For single dwellings and low-rise multi-residential in Australia and New Zealand, cold-formed light-gauge steel (LGS) framing has displaced most timber studwork above two storeys, with AS/NZS 4600 and the NASH Standard governing member design [S4]. The dominant section family is the lipped channel (C-section) in 0.55 to 1.5 mm base-metal thickness (BMT), grade G450 or G550, hot-dip zinc-coated to Z275 or AZ150.
Heavy hot-rolled members (UB, UC, PFC, RHS/SHS to AS/NZS 3679.1) appear only at point loads, transfer beams, and ground-floor podiums, while the wall grid is almost entirely LGS tracks and studs cut to length and screwed with self-drilling hex-head fasteners. Selection therefore reduces to three gates: structural gauge, corrosion coating mass, and fire-resistance level (FRL) lined into the wall assembly.
Section family map: what actually ships to a house site
The hot-rolled families that reach a residential site are limited: 100UB to 250UB for ridge and hip beams, 100PFC to 180PFC for strutting beams over openings, and 200x200x6.0 SHS for verandah posts, all to AS/NZS 3679.1 grade 300PLUS. Most fabricators stock these in 6 m to 9 m lengths, with project-specific plates and cleats cut on automated lines from a 52,000 m² workshop base producing 3,500 t per project [S3].
Light-gauge framing covers the rest of the load path. Standard stud depths are 70, 90, 140, and 250 mm; the trade names C90, C140, and C250 map to nominal 0.45 to 1.5 mm BMT, with track (nogging) sections matching the stud depth. 0.55 mm BMT G300 is the practical floor for external stud framing, 0.75 mm G450 is the default for load-bearing external walls, and 1.0 to 1.5 mm G450/G550 is reserved for multi-storey load cases, lintels, and raking members. Engineers also retain noggings, bracing straps (flat 30x1.0 mm G550), and cyclonic top-hat or tension-rod bracing as separate BOM lines [S4].
Spec gates: BMT, grade, and coating mass
Three numbers decide whether a section is fit for a house. First, BMT must be specified as the base-metal (uncoated) thickness to AS 1397, with the zinc or aluminium-zinc coating mass declared separately: Z275 (275 g/m²) for standard interior and dry-climate exterior, AZ150 (150 g/m² Al-Zn) for moderate marine exposure, and Z350 or Z450 where the dwelling sits within 1 km of a surf coast. A frequent site failure is procurement of AZ50 or Z180 in lieu of the spec, which drops design life below 50 years. [S3]
Second, grade and yield are linked: G300 = 300 MPa yield (ductile, easy to form, used for 0.55 to 0.75 mm BMT trims), G450 = 450 MPa (the workhorse 0.75 to 1.2 mm BMT stud), G550 = 550 MPa (high-strength, used at 0.42 to 0.55 mm BMT where dent resistance is a concern). The Steel Construction New Zealand member directory and SCNZ design guides frame the grade selection against limit-state design per AS 4100 for hot-rolled and AS/NZS 4600 for cold-formed [S4]. Third, geometry: lip size on C-sections is typically 15 to 25 mm and must equal 12 to 25% of the flange width to prevent flange buckling under axial load.
Material context: where structural steel sits in the family
Residential LGS is a thin-coated zinc or aluminium-zinc sheet product, not the same mill feed as the carbon steel plate used in purlins or the alloy steel used in heavy bridge sections. The hot-dip galvannealed or galvanised strip is rolled to AS 1397 and then profiled by a roll-former; the resulting work hardening plus the BMT tolerance (typically ±0.02 mm at 0.75 mm) is what allows designers to publish tabulated capacities rather than run individual calculations on every stud. [S3]
For roof framing, silicon steel is irrelevant, but it is worth contrasting: the stainless steel option in residential is restricted to fixings within 1 km of the surf coast (AS 2312 reference), while standard steel section grades carry the wall frame. Most specifiers therefore buy Z275 G450 cold-formed channels for the bulk of the house and reserve stainless or heavier coatings for fasteners, brackets, and head-flashings, not the primary studs.
