What is AS 4254 and what does it cover?

AS 4254 is the Australian Standard for ductwork construction in air-handling systems. It is published in two parts: AS 4254.1:2012 covers flexible duct (the foil-wrapped reinforced spiral type used for short runs to diffusers and registers), and AS 4254.2:2012 covers rigid sheet-metal ductwork (rectangular and round). AS 4254.2 is the more frequently cited part — it specifies sheet gauge, seam construction, reinforcement, pressure class, leakage class and fabrication tolerance for rectangular and spiral round duct. The standard is called up by the National Construction Code (NCC) Volume One Section J (energy efficiency) and Section F4 (health and amenity), and by most Australian project specifications written by AIRAH-aligned consultants.

What are the AS 4254 pressure classes?

AS 4254.2 defines four pressure classes for rigid duct: Class 1 (low pressure, up to 500 Pa positive / -500 Pa negative), Class 2 (medium pressure, up to 1000 Pa positive / -750 Pa negative), Class 3 (medium-high pressure, up to 1500 Pa positive / -750 Pa negative), and Class 4 (high pressure, up to 2500 Pa positive / -750 Pa negative). Most commercial Australian projects specify Class 2 for low-velocity supply and return systems; Class 3 is common for high-velocity supply trunks in data centres and hospitals; Class 4 is used for industrial dust collection and process exhaust. The pressure class drives the sheet gauge, reinforcement spacing and seam type — higher pressure requires thicker steel and tighter reinforcement.

What are the AS 4254 leakage classes?

AS 4254.2 defines five leakage classes (A through E) measured as L/s per square metre of duct surface at the test pressure. Class A is the tightest (allowing 0.4 L/s/m² at 250 Pa), Class B allows 0.9 L/s/m², Class C allows 1.4 L/s/m², Class D allows 2.4 L/s/m², and Class E allows 4.0 L/s/m². The class roughly maps to SMACNA Seal Class A/B/C: AS Class A ≈ SMACNA Seal Class A, AS Class B ≈ SMACNA Class B, AS Class C-E ≈ SMACNA Class C and unsealed. Data centres and hospitals typically specify Class A or B; commercial office work specifies Class C; residential or low-velocity transfer ducts can be Class D or E. The class determines the seam sealant requirement and the leakage test acceptance criterion.

How does AS 4254 compare to SMACNA and EN 1505?

AS 4254, SMACNA HVAC Duct Construction Standards and EN 1505 cover the same domain but with different terminology and slightly different test methods. AS 4254 Class 2 ≈ SMACNA 1 in. w.g. (250 Pa) ≈ EN 1505 Class B. AS 4254 Class 4 ≈ SMACNA 10 in. w.g. (2500 Pa) ≈ EN 1505 Class D. Leakage class equivalences: AS Class A ≈ SMACNA Seal Class A ≈ EN 1505 Class D, AS Class B ≈ SMACNA Class B ≈ EN 1505 Class C. The Australian, US and European fabricators all produce essentially the same physical duct at the same pressure class — only the certificate language differs. SBKJ machinery is configured to deliver any of the three standards from the same hardware with PLC recipe changes for sheet gauge and reinforcement spacing.

What sheet gauge does AS 4254 require for typical commercial duct?

For Class 2 pressure (most commercial work) AS 4254.2 specifies minimum sheet thicknesses scaled with duct dimension. Up to 750 mm: 0.6 mm. 751 to 1500 mm: 0.8 mm. 1501 to 2250 mm: 1.0 mm. Above 2250 mm: 1.2 mm. For Class 3 pressure, gauges step up by approximately one increment (0.6→0.8, 0.8→1.0, etc.). For Class 4 pressure, 1.0 mm is the minimum for ducts up to 1000 mm and 1.5 mm above that. Stainless steel substitutes follow the same gauge logic but at slightly lower minimums (since stainless has higher tensile strength). All SBKJ duct lines run 0.5 to 1.5 mm galvanized as standard; thicker stainless is a configuration upgrade.

What seam types does AS 4254 accept?

AS 4254.2 accepts four longitudinal seam types for rectangular duct: Pittsburgh lock (snap or hammer), button-punch snaplock, machine-formed grooved seam, and welded longitudinal seam. Pittsburgh lock is the default for galvanized commercial work up to Class 3 pressure. Welded seam is required for Class 4 pressure on small ducts and for any duct in food, pharma, hospital isolation or process exhaust service. Spiral round duct uses the helical lock seam formed continuously during tube manufacture (no longitudinal seam to fail) or a fully-welded longitudinal seam where the application demands it. Transverse joints between duct sections can be S-cleat, TDF flange, angle flange or welded — TDF is the most efficient for high-volume production with auto duct lines.

What does AS 4254 say about duct sealing?

