Why subfloor preparation is the whole job
Subfloor preparation is everything that happens to the surface underneath before a finished floor touches it: measuring how flat it is, testing how wet it is, correcting both, and fixing whatever is loose, damaged or moving. It is invisible when it is done and unmistakable when it is skipped.
Here is the uncomfortable truth of this trade. When a floor gaps, cups, telegraphs, debonds, peaks at a doorway, hollows underfoot or splits along a joint, the cause is almost never the flooring. It is the surface it was laid on, or the moisture coming through that surface, or a fastener that was never driven. The finished floor simply reports the condition of the subfloor, slowly, over the following two years.
Which is why we treat this as a service in its own right rather than a line item hidden inside flooring installation. It gets measured, it gets written down, and it gets quoted openly, because it is regularly the largest variable in a job and it is the one nobody budgets for.
A floating floor telegraphs hollows. It bridges a dip, and every step flexes the locking joint over the void until the joint separates or the board cracks. A glue-down floor telegraphs ridges. It follows the substrate exactly, so a fastener head, a ridge of dried compound or a trowel line reads through as a visible line in raking light. Same subfloor, two different failures, depending on how the floor is attached.
How flatness is actually measured
Flatness is not levelness. A floor can slope steadily across a room and be perfectly flat. It can also be dead level on average and full of humps. Flooring cares about flatness. Levelness is a different conversation and usually a structural one.
Manufacturers write their requirement as a maximum deviation across a stated span. The two figures you will meet most often are 3/16 inch over 10 feet and 1/8 inch over 6 feet, with tighter products, large format tile and some glue-down goods calling for less again. Both numbers are meaningless without their span, which is the part people drop when they repeat them.
The measurement itself is simple and slow. A long, genuinely straight straightedge, 10 feet for the 10 foot spec, is laid on the floor. Feeler gauges or graduated shims are slid under it to find the largest gap. Then it moves and the process repeats, in a grid and on the diagonals, because a floor that passes north to south can fail east to west. On larger areas a rotary laser and a receiver on a graduated rod gets you a map of the surface faster, and the straightedge confirms the spots that look worst.
The point of the span is worth labouring. A four foot level on a gently undulating floor will show a small gap everywhere and pass at every position, because it is short enough to sit inside each wave. Extend to ten feet and the same floor fails badly. Short tool, wrong answer.
Correcting a subfloor: level, patch or grind
Once you know where the floor is out and by how much, there are three tools and choosing between them is mostly about the shape of the problem.
Self-levelling underlayment
A flowable cementitious compound, mixed to a controlled water ratio and poured over a primed surface, which finds its own level and cures hard and flat. It is the right answer when a floor is out of tolerance across a broad area rather than at a handful of points, and it is the only practical way to true a genuinely wavy slab.
Three things decide whether it works. The primer, which controls how fast the substrate pulls water out of the compound and which is the single most skipped step. The water ratio, because over-watering a levelling compound weakens it and causes it to shrink and crack as it cures. And containment, since the material is liquid and will happily find the stairwell, the heating ducts and the room next door if the perimeter is not dammed and every penetration sealed first. Depth limits per lift matter too, and deep fills usually need aggregate added.
What levelling compound is not is a structural product. It adds no stiffness. Pour it over a floor that bounces and you get a flat floor that still bounces, and eventually a cracked layer of compound sitting on a moving deck.
Patching
For isolated dips, gouges, old adhesive ridges and the gaps around a replaced panel, a trowel-applied patching compound is faster, cheaper and cleaner than pouring a room. It is feathered out at the edges so there is no lip, and it is checked with the straightedge once cured rather than by eye. Patching a floor that actually needs levelling is a false economy: you end up with dozens of flat islands in a floor that is still out of tolerance.