Fire and acoustic: lined assemblies, not thicker steel
FRL on a residential LGS wall is set by the plasterboard lining, not by increasing BMT. A standard 13 mm standard plasterboard on a 90 mm stud yields roughly FRL 60/60/60; 2x 13 mm + insulation reaches FRL 90/90/90, and 2x 16 mm fire-rated plasterboard reaches 120/120/120 if the building code requires it (typically Class 1a multi-residential or BAL-FZ construction). Insulating the stud cavity with glasswool or rockwool also lifts Rw by 6 to 10 dB, which is the more common decision driver for attached housing [S5].
For bushfire attack levels above BAL-29, AS 3959 calls for non-combustible framing throughout, and any member within 400 mm of a hearth or solid-fuel appliance needs to comply with AS 1530.1 non-combustibility. Fire-resistant plasterboard from ArcelorMittal's fire-engineered solutions library gives certified FRL ratings per AS 1530.4, and the same FRL logic applies to RHS post-and-beam features in architectural homes [S5]. The cheap shortcut of "bump from 0.75 to 1.2 mm BMT to improve fire" is a mistake: structural capacity rises but FRL is governed by the lining.
Decision comparison: when LGS, hot-rolled, or timber wins
Three criteria sort the section choice on a residential project: span capacity, dimensional stability, and total installed cost per m² of wall. LGS C-studs win spans up to 6 m with 1.2 mm G450 and a built-up box or back-to-back configuration; hot-rolled UB/PFC wins spans above 6 m where deflection governs, particularly over open-plan living or garage door heads; timber wins only where spans are short (under 3 m), the climate is dry (relative humidity below 60% in service), and the builder has no LGS roll-former within 200 km. [S3]
A practical comparison for a 2.4 m stud wall, 0.75 mm BMT G450 LGS, vs 90x45 MGP10 pine, vs 100UB 14.6 hot-rolled: LGS gives non-combustible framing, ±1 mm dimensional tolerance, and Z275 corrosion protection for roughly 10 to 20% more material cost than pine; pine gives slightly better thermal break (R0.4 wall cavity contribution) and is cheaper up front, but requires additional bracing ply on cyclonic sites; 100UB14.6 wins only for portal-framed ground-floor construction or suspended first floors where a 5 m clear span is needed. Fire, acoustic, and termite exposure typically tip the call back to LGS, especially on BAL-12.5 to BAL-29 sites.
Failure modes and inspection gates
The four most common residential LGS failures, in descending order, are: (1) undersized lip on a non-standard roll-former, which lets the flange buckle at the service moment; (2) AZ50 substituted for AZ150 in coastal projects, with red-rust visible inside 24 months; (3) mixed-grade fastener bins, where 10-gauge hex-head screws rated for 1.0 mm G450 are driven into 1.5 mm G550 and snap; (4) field-modified BMT, where a 0.75 mm track is swapped for 0.55 mm to ease a tight fit and the wind-rated shear capacity drops by roughly a third. A field inspection checklist therefore reads: verify the BMT with a micrometer, verify the coating class with a portable XRF or the mill certificate, and verify the grade stamp on each delivered pack against the BOM [S6].
Calculators and cut-list tools are now standard, with 3-minute steel-weight estimators handling the BOM directly off the framing plan, which is how most LGS detailers confirm order mass before procurement [S6]. If the calculated mass does not match the workshop's order within roughly 5%, the specifier has either mis-sized a member or the BOM has drifted to an off-spec section.
For a more direct comparison of how related fasteners and connectors are specified, pneumatic nail-gun and concrete-fastener selection walks through the connector side that pairs with the steel frame, and deformed rebar selection for school buildings covers the companion rebar spec where the residential slab ties into a structural core.