Steel Frames
Getting floor joist sizing right is one of the most consequential structural decisions on any residential or commercial build. Undersized joists deflect excessively, produce springy floors, and can lead to serviceability failures that are expensive to rectify after construction is complete. Oversized joists waste material and add unnecessary cost.
This guide covers the key factors that govern steel floor joist sizing in Australian construction, how to read and apply span tables, standard spacing and sizing for residential and commercial applications, and the Australian standards that underpin it all.
Floor joists are the horizontal structural members that span between bearers (or primary beams) to support the floor deck above. They carry the combined weight of the floor covering, any furniture, equipment, and occupants (the live load), and transfer these loads to the bearers below.
In a typical suspended floor system, the structural hierarchy from ground to finished floor is:
Steel floor joists operate on the same structural principle as timber joists but use cold-formed steel sections instead of solid timber or engineered timber products.
Understanding the components that make up a steel floor joist system helps builders specify the right products and identify what information is needed for an engineering assessment.
Joists: The primary spanning members. In light gauge steel floor systems, joists are typically C-section cold-formed steel members, sometimes with stiffening lips or additional flanges for enhanced performance.
Blocking: Short sections of joist or track installed between joists at bearing points and at mid-span to prevent lateral buckling and provide rigidity to the floor system.
Bearers: Primary beams that support the joists. In steel floor systems, bearers may be cold-formed steel sections, structural steel universal beams, or a combination of both depending on the span and load requirements.
Joist hangers and brackets: Connection hardware that fixes the joist to the bearer or to the wall frame. Using the correct joist hanger type and fixing pattern is a structural requirement.
Bridging: Diagonal or cross-bridging members installed between joists to distribute concentrated loads and reduce vibration. In steel joist systems, bridging is typically flat strap steel fixed at diagonal angles.
Floor deck: The structural panel product fixed to the top of the joists. Must be selected for compatibility with joist spacing (panel span rating) and surface finish requirements.
Light gauge steel floor joists in the Australian market are available in a range of section depths and base metal thicknesses (BMT). The most commonly used standard sizes are:
Joist Depth | Base Metal Thickness (BMT) | Typical Application |
|---|---|---|
150 mm | 1.15 mm | Residential short span joists, non-structural infill |
200 mm | 1.15 mm | Standard residential floor joists |
250 mm | 1.15 mm | Medium span residential and light commercial |
300 mm | 1.15 mm | Longer span residential and commercial applications |
200 mm | 1.50 mm | Heavier residential loads or longer spans |
250 mm | 1.50 mm | Commercial floor systems and longer spans |
300 mm | 1.50 mm | Long span commercial and industrial floor systems |
*Source: Standard section sizes referenced from AS/NZS 4600 Cold-Formed Steel Structures and the NASH Standard for Residential and Low-Rise Steel Framing. Available depths and BMT ratings are indicative of common product ranges in the Australian market and vary by manufacturer. Confirm product availability with your supplier before specifying.* and load-carrying ability. The correct joist size for any application must be confirmed by a structural engineer through load calculation and reference to the manufacturer’s span tables.
Standard joist spacing in Australian residential and commercial construction is:
450 mm centres: Used where the floor deck panel requires closer support, such as with thin plywood panels, tile substrates, or where there is a specific deflection limit requirement (such as under tile finishes, which are sensitive to deflection). Also used in commercial applications with heavier live loads.
600 mm centres: The most common spacing for standard residential applications with structural plywood or particleboard flooring. This is the baseline assumption in most residential engineering design.
900 mm centres: Used with engineered floor panels rated for longer spans, or in commercial applications with structural deck products that can span further between joists. Less common in standard residential construction.
The correct joist spacing for your project depends on the floor deck product specified, the applied load, the joist span, and the deflection limits required. Do not assume a spacing without engineering confirmation.
A span table gives the maximum allowable span for a given joist size, spacing, and load condition. Reading a span table correctly is important for getting the right joist and understanding why a structural engineer’s calculation may differ from the table result.
The four key variables in any span table are:
The following indicative spans are for standard residential loading (1.5 kPa live load, 0.5 kPa dead load) and deflection limit of L/300. These are for general guidance only. Always confirm spans with your structural engineer from manufacturer’s certified span tables.