Grinding
High spots come off rather than getting buried. A ridge of concrete at a trowel line, a hump over a slab joint, hardened adhesive, or a proud fastener are all removed mechanically. Grinding is also how a slab is prepared for adhesive in the first place, and how a curing compound or old glue is taken off. It generates a large amount of silica dust, so it runs with extraction at the tool and the area contained. Removing a high spot is nearly always cheaper than levelling the entire floor up to meet it.
| Problem | How you find it | The fix | If it is ignored |
|---|---|---|---|
| Broad waviness or dips | Straightedge over 6 or 10 feet, or laser | Primer, then self-levelling underlayment | Hollows under a floating floor, joints fail |
| Isolated low spots | Straightedge, feeler gauge | Trowelled patching compound, feathered | Localised flex, cracked planks, loose tile |
| High spots and ridges | Straightedge rocks on the point | Grind flush | Telegraphs through glue-down, tile lippage |
| Squeaks and movement | Walk the floor, listen and feel | Refasten panels into the joists with screws | Squeak persists forever under the new floor |
| Deflection or bounce | Bounce test, joist span and spacing check | Blocking, sistering, or a second subfloor layer | Grout and tile crack, joints work loose |
| Delaminated or water damaged panel | Soft underfoot, swollen edges, visible layers | Cut out and replace full sheets to joist centres | The soft area fails through the new floor |
| High slab moisture | In-situ RH probe or calcium chloride test | Moisture barrier, or a tolerant system | Adhesive fails, cupping, mould, full replacement |
| High slab alkalinity | Distilled water and a pH strip or meter | Barrier or compatible adhesive system | Adhesive chemically breaks down |
| Curing compound or sealer | Water bead test, adhesive scratch test | Shot blast or grind to an open profile | Nothing bonds, floor lifts as a sheet |
| Cracks in the slab | Visual, plus checking for movement and offset | Fill static cracks, isolate or joint moving ones | The crack reappears through the finished floor |
Wood subfloors: plywood, OSB and everything loose
Above a crawlspace or between storeys, the subfloor is panels over joists, and the problems are mechanical rather than chemical.
Deflection
Deflection is how much the floor bends between joists under load. It is a function of joist size, span and spacing, and of panel thickness, and it is why tile fails in some houses and not others. Rigid finishes are unforgiving: a floor that flexes under a person will crack grout and eventually break tile bonds, and no amount of levelling compound changes that because compound adds mass without stiffness. The fixes are structural. Blocking between joists, sistering additional members alongside the existing ones, or adding a second layer of panel to increase the effective thickness. Rigid, forgiving finishes like carpet and vinyl plank tolerate far more movement, which is sometimes the honest answer for a floor that cannot be economically stiffened.
Squeaks and fastening
A squeak is a panel moving against a fastener, or a fastener moving in a joist. Usually it started as a nail that has withdrawn slightly. While the subfloor is open, the fix is quick: locate the joists, drive proper subfloor screws on a regular schedule along every joist line and around every panel edge, and the movement stops. Panel edges are where most squeaks live because that is where support is weakest and where fasteners are most often missed. Adhesive between panel and joist, where panels are being lifted or added anyway, kills it permanently.
This is the cheapest repair on the entire job and it has a hard deadline. The moment the finished floor is down, you have lost access, and a squeak under a glued-down floor is essentially permanent.
Delamination and damage
OSB and plywood both fail when they get wet and stay wet. Plywood delaminates, its plies separating and losing all strength while looking largely intact from above. OSB swells irreversibly, particularly at the edges, so a panel that got rained on during construction leaves a permanent ridge at every joint. Neither can be repaired in place with any honesty. The damaged sheet is cut out to the joist centres either side, so the new piece lands with full bearing, blocking is added along the unsupported edges, and a new panel of matching thickness goes in.
Panel thickness and type. Joist span, spacing and direction. Bounce, walked and felt rather than assumed. Fastener type and how many are missing. Moisture readings on the panels with a pin or pinless meter, and the difference between those and the flooring's readings. Any staining, which is a leak history you are being shown for free.
