Substrate preparation is the least visible and most consequential part of a flooring project. It is also the part homeowners are least prepared for, because nothing in a showroom hints at it: you choose a plank, you agree a price per square foot, and then the old floor comes up and the actual condition of the house becomes the subject.
Almost every flooring failure we are asked to diagnose is a preparation failure rather than a product failure. Hollow spots underfoot, click-lock joints that separate, cracked grout in a tile field, cupped boards over a slab, a floor that squeaks worse than the one it replaced — all of these trace back to something under the finished surface that was not right before the material went down. This guide covers what installers actually require and why.
The first thing to understand is that installers care about flatness, not level. Those are different properties. A floor can slope noticeably across a room — as plenty of older Sacramento and foothill homes do — and still take a rigid plank product without complaint, because the planks sit continuously on a surface that happens to be tilted. The same floor with a dip in the middle and a hump at one end will fail, even if a spirit level reads perfectly at the ends.
Flatness is specified as a maximum deviation over a stated span — a tolerance the flooring manufacturer publishes, and one that binds the warranty. It is measured by laying a long straightedge across the substrate in multiple directions and checking the gap underneath. This is a physical, repeatable measurement, not an opinion, which is why it is the right thing to ask an installer about before work begins.
The tolerances have tightened in practice as the market has moved toward long, wide planks. A short, narrow strip floor bridges minor irregularities easily. A long rigid plank spans them instead, which produces a plank supported at two high points with air beneath it — the source of that hollow, drumming sound and, on a click-lock floor, the repeated flexing that eventually works a locking joint apart. Tile behaves similarly for a different reason: a tile bedded over a hollow has no support beneath it and cracks when someone stands on it.
Levelling a floor to true level is a separate and much larger undertaking, and it is rarely what a flooring project needs. Correcting flatness so the substrate falls within the manufacturer's tolerance is what is actually required, and it is a different conversation about scope and cost.
Filling low areas with self-levelling underlayment
A cementitious self-levelling compound is mixed to a fluid consistency and poured, flowing out to fill depressions and produce a flat plane. It is not a filler you trowel and hope: it has a working time measured in minutes, a mixing ratio that must be respected, and a substrate-preparation requirement of its own — the surface has to be clean, sound, and typically primed so the compound bonds rather than curling at the edges. It also has a cure time before flooring can go over it, which is a scheduling consideration on any project.
Sanding or planing high areas
On a wood subfloor, high spots — a proud panel edge, a crowned joist, a ridge at a seam — are taken down mechanically rather than buried under compound. This is faster and cheaper than filling everything around them and avoids adding unnecessary height to the assembly. On a concrete slab, high areas are ground down.
Most floors need both
A real substrate is rarely uniformly high or uniformly low. The normal sequence is to map the surface with a straightedge, mark the deviations, take down what is high, then fill what remains low. Skipping the mapping step and pouring compound everywhere is expensive and adds height for no reason.
Patching and underlayment panels
Where a wood subfloor is sound but the surface is unsuitable — an old adhesive residue, a deteriorated top ply, or a panel type the flooring will not accept — an underlayment panel over the existing subfloor can be the cleaner answer than trying to rehabilitate the surface. For tile, a cement backer board or an uncoupling membrane is a separate requirement over and above flatness.
Squeaking is movement, and movement has a source. The window to fix it is while the subfloor is exposed — which is precisely the moment most homeowners are thinking about plank colours instead.
Common causes
- Movement between the subfloor panel and the joist beneath it, where the original fastening has loosened.
- Panel edges rubbing against each other at a seam with no support underneath.
- Nails or staples working against the wood as the framing dries and shrinks over years.
- A joist that has cupped or twisted, leaving the panel bearing on it unevenly.
- Blocking or bridging between joists that has loosened over time.
What will not fix it
- The finished flooring itself — replacing the surface without addressing the subfloor rarely fixes it.
- A heavier or thicker flooring product masking the noise reliably.
- Underlayment pads, which quiet impact sound but do not stop structural movement.
If squeaks bother you, say so before demolition rather than after. Re-fastening a subfloor to the framing is straightforward with the surface open and awkward once it is covered.
On any concrete slab, moisture testing is a step, not an option. Concrete releases water vapour as it cures and continues to transmit vapour from the ground beneath indefinitely if there is no effective vapour retarder under the slab or if site drainage sends water toward the foundation. A floor installed over a slab emitting more moisture than the product tolerates can cup, delaminate, lose adhesive bond, or trap moisture beneath an impermeable covering.