Joist Size | At 450 mm Centres | At 600 mm Centres |
|---|---|---|
150 mm / 1.15 mm BMT | Up to 2.8 m | Up to 2.5 m |
200 mm / 1.15 mm BMT | Up to 3.6 m | Up to 3.2 m |
250 mm / 1.15 mm BMT | Up to 4.4 m | Up to 3.9 m |
300 mm / 1.15 mm BMT | Up to 5.1 m | Up to 4.6 m |
250 mm / 1.50 mm BMT | Up to 5.0 m | Up to 4.5 m |
300 mm / 1.50 mm BMT | Up to 5.8 m | Up to 5.3 m |
*Source: Indicative spans derived from design principles set out in AS/NZS 4600 Cold-Formed Steel Structures and the NASH Standard for Residential and Low-Rise Steel Framing. Load assumptions: 1.5 kPa live load, 0.5 kPa dead load, L/300 deflection limit, single-span configuration. Spans are for general guidance only and must not be used as a substitute for engineering design. Always confirm spans with a structural engineer from manufacturer-certified span tables for your specific project, loading, and deflection requirements.* (supported at both ends only) with no mid-span loads and standard residential loading. Continuous joists (supported at three or more points) can achieve longer spans between intermediate supports.
For commercial occupancies with higher live loads, these spans will be significantly shorter. Your structural engineer will calculate the correct spans from manufacturer’s certified tables for the specific load conditions of your project.
For standard Australian residential construction using structural plywood or particleboard floor decking:
The floor deck product must be rated for the joist spacing used. Using a 19 mm plywood on 600 mm joist centres will produce noticeable floor flex and may lead to serviceability failures. Always match the floor deck specification to the joist spacing.
For tile finishes, joist spacing is typically limited to 450 mm centres and deflection limits are specified as L/360 or tighter to prevent tile and grout cracking.
All steel floor joist design in Australia must comply with:
The NASH Standard for Residential and Low-Rise Steel Framing also provides guidance on floor system design for light gauge steel applications.
Engineering certification is required for all structural floor joist systems. Reputable steel floor joist suppliers provide engineering certification as part of the supply documentation.
Span capability: Steel floor joists typically achieve longer spans than equivalent-depth timber joists, which can reduce the number of bearer or post supports required and open up the subfloor space below.
Dimensional consistency: Steel joists are manufactured to precise tolerances and maintain their dimensions over time. Timber joists can shrink as they dry, leading to floor squeaking, nail pops, and plasterboard cracking.
Termite resistance: Steel is completely immune to termite attack. In termite-active areas across Australia, steel floor joists remove the subfloor framing from the termite risk category entirely.
Moisture performance: Steel does not absorb moisture and does not rot. In elevated subfloor environments where moisture levels can be variable, steel joists perform more reliably than unseasoned or treated timber over the long term.
Services integration: Light gauge steel floor joists have pre-punched service holes that allow electrical, hydraulic, and data services to run within the floor depth without additional drilling. This simplifies rough-in coordination and reduces on-site drilling time.
Weight: Steel floor joists are lighter than equivalent timber joists, reducing the loads on bearers, posts, and footings.
Cost: On a supply-only basis, steel floor joists typically cost more than equivalent timber joists. Total installed cost comparisons are narrower when the savings from faster installation, reduced defect rectification, and no treatment requirements are accounted for.
Indicative supply-only costs for light gauge steel floor joist systems in 2025 and 2026:
Residential suspended floor (single storey, standard configuration):
$45 to $90 per square metre of floor area supplied, depending on span, joist size and gauge, and bearer requirements.
Upper floor in a double-storey home:
$55 to $110 per square metre of floor area supplied, depending on span and load requirements.
Commercial floor system:
$80 to $180 per square metre for commercial floor joist systems, depending on live load requirements, span, and fire rating needs.
These figures are for joist supply only and do not include installation labour, floor decking, or engineering fees beyond what is included in the frame package. Contact CMC Steel Solutions with your plans for a project-specific quotation.