Concrete slabs: moisture, alkalinity and what is on the surface
Moisture, and why age tells you nothing
Concrete carries water for a long time, and a slab on grade never finishes drying because the ground beneath it keeps feeding moisture upward. Readings shift with the season and with the drainage around the building. A twenty year old slab can fail a test comfortably. A surface that feels bone dry can be carrying internal humidity well above any adhesive's limit.
Two tests are standard. An in-situ relative humidity probe is drilled into the slab to a specified depth, typically 40 percent of the thickness for a slab drying from one side, sealed into its sleeve, left to equilibrate, then read. It reports the humidity inside the body of the concrete, which is what the adhesive will eventually be exposed to once the floor traps it. A calcium chloride test seals a weighed dish of desiccant to the surface, leaves it for a set period, and weighs it again to give a vapour emission rate. In-situ RH is the more dependable of the two, because a surface test reads the driest part of the slab and can be flattered by a warm dry room.
Alkalinity
Water moving up through concrete carries alkali with it and concentrates it at the surface. High surface pH chemically attacks many adhesives, breaking them down to a soapy residue that has no bond strength left. Testing it takes minutes: distilled water on the clean slab, then a pH strip or meter. Adhesive manufacturers state a maximum, and a slab that passes on moisture can still fail here.
Curing compounds, sealers and hardeners
New concrete is very often sprayed with a curing compound, and floor slabs may also carry a sealer or a densifier. All of them are designed to sit on the surface and control moisture movement, which is exactly the property that stops an adhesive or a primer bonding to the concrete beneath. There is no reliable chemical way to remove them. They come off mechanically, by shot blasting or diamond grinding, until the slab has an open, absorptive profile that drinks a splash of water instead of beading it.
Two quick site tests tell you what you are dealing with. Drop water on the slab: if it beads or sits, something is sealing it. Scratch a coin across it: a glossy film that flakes is a coating, not concrete. On new construction this is a routine finding, and it belongs in the scope before the job starts. On commercial flooring work it is one of the most common causes of a change order nobody planned for.
Cracks
Cracks get sorted into two categories. A static crack, one that is not moving and has no vertical offset, is routed out and filled with a semi-rigid filler or an epoxy that restores continuity across the gap. A moving crack, one that opens and closes seasonally or has one side sitting higher than the other, will reappear through whatever you put over it, because it is still moving. That one needs a crack isolation membrane, sheet or liquid applied, which lets the substrate move without transmitting the movement into the finished floor. Existing control and expansion joints in a slab are a separate matter: they exist because the concrete has to move, and they are carried up through the finished floor as a joint rather than filled and covered.
Vapour barriers, and what they can and cannot do
Where a slab reads above the flooring or adhesive limit, a moisture mitigation system goes down: usually a two-part epoxy membrane rolled onto a prepared, shot blasted slab, which reduces vapour transmission to a level the adhesive can live with. That is a chemical barrier, and it works only if the slab profile underneath it was properly opened up first, which is the whole reason grinding comes before it.
A sheet vapour retarder is a different thing entirely: a polyethylene film laid loose under a floating floor, which stops vapour reaching the underside of the boards. It works for floating installations and it does nothing at all for a glue-down floor, because you cannot glue through it.
Two things a barrier does not do. It does not fix a leak, and if the moisture is coming from a plumbing failure or from surface water running against the building, the barrier is treating a symptom. It also does not level anything. Barrier and levelling are separate operations with a specified order, and the order is set by the products being used, not by convenience.
The warranty question
Flooring warranties are written around the manufacturer's stated installation conditions, and installation outside those conditions is excluded. That means flatness beyond the stated tolerance, moisture readings above the stated limit, a missing vapour retarder where one is specified, and skipped acclimation. All of them are exclusions, and they are the ones actually invoked when a claim is assessed.
What decides a real claim is the failure pattern. Cupping across a whole floor points at moisture. Separated joints in a traffic path point at a hollow underneath. Ridges telegraphing in raking light point at a substrate that was never ground. An assessor reads those the way anyone in the trade does, and if there is no record of a moisture test or a flatness check, the conversation is short.
