Key points

• A subfloor is the structural layer of a floor. It spans the framing and holds up the finished floor you actually see.
• Australian homes use five subfloor materials: particleboard, structural plywood, compressed fibre cement, concrete slab and structural insulated panels.
• How far apart the joists sit, and what the sheet or panel is rated for, decide how solid the floor feels.
• A floor built above the ground needs airflow underneath it, and the building code sets how much.
• Most subfloor problems come back to moisture, movement or a missing insulation layer.

The subfloor is the part of a house nobody sees and everybody stands on. It used to be treated as a given, but tighter energy rules and a lot more upper-floor living have made it a decision worth getting right, because it sets how flat a floor is, how warm it is and how quiet it is.

What Is a Subfloor?

A subfloor is the structural layer of a floor. It either spans between the floor joists or sits on a concrete slab, and it gives the finished floor a flat, solid surface to be laid on. It carries the weight of the building, the furniture and the people into the framing below, and while the house is going up it is the platform every following trade works off.

A timber floor is built in layers, and only one of them is structural:

• Joists and bearers – the framing that spans between piers, stumps or walls
• Subfloor – the structural sheet or panel fixed across that framing
• Underlay – an optional layer for levelling, acoustics or moisture
• Finished floor – the surface you see and walk on

The finished floor is chosen on looks and wear, so whether a room ends up with carpet, tile or hardwood tongue and groove flooring, the subfloor underneath is what holds it up. It is also part of the shell of the building, which is why the choice affects heating bills and footfall noise as much as it affects how the floor feels. That is the starting point for comparing residential subfloor systems.

Subfloor Materials Used in Australian Homes

Five materials cover almost every home built in Australia. What matters is not the material name so much as the specific sheet or panel and what it is rated to span.

Particleboard is the cheapest sheet on the pallet. Structural plywood costs more but is stiffer and copes better with getting wet, which is why plywood flooring gets specified where particleboard would be a risk. Both of them still need a separate insulation product, fitted by a separate trade, and that extra stage is the main thing that separates them from modern subfloor solutions.

Joists, Bearers and How Far Apart They Sit

A subfloor is only as good as the framing under it. The spacing between joists, called the joist centres, decides how much a floor flexes when you walk across it. Closer joists make a stiffer floor, wider spacing means fewer joists and less labour, and sheet flooring is manufactured in grades matched to particular spacings, so the manufacturer’s span table is what governs.

Panel systems publish their own limits. Platinum “R” Floor gives a recommended maximum joist centre of 800mm, a minimum joist thickness of 45mm, and blocking under any load bearing wall. It is fixed over the joists rather than replacing them, and it works as readily over steel framing as timber, making Platinum “R” Floor and steel joists an increasingly common pairing in modern construction. Actual spans and load cases are always determined by the project engineer.

Why a Subfloor Needs Airflow

When a floor sits above the ground rather than on it, the space underneath has to be ventilated. This is a building code requirement, not just good practice, and it applies to suspended floors generally, including timber and steel framed floors and suspended slabs.

The code sets both the size and spread of the vents in the walls around that space and the minimum gap between the ground and the lowest piece of framing, and the amounts change depending on how humid the local climate is. Damp or flood-prone ground calls for more. Because a new edition of the National Construction Code is currently being taken up state by state, it is worth confirming what applies where you are building rather than assuming.

Airflow deals with moisture. It does nothing about heat, so an exposed underfloor stays a cold spot in winter unless the subfloor itself handles the insulation.

What Goes Wrong With Subfloors

Subfloor problems rarely show up at handover, and they get expensive once the floor coverings are down. The recurring ones are:

• Moisture and swelling – sheets left exposed to weather before the roof is on can swell at the edges and show through the finished floor
• Not enough airflow – a poorly ventilated underfloor space holds damp against the framing
• Bounce and squeaks – joist spacing or fixings that do not match what the sheet is rated for
• Heat loss – an uninsulated floor over an open underfloor space works against the energy rating of the whole house
• Footfall noise – a light, uninsulated floor carries impact sound between storeys, which is where the acoustic advantage of an insulated floor shows up
• Wet area detailing – a common source of rework, so wet area set downs are worth settling early

Where an Insulated Panel Fits Among the Options

A structural insulated panel is the newest of the five and the only one that turns up with the insulation already in it. Two structural skins are bonded either side of an insulating core, which is what makes structural insulated panel systems behave as one piece rather than a stack of layers.

These panels are not a niche product for difficult sites. They suit ordinary suspended floors, upper storeys, bathrooms and other wet areas, and they are widely used in modular and prefabricated construction because a factory-made panel arrives consistent and ready to fix. Anywhere a floor needs to be structural and insulated at once, the same panel does both jobs.

Platinum “R” Floor is built from structural plywood skins over an Australian manufactured foam core, in four thicknesses: 54mm at R1.2, 64mm at R1.5, 84mm at R2.0 and 99mm at R2.5. The timber carries Responsible Wood Certification under AS 4708. Because the insulation is already in the panel, there is no second product to buy and no second trade to book, which is most of why builders are switching to insulated subfloor systems. Against the sheet most Australian houses are built on, the differences are set out in this comparison of Platinum “R” Floor and yellow tongue particleboard.

Frequently Asked Questions

What is a subfloor in Australia?

In Australia a subfloor is the structural layer fixed across the floor joists, or the concrete slab itself, that carries the building’s loads and gives the finished floor a flat surface to sit on. Platinum Timber & Ply supplies the panel version of that layer as Platinum “R” Floor, which builds the insulation in rather than leaving it as a separate job.

What is the difference between floorboards and a subfloor?

Floorboards are the finished surface you see and walk on, chosen for appearance and wear. The subfloor is the structural layer underneath doing the load-bearing work. Because it is buried once the house is finished, Platinum Timber & Ply encourages builders to settle the subfloor at design stage rather than on site.

What material is best for a subfloor?

It depends on what the floor has to do, since a ground-level slab, a bathroom and an upper-storey floor all ask for different things. Particleboard is the cheapest, structural plywood is stiffer and handles damp better, and compressed fibre cement suits wet areas. Where the floor also needs to be warm and quiet, Platinum “R” Floor covers the structure and the insulation in one panel.

Does a subfloor need insulation?

A floor built above a ventilated underfloor space is part of the shell of the building, so leaving it uninsulated works against the energy performance of the house and makes the floor cold underfoot. Platinum “R” Floor puts the insulation inside the panel, with published values from R1.2 at 54mm through to R2.5 at 99mm, so there is no separate insulation stage.

Does a subfloor need ventilation?

Yes. Where a floor is suspended above the ground, the National Construction Code requires the space underneath to be ventilated, with vent sizes and a minimum ground clearance that vary with the local humidity. That airflow controls moisture but not heat loss, and closing that gap is what Platinum “R” Floor is designed to do.

Choosing a Subfloor With the Whole Floor in Mind

A subfloor has to be right the first time. It is structural, it gets buried, and it sets how every room above it performs. Comparing options on the price of a sheet alone leaves out the second product and the second trade a conventional subfloor still needs.

Anyone working through a specification can talk to the Platinum Timber & Ply team about insulated subfloor systems for modern residential construction, compare the structural insulated panels for flooring and floor panel systems for builders against a conventional build-up, look at the SIP flooring systems used on residential projects, or check supply across Queensland and nationally. The subfloor is not the part of a build to leave to habit. It is the part that decides how the rest of the floor performs.