In short
Most new floors that fail were let down by what is underneath them, not by the flooring itself. Subfloor preparation means making the surface flat to the manufacturer's tolerance, dry enough for the product, clean and sound, with the right underlayment and room at the edges for the floor to move. Skip those steps and even an excellent floor will gap, crack or squeak.
Key takeaways
- Every flooring product comes with a flatness tolerance, and the subfloor has to meet it before installation starts.
- High spots are ground down and low spots are filled with patch or self-leveling compound, not hidden under padding.
- Concrete slabs should be tested for moisture before wood, LVP or laminate goes down.
- Floating floors need an expansion gap at every wall and fixed object, or they buckle.
A floor that cracks along a grout line, clicks when you walk across it, separates at the seams or lifts in the middle of the room rarely started with a bad product. In most failed floors, the problem was already there on the day of installation, under the new floor. That is what subfloor preparation is for.
It is also the part of the job you never see once it is finished, which makes it the easiest part to rush. Here is what good preparation involves, so you know what to ask about.
What does subfloor preparation actually include?
The subfloor is whatever the new floor sits on. In most Dallas-Fort Worth homes that is a concrete slab. Upstairs rooms and some older homes with pier-and-beam foundations have a wood subfloor of plywood or boards over joists.
Preparing it usually means five things:
- Removing the old floor completely, including carpet tack strips, staples, old adhesive ridges and broken thin-set.
- Checking flatness against the tolerance printed in the installation instructions for the product you chose.
- Correcting it, by grinding high spots and filling low spots.
- Testing for moisture on concrete, and choosing a barrier or adhesive that matches the result.
- Cleaning and planning the edges, so the underlayment lies flat and the floor has room to expand.
How flat does the floor need to be?
Flat and level are not the same thing. Level means horizontal. Flat means no humps or dips over a short distance. A floor can slope gently across a room and still be perfectly acceptable, but it cannot have a dip that the planks bridge over.
Each manufacturer publishes its own tolerance, usually written as a maximum deviation over a set distance, measured with a straightedge. For many floating LVP and laminate products it is commonly around 3/16 inch over 10 feet. Large-format and long plank tile typically calls for a tighter tolerance, because long pieces show every hump as lippage at the edges. The installation sheet in the box is the rule, and it is also what the product warranty is judged against.
What happens when a floor is not flat enough depends on the material:
- Floating LVP and laminate flex over dips. The locking joints work loose, seams open, and the floor sounds hollow or clicks underfoot.
- Tile ends up with uneven edges and hollow spots in the mortar, which is where cracks start.
- Glue-down wood and vinyl cannot make full contact with the adhesive, leaving soft spots that can release over time.
Grinding, patching and self-leveler
High spots on a slab, such as a trowel ridge or a lip at an old crack, are ground down with a diamond grinder connected to a dust extractor. That is loud work but it is short, and it is far better than trying to raise the whole room to meet the high point.
Low spots are filled. Small dips are feathered out with a cement-based patch and troweled smooth. Larger areas get a self-leveling compound, a pourable cement mix that flows out and finds its own flat surface. The slab is cleaned and primed first so the leveler bonds, and dams are set at doorways to keep it where it belongs. Leveler has its own drying time before flooring can go over it, and that time is printed on the bag.
In older homes, one caution matters before any grinding starts. Some black adhesives used under older floors can contain asbestos, so if we find old dark mastic under a floor in an older house, it should be tested before anyone sands or grinds it.
Wood subfloors upstairs
On a wood subfloor, preparation means walking every square foot. Loose panels are screwed down to the joists to stop squeaks, damaged or water-stained sections are cut out and replaced, and raised seams are sanded flat. Soft spots near toilets and tubs are a warning sign of an old leak and are opened up rather than covered.
Why moisture testing on a slab matters
Concrete looks dry but it carries moisture from the ground below, and that moisture moves up through the slab as vapor. It does not hurt the concrete. It does hurt what sits on top: wood cups and swells, adhesives lose their grip, and moisture trapped under a floating floor can cause odor and mold.
Moisture on a slab is measured, not guessed. The two common methods are relative humidity probes set into small holes drilled in the concrete, and a calcium chloride test sealed under a dome on the surface. The reading is compared with the limit the flooring or adhesive maker publishes. A plastic sheet taped to the floor overnight is a rough screen at best and not a substitute for a real test.