So we take the readings and we write them down, before anything covers them. It protects you, and honestly it protects us, because the alternative is arguing about a floor nobody measured.
Common questions about subfloor preparation
How flat does a subfloor have to be?
Manufacturers state a maximum deviation across a span. The common figures are 3/16 inch over 10 feet and 1/8 inch over 6 feet, tighter for some products. It is measured with a long straightedge and feeler gauges, or a rotary laser over bigger areas, in a grid and on the diagonals. A short level passes on a wavy floor and is still wrong, so the span matters as much as the number.
What is self-levelling underlayment and when do we need it?
A flowable cementitious compound poured over a primed subfloor that finds its own level and cures flat. It is the right tool when a floor is out of tolerance over a broad area. Isolated dips are cheaper to patch and localised high spots are cheaper to grind. It adds no stiffness, so a floor that bounces has to be stiffened structurally first.
Why test a slab that is already years old?
Because age tells you very little. A slab on grade sits on ground that continuously feeds moisture upward, so it never fully dries and its readings move with the season. A slab can feel dry at the surface while carrying enough internal humidity to destroy an adhesive bond. An in-situ relative humidity probe reads the body of the slab rather than its face, which is why we use one.
Why do curing compounds have to be removed?
They are sprayed on fresh concrete to hold moisture in and control curing, so they form a film on the surface. That film is exactly what stops an adhesive or primer bonding to the concrete underneath. They cannot be dissolved reliably, so they come off mechanically by shot blasting or grinding until the slab drinks water instead of beading it.
What happens to the warranty if prep is skipped?
Most flooring warranties exclude installation outside the manufacturer's stated conditions, which covers flatness beyond tolerance, moisture above the limit, a missing vapour retarder and skipped acclimation. In practice the claim is declined because the failure pattern points at the subfloor and there is no test record to argue otherwise. That is why readings get taken and written down before anything covers them.
Is a squeak worth fixing before the new floor goes on?
Always. A squeak is a panel moving against a fastener, which means something is loose, and a new floor on top will not silence it. While the subfloor is exposed it is a fast job to drive proper screws into the joists on a regular schedule and stop the movement. Once the finished floor is down that access is gone and the repair becomes far more invasive.
What we do on site, in order
- Walk it. Feel for bounce, listen for squeaks, look for staining and past repairs, note where the floor changes construction.
- Measure flatness. Straightedge and feeler gauges in a grid and on the diagonals, or a laser on larger areas. Deviations get marked on the floor itself.
- Test moisture. Meters on wood, in-situ RH or calcium chloride on concrete, plus a pH reading on any slab that is going to be glued.
- Check the bond surface. Water bead and scratch tests for curing compounds, sealers and old adhesive.
- Fix what is loose. Refasten panels, replace damaged sheets to joist centres, add blocking where edges are unsupported.
- Correct the profile. Grind the high spots, patch the isolated dips, level the broad areas, prime everything first.
- Recheck, then hand over. The straightedge goes back on the floor after the compound cures. Numbers are recorded before anything covers them.
If the old floor is still down, the tear-out comes first and our floor removal service handles it, because you cannot assess a subfloor you cannot see. Prep almost always reveals something once the covering is off, which is why we scope it as a range at the measure rather than pretending to a certainty nobody has.
What to send us
Tell us what is on the floor now, what is underneath it if you know, whether the space is on a slab or over joists, and whether anything about the existing floor bothers you: a bounce, a squeak, a dip, a stain. Photographs of any damage help. If the building is new, tell us who poured the slab and when, because that decides whether we are expecting a curing compound.
Material choice interacts with all of this, so it is worth deciding together. Tile is the least tolerant of deflection and the most demanding on flatness. Hardwood is the most sensitive to moisture. Luxury vinyl plank and carpet forgive the most movement, and the flooring comparison lays the trade-offs out. Whether the job is a house, covered on our residential flooring page, or a workplace under commercial flooring, the prep decides the outcome either way. Start the estimate here.