Subfloor Preparation and Slab Levelling: What It Is and Why It Matters

The short answer

New flooring is only as good as what sits underneath it. A beautiful hybrid or vinyl plank laid over an uneven slab will flex at the joints, sound hollow in places and can eventually peak or separate. Carpet is more forgiving, but even carpet shows ridges over lumps of old adhesive or a step at a doorway.

Subfloor preparation is the step that makes the surface flat, sound, clean and dry enough for the floor you have chosen. On the Gold Coast it is part of nearly every job we quote, whether the home is a new estate house on a slab, a 1970s lowset brick home with old tiles and lino, or a Queenslander-style home on a raised timber floor. It is not an optional extra: it is what lets the new floor perform the way the manufacturer intends.

What counts as the subfloor

The subfloor is the structural surface that the new floor sits on. In most Gold Coast homes and apartments it is one of two things:

• A concrete slab: almost every house built on the Gold Coast in recent decades, every apartment building, and most townhouses. Slabs can have high spots, dips, cracks, old adhesive, paint or tile residue on them.
• A timber subfloor: hardwood boards, particleboard or plywood sheets on joists, common in older homes, raised homes on the hinterland edge and upper floors of some townhouses. Timber subfloors can move, squeak, have proud nails or damaged sheets, and need good ventilation underneath.
• Existing tiles can also become the base for a new floor in some cases, which is covered in our guide to laying flooring over tiles.

Why flatness matters so much

Every hard floor comes with installation instructions that set a flatness tolerance for the subfloor. It is usually written as the maximum gap allowed under a straight edge of a set length. The exact numbers differ from product to product, and the tolerance for a thin vinyl plank is generally tighter than for a thick rigid-core hybrid, so the instructions for the specific floor you choose are the reference point.

When a floating floor such as hybrid sits over a dip, the planks bridge the gap. Every time someone walks across that spot, the click joints flex slightly. Over months that movement can produce a hollow sound, a soft spot, creaking or, in the worst cases, joints that lift or open. Over a hump, planks rock and can stress the locking system. Thin vinyl plank, especially glue-down, does not bridge anything: it follows the surface underneath, so grout lines, trowel marks and lumps of old glue can show straight through the finished floor.

Flatness is also why large open-plan areas need extra attention. A slab that looks fine in one room can rise or fall by a few millimetres along a ten-metre run from the front door to the alfresco, and a floating floor laid in one continuous run will reveal it.

Step 1: removing the old floor properly

Preparation starts with what comes up. Carpet, underlay and smooth edge are removed, along with the staples and tack strips that hold them. Old floating floors and their underlay come up in one piece. Glued floors take more work: old vinyl, cork or glue-down planks leave adhesive behind, and that residue needs to be scraped or ground off so it does not create lumps or react with new adhesives.

Removal and disposal of the old floor is included in our quotes, so it is planned at the measure rather than left for you to organise.

One caution for older homes: vinyl tiles, sheet vinyl and the adhesive or backing under them in buildings from before 1990 can contain asbestos. If there is any doubt, the material should be tested, and removed by a licensed asbestos removalist where required, before any preparation work starts. We point this out at the measure so it can be planned properly. We do not remove asbestos ourselves.

Step 2: checking the slab

With the old floor gone, the slab is checked across every room with a long straight edge and by eye and hand. The things we look for:

• High spots and ridges, including old screed lines, concrete splatter and the edges where rooms were poured separately
• Dips and hollows, especially in the middle of large rooms and near doorways
• Cracks: fine shrinkage cracks are common and usually just need filling; wider cracks or cracks with one side higher than the other can point to movement and should be understood before a floor goes down
• Contaminants such as old adhesive, paint, plaster, oil or sealers that stop levelling compound or glue from bonding
• Signs of moisture such as damp patches, efflorescence (white powdery deposits), musty smells or lifting old floor coverings

Step 3: grinding high spots

High spots are ground down with a concrete grinder fitted with dust extraction. Grinding also removes thin layers of old glue and paint and opens up the surface so levelling compound bonds well. On most jobs grinding is targeted at specific ridges and lips rather than the whole slab, but older slabs with a lot of residue sometimes need a full surface grind.

