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The need usually shows up when a new floor is being quoted: click planks, large tiles and thin vinyl demand a flatter base than carpet or old sheet flooring ever did. Older houses add settling and notched joists, and basement slabs add cracks and moisture that must be understood before anything covers them.
This page covers flat versus level, levelling concrete, fixing wood subfloors, when a sag is structural, moisture testing and basement slabs, silica dust and drying time.
Signs your floor needs levelling, and why flat matters more than level
Installers treat flat and level as different things. Level means horizontal; flat means free of dips and humps over a given distance. Flooring manufacturers set a flatness tolerance in their instructions, expressed as the largest gap allowed under a straightedge of a set length, and the floor must meet it for the warranty to apply. A floor can slope gently across a room and still be flat enough, while one that's level on average can hide a hollow that breaks a click joint.
- Joints in a floating floor that open, peak or creak in the same spots.
- Cracked tiles or grout, or tile edges sitting higher than their neighbours.
- Furniture that rocks, or a ball that always rolls to the same side of the room.
- Humps over the joists, or a dip where an old wall once stood.
- Soft, springy or hollow-sounding areas underfoot.
- Doors that swing on their own, or gaps opening under the baseboards.
Levelling a concrete floor: grinding, patching and self-levelling underlayment
Concrete is levelled by taking high spots down and filling low ones. Small hollows, cracks and joints get a trowelled patching compound feathered out to meet the slab. Larger areas get self-levelling underlayment, a cement-based mix poured fluid so it flows out into a flat surface. It isn't magic: it follows a sloping slab unless the installer sets the target height and guides it, and it only bonds to a well-prepared surface.
- Removal: old flooring, adhesive and paint come off; in older homes, vinyl tiles, sheet flooring and black adhesive may contain asbestos and are tested before anyone grinds or scrapes them.
- Grinding: ridges, high spots and weak surface material are ground away until the slab is clean and sound.
- Cracks and joints: cracks are cleaned and filled, while joints meant to move are carried through rather than buried.
- Primer and dams: the primer named by the maker seals the slab and helps bonding, and foam or tape closes gaps at walls, drains and doorways.
- Pour: the mix goes down in one continuous operation, spread with a gauge rake and rolled with a spiked roller to release trapped air.
- Curing: drafts, direct sun and heaters blowing on the surface are kept away while it sets, as the label directs.
Levelling wood subfloors: sanding seams, shims, sistered joists and new plywood
On a wood-framed floor, the fix depends on where the problem sits: in the subfloor panels, in individual joists, or in the beams and posts that carry them. A careful contractor looks from below when the basement ceiling is open before choosing a method.
- Sanding and refastening: ridges at panel seams are sanded flat, and loose or squeaky panels are screwed down to the joists.
- Shimming: with the subfloor off, tapered wood strips are fastened to the tops of low joists to bring them into one plane.
- Sistering: a new joist is fastened alongside a crowned, cracked or twisted one to straighten or strengthen it; this is framing work for a qualified carpenter, following an engineer's design where one is needed.
- New plywood: an underlayment panel glued and fastened over the subfloor stiffens it and gives thin flooring a smooth base.
- Compound over wood: only products approved for wood, with the primer and any reinforcing mesh they require, and only over a floor that doesn't flex.
When a sagging floor needs structural repair, not levelling
Compound and plywood correct the surface; they can't stop a floor that is still moving, and piling compound into a sag adds weight to framing that may already be struggling. Warning signs include a slope that keeps getting worse, a sag toward a centre beam or where a wall was removed, cracks in the walls or ceiling above, doors that have gone out of square, joists cut for plumbing or ducts, and soft wood near the foundation.
Those call for a structural engineer or a structural repair contractor, who may recommend new posts, beams or reinforced joists; see the structural repair page. Once the framing is sound, the flooring contractor takes care of the final flatness.
Basement slabs: cracks, dampness and moisture testing before new flooring
Concrete holds water long after it looks dry, and a basement slab resting on the ground can keep drawing moisture from below. That moisture can break down adhesives, swell wood-based floors, feed mould under pads and loosen levelling layers. Flooring and adhesive manufacturers therefore require a moisture test on the slab and set their own limit for the result. The installer should name the test, record the readings and compare them with that limit before anything goes down.
Cracks need reading too. Fine, stable shrinkage cracks are common and get filled during preparation. A crack that is wide, lets water in, keeps growing or has one side higher than the other points to a drainage, frost or foundation problem, and levelling over it only hides it until it shows through the new floor. Have it assessed first, starting with the foundation cracks page, and stop any water entry before the basement is finished.
Silica dust when grinding concrete and sanding levelling compound
Grinding a slab, cutting out cracks and sanding cured patching compound all turn cement-based material into airborne dust containing crystalline silica. According to the Canadian Centre for Occupational Health and Safety (CCOHS), inhaling it can harm the lungs, and crystalline silica is a recognized cause of lung cancer. CCOHS advises keeping the dust down with water or collecting it with an approved HEPA vacuum rather than sweeping it dry or blowing it with compressed air. On a levelling job, that looks like this:
- Grinders connected to a dust extractor with a HEPA filter, or wet methods where water won't cause harm.
- Respirators suited to silica dust for everyone doing the work.
- Plastic sheeting over doorways, with heating and ventilation ducts covered while grinding.
- Children and pets kept out, and cleanup with a HEPA vacuum, never a broom.
How long floor levelling takes to dry before new flooring goes down
There's no single drying time. Each product label gives one time for walking on it and another for installing flooring over it, based on the temperature and humidity it states. What stretches the wait:
- Thickness: deep pours hold more water and take longer than thin skim coats.
- Product: rapid-setting compounds are made for a quick turnaround, while standard mixes need more patience.
- Room conditions: cold slows curing and humid summer air slows drying; steady heat and gentle ventilation help, but heaters aimed at the surface can crack it.
- What's underneath: a new or damp slab keeps releasing moisture after the compound sets, so moisture-sensitive floors wait for a passing reading, not a date on the calendar.
Floor Levelling: frequently asked questions
Can I level just part of a floor?
Yes, if the problem is local: patching compound can fill a single hollow, and a high seam can be sanded down. The transition into the rest of the floor still has to meet the flooring's flatness tolerance, so the installer checks well beyond the repair.
How flat does a floor need to be for vinyl plank or tile?
The flooring manufacturer sets the tolerance, and large tiles and click floors are the strictest. Ask the installer which figure applies to the product you chose, and how and where it will be checked.
Will levelling raise my floor?
Usually a little, since compound, plywood or shims add material. Check doors, thresholds, transitions to other rooms, the toilet flange and under-counter appliances beforehand: a few added layers can leave a dishwasher stuck in place.
Why do floors slope in older houses?
Wood beams shrink and settle, posts sink, and joists were sometimes cut for plumbing or overloaded by later renovations. A gentle, stable slope can often be corrected under new flooring, but one that's getting worse needs a structural assessment first.
Sources
Written by the LokalMatch editorial team. Reviewed by Zubair Afzal on September 10, 2026. How we write and check our guides