Subfloor preparation is the work done to the slab, joists, sheet flooring or old floor before a new timber floor goes down. It usually involves checking moisture, levelling or grinding the surface, fixing loose sheets or boards, and removing old coverings so the surface meets the flooring manufacturer's flatness and moisture requirements. The scope is set by an on-site assessment, because no two subfloors need exactly the same work.
Design Timber Flooring is a mobile, owner-operated business working across Greater Sydney. Jimmy assesses the subfloor as part of every installation quote, and explains what needs doing and why before anything is ordered. This page covers preparation in detail; our main timber floor supply and installation service explains the full installation process.
What subfloor preparation is
The subfloor is whatever the new floor will sit on or be fixed to. In Sydney homes that is usually one of four things:
- Concrete slab. Common in apartments, newer houses, extensions and ground floors.
- Timber joists. Older homes and many raised floors, where boards are fixed directly to the framing.
- Particleboard or plywood sheets. Common in project homes and upper floors, laid over joists.
- An existing floor. Old timber, tiles or vinyl that may be retained as a base, or removed.
Each needs different checks. A floor is only as good as what is under it, and most installation problems that show up later, such as hollow spots, squeaks, cupping or joints opening, can be traced back to the subfloor.
Common preparation work
| Subfloor | Typical checks | Possible preparation |
|---|---|---|
| Concrete slab | Moisture, flatness, cracks, old adhesive, contamination | Moisture testing, grinding high spots, levelling compound in low spots, removing adhesive, moisture barrier if the product requires it |
| Timber joists | Level across joists, spacing, condition, ventilation below | Packing or planing joists, adding noggings where needed, checking for rot or pest damage |
| Particleboard or plywood | Loose or swollen sheets, squeaks, joins, flatness | Re-fixing loose sheets, replacing damaged sections, sanding or levelling joins |
| Existing timber floor | Loose boards, cupping, height, condition | Re-fixing boards, sanding flat, deciding whether to retain or remove |
| Tiles or vinyl | Bond, flatness, height, what is underneath | Removing, grinding or levelling, or retaining where suitable for the product |
Flatness and moisture limits vary between products, so the preparation is matched to the floor you choose rather than a one-size rule.
When preparation is straightforward, and when it is not
Preparation is often light when:
- The slab is dry, sound and close to flat.
- Sheet flooring is firmly fixed and undamaged.
- The existing covering lifts cleanly without heavy adhesive.
It becomes more involved when:
- The slab has moisture issues, cracks or significant dips and humps.
- Joists are out of level, undersized, damaged or poorly ventilated.
- Particleboard has swollen from past leaks or is coming loose.
- Old coverings are glued down, or several layers have been laid on top of each other.
Some issues are beyond flooring preparation. Structural movement, failed bearers, drainage problems, termite damage or rising damp need the right trade or inspector first. We will tell you plainly if a cause sits outside our scope.
Moisture checks
Timber reacts to moisture, so moisture in the subfloor is one of the first things to check. On slabs, new concrete and screeds can hold moisture for a long time, and older slabs can draw moisture from below. On raised floors, poor ventilation or water pooling under the house can affect the joists and the new boards.
Where the product or the conditions call for it, Jimmy uses moisture testing and compares readings with the flooring manufacturer's requirements. If readings are outside those limits, options can include waiting for the slab to dry, a suitable moisture barrier, or improving ventilation below a raised floor. Installing over a subfloor that is too wet is a common cause of gaps and cupping in floorboards.
Levelling and grinding
Most products set a flatness tolerance over a given distance. Concrete slabs are rarely perfect, so preparation might include:
- Grinding down high spots, ridges and old trowel marks.
- Filling dips and low areas with a suitable levelling compound.
- Removing old adhesive, paint or residue that would affect bonding.
- Repairing or filling cracks where appropriate.
Levelling compounds need drying time before the floor goes down, and the time depends on the product, depth and conditions. On timber subfloors, levelling may mean planing or packing joists, or sanding sheet joins flat. Patterned floors such as herringbone and chevron are especially sensitive to an uneven base.
Fixing loose sheets and boards
Loose particleboard or plywood sheets move underfoot and can cause noise and movement in the floor above. Preparation may involve re-fixing sheets to the joists, replacing water-damaged or swollen sections, and checking joins. On old timber floors being retained as a base, loose boards are re-fixed first. Our page on squeaky floorboards explains how movement in boards is traced and fixed.
Removing old coverings, and an asbestos caution
Carpet, underlay, gripper strips, vinyl, cork, tiles and old adhesives often need to come up before the new floor goes down. The effort depends on how they were fixed and how many layers there are.
