What Is a Subfloor? Types, Problems and How to Prepare One
A subfloor is the structural layer your new flooring sits on. In most Sydney homes it is a concrete slab, or timber-framed sheeting such as particleboard, plywood or old hardwood boards fixed to joists. Whatever you lay on top will copy its faults, so its condition decides how much prep happens before the new floor goes down.
Below: the subfloor types you will find, how to tell which one you have, how flat it needs to be for each kind of floor, and what prep usually involves.
What is a subfloor, exactly?
Think of a finished floor as three layers. At the bottom is the subfloor, the part of the building that carries the load. On top of that sits any underlay or moisture barrier. The top layer is the floor covering you see and walk on.
The subfloor belongs to the house, so it stays when you change floors. Carpet comes up. A floating floor comes up. Old vinyl gets scraped off. What is left underneath is the subfloor.
In Australia you will also hear "subfloor" used for the space under a raised timber floor, the area that needs ventilation and gets checked in a building inspection. That is a related idea. This post is about the surface your flooring is laid on.
The five subfloors you find in Sydney homes
| Subfloor | Where you usually find it | Typical prep before new flooring |
|---|---|---|
| Concrete slab | Ground floors, apartments, most newer houses | Grind high spots, fill cracks, level dips, check moisture |
| Particleboard sheeting (often called yellow tongue) | Upper floors of newer timber-framed houses | Sand the sheet joins, re-screw loose sheets, fill gaps |
| Plywood sheeting | Renovations and some upper floors | Re-fix loose sheets, sand joins, look for old water damage |
| Hardwood floorboards on joists | Older houses and cottages | Re-fix loose or cupped boards, sand or pack low spots |
| Existing tiles | Kitchens, living areas, older units | Find loose or drummy tiles, fill deep grout lines |
Yellow tongue is the name most Sydney installers use for particleboard flooring sheets, after the coloured tongue along each edge. Carpet, underlay and floating floors never count as a subfloor. They always come up first.
How to tell which subfloor you have
You can usually work it out without pulling anything up:
- Any apartment, or the ground floor of a house built on a slab: almost always concrete.
- A house with steps up to the front door and a floor that sounds slightly hollow: a raised timber frame, with sheeting or boards on joists.
- Upstairs in a two-storey house built in the last few decades: usually particleboard sheeting.
- Still unsure? Lift a corner of carpet inside a wardrobe. Grey concrete, pressed chip sheets, layered plywood or long timber boards will tell you straight away.
Prep is priced at the measure along with the floor itself, so it is something we check on site rather than guess from a photo.
How flat does a subfloor need to be?
Flatness is the number that matters most, and each floor sets its own limit on its spec sheet. Lay a floor outside that limit and you put the warranty at risk. Here is what some of the ranges we sell ask for:
| Floor | How it is laid | What the product spec says |
|---|---|---|
| Guardian 6.5mm hybrid | Click, floating | Substrate sound and flat to within 3mm over 3m; maximum raft 15m x 15m |
| MTF 12mm laminate | Click, floating | The thicker board copes better with a subfloor that is not perfectly level |
| Quercus 15/4mm engineered oak | Direct stick, or floating with glued joints | With underfloor heating, sub-floor temperature must not exceed 35 degrees |
| Olympus Neo 2.5mm vinyl plank | Direct stick | A flat, dry, properly prepared substrate is essential |
"3mm over 3m" means a 3m straightedge laid on the floor should show no gap deeper than 3mm underneath. Across a whole room, that is tight. Plenty of slabs and older timber floors miss it somewhere, often along walls, at doorways or where a wall was once removed.
Thin glued floors are the least forgiving. At 2.5mm, a vinyl plank shows every ridge beneath it once low light rakes across the room. A 12mm laminate board bridges small hollows far better, which is one reason it suits older houses.
Common subfloor problems and what they cause
Most flooring faults that appear in the first year trace back to one of these:
- High spots and dips. Click joints flex every time someone walks over them. In time they open up or break.
- Loose sheets or boards. Squeaks and a bouncy feel that no new floor will fix.
- Old adhesive, paint or underlay residue. Glue-down floors will not bond to it, and floating floors sit unevenly on it.
- Moisture in a slab. Timber and laminate can swell or cup. Adhesive under vinyl or carpet tiles can let go.
- Cracks. Fine shrinkage cracks in slabs are common and usually filled. Wide or moving cracks need a builder to look at them first.
None of this tends to show on day one. It turns up months later as a gap, a creak or a board edge that catches the vacuum.
How subfloor prep is done
Prep is everything between the old floor coming up and the new one going down. Depending on what is there, it usually means some of these steps:
- Uplift the old floor covering, including any underlay stuck to the subfloor.
- Scrape or grind off old adhesive, paint and leveller.
- Re-screw or re-nail loose sheets and boards.
- Grind high spots in concrete; sand proud joins in timber sheeting.
- Fill cracks and low spots, then feather or self-level any larger area that is out.
- Check moisture on slabs and lay the barrier or underlay the product calls for.
- Vacuum the whole area before the first board goes down.
Laying a hybrid floor yourself? Our step-by-step guide to laying hybrid flooring picks up from the point where the subfloor is ready.
What prep looked like on four recent jobs
Some jobs need a day of prep. Some need none. Four examples laid by our regular installers:
- The Ponds. The old carpet underlay had fused to the sheet floor and came away in strips, so it was scraped back by hand. Two rooms were packed up so the new floor sits flush with the corridor tiles.
- Hoxton Park, a new build. The yellow tongue subfloor was sanded at every sheet join and a beam spot-levelled before a single board went down.
- Neutral Bay, an office. The subfloor was uneven and needed minor grinding, partial levelling and a feather finish.
- Kellyville. No prep at all. The subfloor was already level, so the carpet came up and the new floor went straight on.
New builds are not exempt. Sheet joins on a new timber frame can sit a fraction proud of each other, and if nobody sands them they show through the new floor within a year.
Can you lay new flooring over old flooring?
Sometimes. As a general rule:
- Over carpet: no. Carpet and its underlay always come up.
- Over a floating floor: no. Two floating layers move against each other. Our post on what a floating floor is explains why the underlay matters.
- Over tiles: often yes for floating hybrid or laminate, if every tile is firmly stuck and the grout lines are shallow.
- Over old vinyl or lino: only if it is well bonded and flat. Lifting edges or soft spots mean it comes up.
Height is the other check. Every layer you keep raises the floor, which can foul doors, kitchen kickboards and the step into the next room.
Does subfloor prep cost extra?
Usually, and how much depends entirely on what is under the old floor. That is why there is no fixed published price for it. Removal of the old floor, slab work and stairs are all priced at the measure, once someone has seen the subfloor.
The measure itself is free and takes about 20 to 30 minutes. You get the price before we leave: supply and install, itemised, in writing.
If the budget is tight, prep is the last place to cut. A modest board on a flat subfloor will outlast an expensive one laid over humps, whether it is hybrid flooring or engineered timber.
Ready to plan a new floor? Book a free measure and quote and the subfloor gets checked as part of it.
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