what else to check before you appoint a supplier is a separate question. The Ducero roofing page covers what a roofing package is checked against, with the system-level questions in our guide to choosing a steel roof truss system.
Which sections does Ducero supply?
Ducero supplies light-gauge steel roof trusses and roof covering, and its sister company Superspan supplies the light-gauge steel profiles that go with them. The group's published steel listing includes C-section channels C75 and C73 and roof battens B30 and B35. The group also builds mild-steel portal frames and metal deck roofs.
Most of the group's published roof work is the battened kind. Of the lines published in the project archive, 465 mention light-gauge steel trusses and 11 mention mild-steel trusses - so on the great majority of those roofs it is a batten, not a purlin, that carries the sheet. The steel in the frames and trusses is BlueScope TRUECORE® and Blue ZACS®.
The group works the other way round as well. Mild-steel portal frames, metal deck roofing and any design, supply and install solution your structure needs sit alongside the light-gauge line. Ducero is a CIDB-registered contractor, Grade G5, on a certificate valid to 18 June 2028. Separately, CIDB publishes a quality-assurance standard for the product class itself, CIS 12:2009 - worth naming in a specification.
Truss and section packages are quoted from your drawings, and size and thickness are confirmed against them. Send the roof layout and the sheet profile through the contact page, or read what the Ducero steel page covers first.
Frequently asked questions
Can the existing purlins be reused when an old metal roof is replaced?
Sometimes, but it has to be checked rather than assumed. The new sheet may have a different maximum support spacing from the one the purlins were set out for, and they may have corroded around the old fixing holes. Our guide to repairing, reroofing or replacing a metal roof covers the wider decision.
Do roof battens have to line up with the truss top chords?
Yes. A batten carries the roof sheet only as far as the next truss, so every batten run needs a top chord beneath it at the centres the design assumed. Shift a truss on site to clear a penetration and that bay has to be re-checked.
What coating do purlins need near the Malaysian coast?
A heavier one than inland, and the manufacturer's own environment classes are the guide. Lysaght supplies its MAX-SPAN purlins with a Z350 zinc coating as standard and calls for a Z450 minimum coating mass in severe corrosive environments, treating an environment as non-aggressive only more than 1,000 m from rough surf, 750 m from industrial emission and 300 m from calm salt waters - figures from its Australian manual, so confirm the equivalent with your supplier.
Does a solar array change the purlins or battens a roof needs?
It can. Mounting rails transfer the array's weight and additional wind uplift into the sheet's supports, often at points the original design did not allow for, so capacity and fixings are checked before the array is specified. See our guide to solar mounting on a metal roof.
Sources
- Department of Standards Malaysia - MS 1553:2002, Code of Practice on Wind Loading for Building Structure
- Department of Standards Malaysia - MS EN 1993 Part 1-3:2019, Eurocode 3: Supplementary rules for cold-formed members and sheeting
- Lysaght - TRIMDEK Design & Installation Guide
- Lysaght - MAX-SPAN Purlins and Girts Design Manual
- CIDB Malaysia - Construction Industry Standard (CIS), including CIS 12:2009