AS 4254.2 references duct sealants by performance, not by brand: the sealant must meet AS 1530.3 (fire-hazard properties), AS 4254.2 Appendix B (sealant test for leakage), and the project's air-quality requirements (low-VOC for indoor zones). Common sealants are water-based mastic for Class B and C leakage, butyl rubber tape and brush-on sealant for Class A. Pressure-sensitive duct tape alone is NOT acceptable to AS 4254 in any leakage class — it must be combined with a brush-on or trowel-on sealant. SBKJ duct ships with all seams formed but unsealed; the contractor applies sealant on site after installation and before insulation.

How is AS 4254 leakage testing performed?

AS 4254.2 Appendix C specifies the leakage test: a section of duct is pressurised to the project's specified test pressure (typically 250 Pa for Class 2, 1000 Pa for Class 3, 2500 Pa for Class 4) using a calibrated fan, the leakage is measured with an orifice plate or flow meter, and the result is divided by the duct surface area to give L/s per square metre. The result is compared against the project leakage class limit. Testing is usually performed on 10 to 25 percent of installed duct selected randomly; if more than 10 percent of tested sections fail, the contractor must remediate and retest. SBKJ duct manufactured on SBAL-V at SMACNA Seal Class A typically passes AS Class A or B without remediation, provided the contractor applies sealant correctly during installation.

Standards Reference

AS 4254 Australian Ductwork Construction Code — Reference Guide for HVAC Duct Fabrication

AS 4254.2 is the working specification every Australian HVAC duct fabricator reads on day one and every project consultant cites in every duct schedule. This reference distils the parts that matter for shop-floor production — pressure class, leakage class, sheet gauge, seam construction, reinforcement and tolerance — into the format SBKJ engineering uses internally to scope projects in the Australian market and to configure PLC recipes for AS-compliant first-article duct.

Maintained by SBKJ Engineering Team · Published May 9, 2026 · Reviewed by SBKJ Senior Engineers

What AS 4254 covers and what it doesn't

AS 4254 is the Australian Standard titled Ductwork for air-handling systems in buildings. It is published in two parts:

AS 4254.1:2012 — Flexible duct. Covers foil-wrapped reinforced spiral flex duct used for short branch runs to ceiling diffusers and registers. Specifies materials, length limits (typically 3 m maximum), bend radius, support intervals, and termination requirements.
AS 4254.2:2012 — Rigid duct. Covers rectangular and round sheet-metal ductwork. Specifies sheet gauge, seam construction, reinforcement, pressure class, leakage class and fabrication tolerance. This is the part SBKJ duct lines manufacture against.

What AS 4254 does not cover:

Mechanical ventilation rates — that's AS 1668.2
Fire and smoke control — that's AS 1668.1
Fire dampers themselves — that's AS 1682 series
Acoustic performance — that's typically per project specification, often referencing AS/NZS 2107
Insulation — that's AS/NZS 4859
Hospital pressure relationships — that's AS/NZS 2243.3 and ASHRAE 170
Energy efficiency — NCC Section J handles that

Pressure classes (AS 4254.2 Table 3.1)

AS 4254.2 defines four pressure classes:

Class 1 (low pressure) — up to 500 Pa positive / -500 Pa negative. Used for low-velocity transfer ducts, residential return air, and exhaust ducts at the diffuser end. Roughly equivalent to SMACNA 2 in. w.g.
Class 2 (medium pressure) — up to 1000 Pa positive / -750 Pa negative. The default for commercial office and retail supply / return systems. Roughly equivalent to SMACNA 4 in. w.g. Most Australian commercial duct is Class 2.
Class 3 (medium-high pressure) — up to 1500 Pa positive / -750 Pa negative. Common for high-velocity supply trunks in data centres, hospitals, large retail, and pressurised stair shafts. Roughly equivalent to SMACNA 6 in. w.g.
Class 4 (high pressure) — up to 2500 Pa positive / -750 Pa negative. Used for industrial dust collection, process exhaust, kitchen exhaust risers and some smoke spill systems. Roughly equivalent to SMACNA 10 in. w.g.

Leakage classes (AS 4254.2 Table 4.1)

Five leakage classes, measured at the project's specified test pressure:

Class A — 0.4 L/s/m² at 250 Pa. Tightest. Required for data centres, semiconductor cleanrooms, hospital isolation rooms.
Class B — 0.9 L/s/m². Common for commercial office, hotel and retail VAV systems where energy efficiency matters.
Class C — 1.4 L/s/m². Common for commercial office and retail at standard specification.
Class D — 2.4 L/s/m². Residential, transfer ducts, low-velocity exhaust.
Class E — 4.0 L/s/m². Loose tolerance, used for non-critical exhaust where leakage is acceptable.

