Floor Levelling near you

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.

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.

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.

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.

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.

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:

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: