Rigid Ducting. Proven Performance.
Spiro-Loc rigid steel ducting is a lightweight galvanised steel duct system, machine-wound in a continuous spiral from metal strip coil stock. The four-ply lock seam maintains a rigid circular cross-section, delivering structural integrity and predictable airflow performance across the full life of the installation.
Spiro-Loc is the preferred choice for main duct runs, high-pressure systems, and applications where semi-rigid or flexible ducting will not meet the performance or longevity requirements. Beyond HVAC, it is also used for concrete void forming, irrigation pipe, and rubbish chutes on high-rise buildings.
Why Choose Spiro-Loc Rigid Steel Ducting?
- Four-ply lock seam construction maintains a rigid circular shape under load
- Available in diameters from 100mm to 1500mm
- Section lengths from 750mm up to 6m
- Plain, single swage, or double swage profiles for enhanced rigidity and crush resistance
- Plain or insulated options available
- Suitable for high-pressure HVAC systems up to 2500Pa positive pressure
- Complies with AS 4254, AS 1530.3, and UL 181
Ideal Applications
- Commercial and industrial HVAC main duct runs
- High-pressure air conditioning supply systems
- Long horizontal duct spans requiring structural self-support
- Underground ventilation for mine shafts and tunnels
- Concrete void forming in structural slabs
- Irrigation pipe and rubbish chutes on high-rise buildings
Rigid vs Semi-Rigid vs Flexible Ducting
| Criteria |
Spiro-Loc |
Spiro-Set |
Spiro-Flex |
| Type |
Rigid steel ducting |
Semi-rigid aluminium ducting |
Flexible ducting |
| Airflow consistency |
Highest |
High |
Lower |
| Installation |
Requires fittings and fabrication |
Hand-bendable on site |
Compressible and flexible |
| Self-supporting span |
Yes – up to 3,600mm hanger spacing |
Yes |
No |
| Pressure rating |
Up to 2500Pa positive pressure |
±1.5 kPa |
Low pressure only |
| Best suited for |
Main duct runs, high-pressure systems, and long spans |
Commercial HVAC, retrofits, exposed ceilings, and VAV connections |
Short flexible duct runs and terminal connections |
Spiro-Loc Rigid Steel Ducting Features
Structural Rigidity Under Load
The four-ply lock seam adds sufficient strength to maintain a rigid circular shape up to 600mm diameter in 0.95mm material, or 500mm in lighter gauge. For diameters 450mm and above, one or two external swages can be incorporated to significantly improve rigidity and crushing resistance. This makes Spiro-Loc well suited to high-pressure duct systems and long unsupported spans where duct deformation is not acceptable.
Wide Size and Length Range
Standard forming dies are held for common sizes from 100mm to 1500mm diameter. Section lengths range from 750mm to 6m, subject to practical handling considerations. Special dies can be produced for non-standard diameters. For projects requiring large quantities in relatively inaccessible locations, on-site manufacture with machines and operators is available — contact your local Holyoake branch.
Leakage Control and Sealing
The longitudinal four-ply seam and circumferential traverse joints must be sealed after installation to meet the pressure classification requirements of SMACNA and AS 4254. In critical high-pressure conditioned air supply ducts, each duct length should be inspected for impact or fatigue damage before installation. Any looseness in the seam — identifiable when a section bends under its own weight when lifted by its ends — indicates the section should not be used in high-pressure applications.
Black Finish Options for Exposed Installations
Spiro-Loc is supplied with a black inner finish as standard. For exposed ceiling and architectural applications where the duct is visible, a black outer finish is available on request — allowing Spiro-Set to meet both performance and aesthetic requirements without additional painting or cladding.
- Black inner finish: standard on all Spiro-Set ducting
- Black outer finish: available for exposed ducting applications
- Suited to hospitality, retail, and commercial fit-outs where ductwork is left visible
Available Rigid Steel Ducting Options
- Type SPU – Plain, uninsulated: lightweight galvanised steel, standard plain profile. Available in diameters 100mm to 1500mm.
- Type SPI-FP – Plain, insulated: galvanised steel liner with lofted polyester blanket and 100-micron polyethylene vapour barrier outer sleeve. Thermal resistance R1.0 m²K/W. Available in insulated die sizes 100mm to 500mm.
- Type SAI-FP – Acoustic, insulated: as SPI-FP with acoustic insulation treatment. Available to special order.
Spiro-Loc Technical Specifications
| Material |
Plain galvanised steel, spirally wound, four-ply lock seam |
| Wall thickness (BMT) |
0.45mm, 0.55mm, or 0.95mm standard; special orders available |
| Standard diameters |
100, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1300, 1400, 1500 mm |
| Section length |
Standard 3m; special orders 750mm to 6m |
| Profile options |
Standard plain, single swage, or double swage |
| Insulation |
Lofted polyester blanket, R1.0 m²K/W, with 100-micron polyethylene vapour barrier outer sleeve |
| Maximum hanger spacing (horizontal) |
3,600mm for all diameters, per AS 4254.2-2012 |
| Maximum hanger spacing (vertical) |
4,000mm, coinciding with floor slabs |
Wall Thickness and Pressure Ratings
The following minimum metal thicknesses are required under AS 4254.2-2012, Ductwork for air-handling systems in buildings, Part 2: Rigid duct.
| Duct Size (mm) |
Max. 500Pa positive |
Max. 500Pa negative |
Max. 1000Pa positive |
Max. 2500Pa positive |
| Up to 200 |
0.4mm |
0.4mm |
0.4mm |
0.5mm |
| 201–350 |
0.4mm |
0.5mm |
0.4mm |
0.5mm |
| 351–650 |
0.5mm |
0.6mm |
0.5mm |
0.6mm |
| 651–900 |
0.6mm |
0.8mm |
0.6mm |
0.8mm |
| 901–1250 |
0.8mm |
1.0mm |
0.8mm |
1.0mm |
| 1251–1500 |
1.0mm |
1.2mm |
1.0mm |
1.2mm |
Testing and Compliance
- Complies with AS 4254, AS 1530.3, and UL 181 where applicable
- Installation in accordance with AS 4254.2-2012, Ductwork for air-handling systems in buildings, Part 2: Rigid duct
- Joint sealing in accordance with SMACNA high and low pressure ducting standards