Step 4: filling and levelling

Dips are filled with a cement-based levelling compound after the slab has been cleaned and primed. Primer matters: it seals the concrete so water is not drawn out of the compound too quickly, and it helps the compound bond.

There are two broad approaches, and the right one depends on the floor you have chosen and the condition of the slab:

• Patch levelling: compound is feathered into individual dips, cracks and low spots. This is common for hybrid and carpet over a slab that is generally in good shape.
• A flood coat: self-levelling compound is poured across the whole area to create a new, smooth, continuous surface. This is common for glue-down vinyl plank and sheet vinyl, for vinyl plank over tiles, and for slabs with widespread unevenness.
• Once dry, levelled areas are checked again with the straight edge and sanded smooth where needed before the floor goes down.

Timber subfloors: a different kind of preparation

Timber subfloors are usually less about flatness and more about movement. Squeaks and soft spots come from boards or sheets that have worked loose from the joists over decades. The usual fixes are screwing boards and sheets down to the joists, replacing damaged or water-affected sections, punching proud nails, and sanding or planing high board edges.

For hybrid, carpet and floating engineered timber, a sound and flat timber subfloor is a good base. Some floors need an extra layer such as plywood over old boards to give a smooth, continuous surface, and the installation instructions for the floor will say whether that is required. For raised homes, ventilation under the floor matters too: a damp, poorly ventilated space under a timber floor can affect any floor laid above it.

Moisture: the problem levelling cannot fix

Levelling compound fixes shape, not moisture. Concrete slabs hold water for a long time after they are poured, and slabs on the ground can draw moisture up from below if the original vapour barrier was damaged or missing. Too much moisture can cause hybrid and timber to swell or cup and glue-down floors to lose their bond.

Floor manufacturers set limits for slab moisture, and new slabs need time to dry before flooring goes down. If there are signs of a moisture problem at the measure, such as damp patches, efflorescence or a history of leaks, we will tell you, and it should be investigated and resolved before a moisture-sensitive floor is installed. In some cases a floor that tolerates more moisture is the better choice for that room.

Problems that poor preparation causes

When a new floor fails early, the cause is very often underneath it. Common examples:

• Hybrid floors that sound hollow or click underfoot in certain spots, usually over a dip that was never filled
• Peaking, where two rows of planks push up into a ridge, often a mix of an uneven subfloor and missing expansion gaps
• Vinyl plank that shows grout lines, trowel marks or lumps of old glue through the surface
• Glue-down floors lifting at the edges where adhesive could not bond to dusty or contaminated concrete
• Carpet with ridges or a visible step at a doorway where an old floor height was not dealt with
• Squeaks under carpet or floating floors on a timber subfloor that was not screwed down first
• Our guide to hybrid flooring problems explains peaking, gaps and hollow sounds in more detail.

How preparation should appear in your quote

A common complaint about flooring jobs is a price that grew on installation day because the slab "needed levelling". Preparation is very difficult to price accurately after the old floor is already up and the installer is standing on site, which is exactly why it should be assessed before you commit.

At a free in-home measure we check what we can see and feel: the existing floor, the slab or timber at doorways and edges, the age and type of home, and any signs of movement, tiles or old vinyl. That assessment goes into a fixed written quote that includes removal of the old floor, subfloor preparation and levelling, stairs and trims. If something is hidden under the existing floor that changes the scope, such as asbestos-containing material or a wet slab, we explain it and what it means before any further work is done.

When you compare quotes from different flooring companies, ask each one what preparation is included and how they will deal with the slab or timber floor. A lower price that leaves preparation out is not a like-for-like quote.

What you can do before the measure

You do not need to do anything technical, but a few things help us give you an accurate quote first time:

• Tell us the age of the home and whether you know what is under the current floor
• Mention any squeaks, soft spots, cracks in tiles, damp smells or past leaks
• Let us know if you plan to run one floor through several rooms, as long runs need the most careful levelling
• For apartments and townhouses, have your body corporate or strata by-laws handy, because the acoustic underlay affects the build-up height
• If you have the product name of a floor you like, the manufacturer's installation instructions will tell us its flatness and moisture requirements