The leakage class drives the sealant requirement: Class A demands brush-on butyl mastic on every seam and joint; Class B and C accept water-based mastic; Class D and E accept duct tape over sealed seams. SBKJ duct ships with seams formed but unsealed — the contractor applies project-specified sealant on site.

Sheet gauge (AS 4254.2 Table 3.2)

Minimum sheet thickness for rectangular duct in galvanized steel, by pressure class and duct dimension (longer side):

Duct longer side (mm)	Class 1	Class 2	Class 3	Class 4
Up to 300	0.5 mm	0.6 mm	0.8 mm	1.0 mm
301–750	0.6 mm	0.6 mm	0.8 mm	1.0 mm
751–1500	0.6 mm	0.8 mm	1.0 mm	1.2 mm
1501–2250	0.8 mm	1.0 mm	1.2 mm	1.5 mm
Above 2250	1.0 mm	1.2 mm	1.5 mm	1.6+ mm

For stainless steel, the gauge can step down by approximately one increment (e.g., 0.8 mm galvanized → 0.7 mm stainless) due to higher tensile strength, but most fabricators run stainless at the same gauge as galvanized for production simplicity. Round duct gauges scale similarly with diameter; SBKJ spiral tubeformers handle 0.4–2.0 mm galvanized (SBTF-1602 and SBTF-2020) or up to 1.2 mm galvanized (SBTF-1500 and SBTF-1500C).

Reinforcement (AS 4254.2 Section 5)

Larger ducts and higher pressure classes require external reinforcement to prevent panel deflection. AS 4254.2 specifies reinforcement intervals as a function of duct dimension and pressure class. Typical practice for Class 2 (1 kPa):

Duct up to 600 mm: no reinforcement required (sheet stiffness adequate)
600–1200 mm: reinforce at 1500 mm intervals with cross-break or galvanized angle
1200–1800 mm: reinforce at 1200 mm intervals with welded angle iron or formed channel
Above 1800 mm: reinforce at 900 mm intervals, double angle iron typically

For Class 3 and Class 4 pressure, intervals tighten by 25–50 percent and angle sections increase from 25 × 25 × 3 to 40 × 40 × 5 mm. SBKJ SBAL-V auto duct line integrates cross-break and bead reinforcement inline (replacing manual angle iron on lower-pressure duct); the TDF flange itself provides transverse reinforcement at every section joint.

Longitudinal seam types (AS 4254.2 Section 4.3)

Pittsburgh lock (hammer or machine-formed) — the dominant seam for galvanized commercial duct up to Class 3 pressure. SBKJ SBLC lockformer family produces Pittsburgh in the 0.4–1.2 mm range.
Button-punch snaplock — faster than Pittsburgh, no hammer or lockformer required, but slightly less strong. Acceptable for Class 1 and Class 2.
Machine-formed grooved seam — used on heavier-gauge duct (1.0+ mm); requires a dedicated forming machine. Less common than Pittsburgh.
Welded longitudinal seam — required for Class 4 high pressure on small ducts and mandatory for food processing (EHEDG), pharma cleanroom (GMP Annex 1), hospital isolation (AS/NZS 2243.3), kitchen exhaust (AS 1668.2 Section 6 / NFPA 96), and any chloride or aggressive chemical exposure. SBKJ SBAL-V with integrated TIG seam welder, or SBAL-III plus standalone TIG welder, delivers welded-seam duct for these applications.

Transverse joint types

S-cleat — the traditional Australian transverse joint for rectangular duct. Acceptable for Class 1 and 2 leakage class C/D. Cheap to fabricate but limited to lower pressure classes.
TDF flange (rolled transverse flange) — the dominant joint for Class 2 and above. Formed inline on SBAL-V or standalone on SBTDF flange formers. Mates with mating TDF flange + corner block + gasket. Used on most Australian commercial work since 2005.
Angle flange (bolted) — the heavy-duty transverse joint for Class 3 and Class 4 pressure, large duct, and welded-seam specifications. Acceptable on every leakage class. Slower to fabricate and install but the most robust.
Welded transverse joint — for the most stringent specifications. Continuous TIG weld at every section interface. Used only where AS 4254.2 demands it (food, pharma, hospital, industrial chemical exposure).

Fabrication tolerance (AS 4254.2 Section 6)

AS 4254.2 specifies dimensional tolerance for fabricated duct:

Length tolerance — ±5 mm on sections up to 1500 mm long; ±10 mm above 1500 mm. SBKJ SBAL-V achieves ±1.0 mm on length (well inside specification); SBAL-III achieves ±1.5 mm.
Width / height tolerance — ±3 mm on dimensions up to 750 mm; ±5 mm above 750 mm. SBKJ achieves ±0.5 mm to ±1.0 mm depending on machine.
Squareness / diagonal tolerance — difference between diagonals <5 mm on rectangular sections up to 1500 mm. SBKJ achieves <0.8 mm.
Round duct circularity — SBKJ spiral tubeformers achieve roundness within 0.2 percent of diameter (e.g., 600 mm diameter duct varies by less than 1.2 mm).