Using residential span tables for commercial loads. Residential span tables assume 1.5 kPa live load. Commercial occupancies have live loads of 2.5 kPa to 5.0 kPa or higher. Using a residential table for a commercial application will result in undersized joists that fail serviceability or strength limits.
Ignoring point loads. Distributed load tables do not account for heavy concentrated loads such as baths, spa baths, mechanical plant, or built-in cabinetry. Where heavy point loads are expected, your structural engineer must check the joist capacity specifically for those load conditions.
Using the wrong deflection limit for the floor finish. Designing to L/300 deflection for a floor that will be tiled is a common error that leads to tile and grout cracking. Confirm the required deflection limit with your tile supplier and structural engineer before finalising joist sizing.
Installing joists upside down. Steel C-section floor joists have a specific orientation that must be maintained during installation. Installing a joist inverted changes the load path and can significantly reduce the structural capacity. Always install joists with the open face of the C-section oriented as specified in the engineering drawings.
Cutting through the joist web without engineering approval. Pre-punched service holes in steel joists are located and sized to maintain structural performance. Field-cut holes in the joist web can significantly reduce the joist’s load capacity. Never cut holes in a steel floor joist without written engineering approval.
Maximum spans for steel floor joists in residential construction depend on the joist size, BMT, spacing, and load. For standard residential loading at 600 mm centres, spans typically range from about 2.5 m for a 150 mm / 1.15 mm joist to about 5.3 m for a 300 mm / 1.50 mm joist. These are indicative only. Your structural engineer will confirm the maximum span from manufacturer-certified span tables for your specific project conditions.
For a 4-metre span at 600 mm centres under standard residential loading and L/300 deflection limit, you would typically need a 250 mm / 1.15 mm or 300 mm / 1.15 mm light gauge steel joist. However, floor joist sizing must always be confirmed by a structural engineer from manufacturer-certified span tables, taking into account your specific load conditions, deflection requirements, and floor finish.
The most common joist spacing for Australian residential construction is 600 mm centres, used with 22 mm structural plywood or 25 mm particleboard flooring. For tile finishes or where a stiffer floor is required, 450 mm centres are commonly specified.
Joist depth depends on the span, load, and spacing. For standard residential spans of 3 to 4 metres at 600 mm centres, 200 mm to 250 mm joist depths are typical. For longer spans or heavier loads, 300 mm joists are used. Your structural engineer will confirm the required depth.
Yes. Steel floor joists are not affected by condensation or moisture in the way that timber joists can be. However, adequate subfloor ventilation is still important for the general health of the building and for the performance of any timber elements in the floor system such as the floor deck or any remaining timber framing. AS 3660 Termite Management and the NCC set requirements for subfloor ventilation that apply regardless of the framing material used.
A bearer is a primary structural beam that spans between posts or stumps and supports the joists above it. A joist is a secondary structural member that spans between bearers and supports the floor deck. Bearers run perpendicular to joists. The bearer is deeper and stronger than the joist it supports, as it carries the combined load of all the joists above it across its span.
No. Steel joists are not a food source for termites and do not require chemical treatment. However, a compliant termite management system is still required under the NCC for the building as a whole, as termites may use the subfloor as a pathway to attack other timber elements such as the floor deck, skirting boards, and internal fitout. Physical termite barriers at the perimeter are the standard management approach for steel-framed buildings.
Yes. Steel floor joists are compatible with hydronic underfloor heating systems. The floor deck product and the hydronic system installation must be coordinated to ensure the heat transfer is adequate and the floor deck temperature does not exceed the product manufacturer’s limits. An engineered floor system with appropriate insulation below the joist will improve the efficiency of the heating system.
Minimum bearing length for light gauge steel floor joists at support points is typically 40 to 50 mm on each end, depending on the joist size and the load. Bearing stiffeners may be required at heavily loaded end supports to prevent web buckling. Your structural engineer will specify minimum bearing lengths and any stiffener requirements in the engineering drawings.
Yes. CMC Steel Solutions supplies light gauge steel floor joist systems for residential and commercial projects across Melbourne and regional Victoria. Our supply packages include full engineering certification to AS/NZS 4600. Contact our team with your plans to receive an obligation-free quotation.
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