Some older vinyl sheet, vinyl tiles, underlays and adhesives, particularly in homes built or renovated before the late 1980s, can contain asbestos. If there is any chance an old covering contains asbestos, it should not be cut, sanded, scraped or lifted until it has been tested. Where asbestos is present, removal must be carried out by a licensed asbestos removalist in line with NSW requirements. We do not remove asbestos-containing material, and we schedule our work after the area has been cleared.
How Jimmy assesses the subfloor
On site, Jimmy will:
- Identify the subfloor type and how the existing covering is fixed.
- Check flatness with a straight edge across the main areas and doorways.
- Look for cracks, adhesive residue, loose sheets, damaged boards and signs of past leaks.
- Recommend moisture testing where the product or the conditions call for it.
- Look underneath raised floors where there is safe access, checking joists, ventilation and any obvious pest or rot issues.
- Measure heights at doors, thresholds and adjoining floors, since preparation affects the final build-up.
The findings are written into the quote, so you can see what preparation is included and why.
The process
- Assessment. Subfloor type, moisture, flatness and condition are checked on site.
- Written scope. Preparation steps are listed separately in the written quote.
- Removal. Old coverings are lifted and disposed of, after any asbestos testing and licensed removal if required.
- Repairs and fixing. Loose sheets and boards are re-fixed and damaged sections replaced.
- Levelling and moisture control. Grinding, levelling compound or moisture barriers are applied where needed and allowed to dry or cure.
- Final check. Flatness and moisture are rechecked against the product's requirements before installation starts.
What affects timing
There is no fixed duration for subfloor preparation. It depends on:
- The type and number of old coverings, and how they were fixed.
- How much grinding or levelling is needed, and the drying time of compounds.
- Slab moisture, which may mean waiting before installation.
- Whether testing or licensed asbestos removal is needed first.
- Access, especially in apartments with lift and hours restrictions.
Preparing for the work
- Tell us the age of the home and anything you know about past renovations or leaks.
- If you suspect an old covering may contain asbestos, arrange testing before any removal.
- Clear rooms of furniture and fragile items.
- Arrange safe access to under-floor areas if the home is raised.
- Expect some noise and dust from grinding and removal.
Our guide to preparing your home for a floor install has more practical steps, and timber flooring over tiles covers when tiles can stay.
Subfloor preparation checklist
- Subfloor type identified
- Moisture checked against the product's requirements
- Flatness checked across rooms and doorways
- Loose sheets or boards re-fixed, damaged sections replaced
- Old coverings removed, with asbestos testing where relevant
- Levelling or grinding done and compounds dry
- Door and threshold heights confirmed for the new floor
Preparation across Sydney
Jimmy prepares subfloors for new timber floors in homes and apartments across Sydney's suburbs. For species choices once the base is ready, see spotted gum and blackbutt, or compare products on our engineered timber flooring page.
Frequently asked questions
Why is subfloor preparation needed before a timber floor?
The new floor relies on the subfloor being dry, flat and stable. Moisture, dips or loose sheets can lead to hollow spots, movement, noise or cupping later. Preparation brings the base within the product manufacturer's requirements.
Do all concrete slabs need levelling?
Not always. Some slabs are close enough to flat for the chosen product with only minor grinding. Others need levelling compound in low areas. It is assessed on site against the product's flatness requirement.
How do you check for moisture in a slab?
Jimmy checks for visible signs and recommends moisture testing where the product or conditions call for it. Readings are compared with the manufacturer's limits before installation is confirmed.
Can new timber go over old floorboards?
Sometimes. Sound, flat and firmly fixed boards can be a suitable base for some products. Loose, cupped or damaged boards need repair or removal first. The decision depends on the boards, the product and the finished height.
Can old vinyl or lino contain asbestos?
Some older vinyl, vinyl tiles, underlays and adhesives can. If there is any chance, have it tested before it is disturbed. Asbestos-containing material must be removed by a licensed asbestos removalist.
Is subfloor preparation included in the quote?
Preparation identified during the on-site assessment is listed in the written quote. If hidden issues appear once old coverings are lifted, we discuss them with you before going further.
Can you fix structural problems under the floor?
We handle flooring preparation such as re-fixing sheets and packing joists. Structural issues, such as failed bearers or stumps, drainage problems or termite damage, need a builder or inspector first.
Ask about your subfloor
Tell us your suburb, the existing floor and the new floor you have in mind through our online quote form, or call Jimmy on 0404 906 432 to book a free on-site assessment.