Material standards cross-referenced by AS 4254

AS 1397 — Hot-dip metallic-coated steel sheet and strip. Specifies coil galvanizing for ductwork.
AS 1449 — Stainless steel sheet and strip (grade 304, 316 typically).
AS 1734 — Aluminium sheet (where aluminium duct is used — cleanrooms, lab fume exhaust).
AS 1530.3 — Fire-hazard properties of building materials — for sealants and gaskets.
AS/NZS 4859 — Insulation materials — for duct external insulation thermal properties.

Cross-references to other Australian standards

AS 1668.1 — Fire and smoke control. Smoke spill ducts need FRL rating per AS 1530.4; AS 4254 specifies the construction, AS 1668.1 specifies the fire performance.
AS 1668.2 — Mechanical ventilation. AS 1668.2 sets airflow rates; AS 4254 sets how the duct that carries that airflow is built.
NCC Section J — Energy efficiency. NCC J6.4 sets duct sealing requirements (specifies AS 4254 leakage class).
NCC Section F4 — Health and amenity. Calls up AS 1668.2 which calls up AS 4254.
AS 1530.4 — Fire-resistance level (FRL) testing for duct passing through fire compartments.

How SBKJ machinery supports AS 4254 compliance

SBKJ's machine platform is designed to deliver every AS 4254 pressure class and leakage class from a single shop:

SBAL-II (5.5 kW, 18 m/min, 0.5–1.2 mm, up to 1500 mm) — Class 1 / Class 2 Pittsburgh lock galvanized commercial duct, leakage Class C/D.
SBAL-III (15.7 kW, 14 m/min, 0.5–1.2 mm, up to 1500 mm) — Class 2 / Class 3 Pittsburgh + TDF galvanized commercial duct, leakage Class B/C.
SBAL-V (87 kW, 16 m/min, 0.5–1.5 mm, up to 1500 mm) — Class 2 / Class 3 / Class 4 TDF galvanized or stainless duct, leakage Class A/B. The flagship for data centres, hospitals, cleanrooms.
SBTF-1500 / 1500C / 1602 / 2020 spiral tubeformers — round duct from Φ80 to Φ2500 mm, galvanized 0.4–2.0 mm, stainless 0.4–1.2 mm. AS 4254 Class 1 to 4 round duct.
SBTDF-12 / SBTDF-1.5-16 transverse duct flange formers — standalone TDF flange production for SBAL-III or upgrade workflows.
F350 nail-free riveting machine — corner block and TDF flange corner assembly.
SBHF-I seam closing machines — final closing of Pittsburgh longitudinal seam after duct is folded.
TIG seam welder + plasma cutter — welded-seam duct for Class 4 pressure or hygienic / industrial chemical applications.

Common AS 4254 fabrication mistakes

Specifying Class 4 pressure but Pittsburgh seam. AS 4254.2 requires welded seam at Class 4. Pittsburgh duct fails the leakage test at 2500 Pa even with sealant.
Forgetting reinforcement above 1500 mm duct. Panel deflection becomes audible “oil-can” noise during fan modulation and erodes leakage class. Cross-break or angle iron at the specified interval.
Using duct tape as primary sealant. AS 4254.2 does not accept tape alone — must be combined with brush-on mastic. Most Class A leakage failures trace to this.
Specifying Class A leakage on residential or low-velocity transfer duct. Over-specification adds 5–10 percent to capital cost without delivering occupant value. Match class to the actual energy performance need.
Mixing TDF flange systems. AS 4254 does not standardise TDF profile dimensions; different manufacturers have slightly different rolled profiles. Specify a single TDF system (SBKJ in-house, or specific aftermarket brand) and stick with it across the project.
Skipping the AS 4254 first-article leakage test. The standard requires the first 10 percent of installed duct to be leak tested. Skipping it during the early project phases means rework discovered late.
Galvanized in chloride environments. AS 4254 doesn't prohibit galvanized in coastal or chlorinated environments, but the duct will fail within 5–7 years. Specify stainless from the start in coastal Class C5-M atmospheres.

Talk to SBKJ engineering

SBKJ engineering supports Australian fabricators on AS 4254 compliance:

PLC recipe configuration for AS Class 1 through Class 4 pressure
Sheet gauge selection sheets per project specification
WPQR (Weld Procedure Qualification Record) for 304-2B and 316-2B stainless
First-article leakage testing assistance and acceptance documentation
Cross-reference of AS 4254 to SMACNA, EN 1505 and DW/144 for export projects

Contact SBKJ engineering