If the reading is high, the fix depends on the floor: a vapor barrier underlayment under floating floors, a moisture-control adhesive or sealer under glue-down floors, or choosing a product rated for those conditions. Our page on engineered and solid hardwood installation explains why wood is the most sensitive of all to this step.
Underlayment and expansion gaps
Underlayment is the thin layer between the subfloor and a floating floor. Depending on the product, it cushions, quiets footsteps, smooths very small imperfections and blocks moisture. It does not fix a floor that is out of flat, and a thick soft pad under a rigid plank can make the locking joints flex and fail. Many luxury vinyl plank floors come with a pad attached, and those manufacturers commonly say not to add a second one.
| Floor type | What usually goes under it on a slab |
|---|---|
| LVP with attached pad | A vapor barrier sheet if the maker requires it, nothing else |
| LVP without a pad | An underlayment approved by the plank maker |
| Laminate | Underlayment with a built-in vapor barrier |
| Floating engineered wood | Vapor barrier plus a foam or cork underlayment |
| Glue-down wood or tile | No pad, adhesive or thin-set directly on the prepared slab, with a membrane where needed |
Expansion gaps are the other quiet rule. Floating floors grow and shrink with temperature and humidity, so they need a gap at every wall, cabinet run, fireplace hearth and door jamb. The baseboard and trim cover it. When a floating floor is pinned down by cabinets set on top of it, a heavy island, or tight trim, it has nowhere to go and it peaks at the seams. Very long runs and some doorways may also need a transition joint, according to the manufacturer's limits. Tile has its own version: movement joints at the perimeter and in large fields, filled with flexible sealant rather than grout.
Door jambs, height and the next room
Preparation also covers the places where the new floor meets the rest of the house. Door casings and jambs are undercut with a flush saw so the new floor slides underneath, which looks cleaner than scribing planks around the trim and leaves room for movement. Door bottoms may need trimming if the new floor sits higher than the old one.
Height matters at every transition. Leveler, underlayment and the floor itself all add thickness, and the finished surface has to meet the next room, the stair nosing and exterior thresholds without an awkward step. Planning those heights before anything is poured or installed avoids steep reducers and trip edges later. Appliances count too: a dishwasher that fits under the counter today may not slide out once a thicker floor goes in front of it.
Signs your last floor failed from below
- Seams opening or planks peaking along a line.
- Clicking, crackling or a hollow sound in one area.
- Cracked tiles or grout following a line across the room.
- Wood boards cupping, with edges higher than the centers.
- A musty smell when floating planks are lifted.
Each of these points to flatness, moisture or edge problems, and each should be diagnosed before a new floor goes in, or the new floor inherits the same fate. You can read how we handle the rest of the job on our flooring installation services page and in our step-by-step project process.
What to ask before you call for an estimate
Preparation should be written into your estimate, not treated as a surprise later. When you compare quotes, ask:
- Will the slab or subfloor be checked for flatness against the product's tolerance?
- Is moisture testing included on concrete, and what happens if the reading is high?
- How are high and low spots corrected, and is self-leveler included?
- Which underlayment will be used, and does the plank maker approve it?
We check all of this during the visit and spell it out in the written price. Book a free flooring estimate and we will look at what is under your floor before recommending what goes on top.
Questions homeowners ask
Is subfloor preparation really necessary for LVP?
Yes. LVP is often sold as forgiving, but floating click planks depend on a flat surface to keep their joints locked. Dips cause flex, separation and a hollow sound. The plank maker's installation sheet lists a flatness tolerance, and installing over a floor that does not meet it can void the product warranty.
Can new flooring go over old tile?
Sometimes. The old tile must be well bonded, flat and free of cracks, and the added height has to work at doors, appliances and the next room. Grout lines may need to be filled so they do not print through thin vinyl. In many cases removing the old tile gives a better result.
How long does self-leveler need before flooring goes on?
It depends on the product, its thickness and the conditions in the house. Some compounds accept foot traffic within hours but need longer before flooring or adhesive goes over them. The drying time on the bag is the rule, and moisture-sensitive floors should wait until the leveler meets the limit.