There are two widely used methods. An in-situ relative humidity test places probes into holes drilled into the slab and measures the humidity within the concrete itself once equilibrium is reached — this is generally regarded as the better indicator of what the slab will do over time, because it reads conditions at depth rather than only at the surface. A calcium chloride test measures the rate of moisture vapour emission from the slab surface over a set period by weighing a sealed dish of desiccant before and after.
Each flooring manufacturer publishes limits for its products against these methods, and the warranty is conditioned on meeting them. That is the practical reason not to skip the test: it is not that testing prevents moisture, it is that testing tells you which products you can install and preserves your recourse if something goes wrong.
Where a slab reads high, the answer is usually a moisture-mitigation system — a topically applied membrane or coating designed to reduce vapour transmission — installed before the flooring. That is real added scope, which is why it is far better discovered during assessment than after the material has been ordered.
Raised-foundation homes over vented crawlspaces have the same question in a different place. There, the moisture content of the subfloor is fed from below, and a crawlspace with proper ground cover, functioning ventilation, and drainage that moves water away from the foundation is doing work for every floor above it. Federal building-science guidance on foundation and crawlspace moisture management is the reference worth reading before installing wood over one.
For the underlying building science, see the U.S. Department of Energy's Building America guidance on foundations and crawlspaces, the Building Science Corporation's work on moisture control, and the EPA's guide to mold, moisture and your home.
- 01Flat within the flooring manufacturer's published tolerance, verified with a straightedge in multiple directions.
- 02Structurally sound — no delaminated panels, soft spots, or water-damaged sheathing left in place.
- 03Dry — slab moisture tested against the product's stated limit, or crawlspace conditions addressed on a raised floor.
- 04Clean — old adhesive, fasteners, staples, and debris removed so the new assembly bonds or sits correctly.
- 05Secure — subfloor re-fastened to the framing where it has loosened, before anything covers it.
- 06Stiff enough for the covering — tile in particular depends on floor framing that limits deflection.
An honest proposal names substrate work as its own line rather than folding it into a square-foot rate, because part of it genuinely cannot be known until the old floor is out. We tell you what we expect to find, and we tell you when what we find changes the number.
How flat does my subfloor actually need to be?
The requirement comes from the flooring manufacturer rather than a single universal figure, and it is stated as a maximum deviation over a given span — measured by laying a long straightedge across the substrate and checking the gap beneath it. What matters more than memorising a number is knowing that tolerances tighten as planks get longer and wider, and the long wide planks currently in fashion are the least forgiving products on the market. Ask your installer what tolerance the specific product you have chosen requires, and ask how they intend to verify it. That is a straightforward question with a straightforward answer, and it is the best predictor of whether the installation will hold up.
My floor slopes. Does it need to be levelled before new flooring?
Probably not, and this is one of the most common misunderstandings in flooring. Installers require flatness, not level. A floor that slopes consistently across a room but has no dips or humps can generally take a rigid plank product without issue, because every plank sits continuously on the surface. What causes failures is local deviation — a dip in the middle of a run leaves a plank bridging air and flexing, which produces hollow sound and eventually separates a locking joint. Bringing a sloped floor to true level is a structural undertaking of an entirely different scale, and it is rarely what a flooring project needs.
Will new flooring fix my squeaky floors?
Only if the squeak is addressed while the subfloor is exposed. Squeaks come from movement — between the subfloor panel and the joist beneath it, or between panel edges at an unsupported seam — and a new surface laid over that movement does not stop it. The good news is that the window to fix it is exactly when the old floor is up: re-fastening the subfloor to the framing at the noisy areas is comparatively easy at that moment and difficult afterwards. If squeaks are a priority, say so before demolition so the crew addresses them rather than covering them, and understand that some causes rooted in the framing itself may need more than surface re-fastening.
What is a slab moisture test and do I really need one?
It measures how much water vapour a concrete slab is giving off, using one of two recognised methods: in-situ relative humidity probes placed in holes drilled into the slab, or a calcium chloride test that weighs moisture absorbed by a sealed desiccant dish over a set period. You need it because flooring manufacturers set limits and condition their warranties on meeting them — installing over an untested slab does not avoid the risk, it just transfers it to you. If the slab reads above the product's limit, the usual remedy is a moisture-mitigation membrane or coating applied before installation, which is real added scope and much better discovered during assessment than after the material arrives.
How long does self-levelling underlayment take to cure?
It varies by product, thickness, and site conditions, and the manufacturer's data sheet is the authority — some products accept foot traffic within hours while requiring longer before flooring goes over them, and thicker pours take longer than thin ones. What matters for planning is that it is a real interval in the schedule, not an afternoon. It also has requirements on either side: the substrate has to be clean, sound, and usually primed for the compound to bond properly, and the compound has to be mixed to the specified ratio and placed within a short working time. Rushing any part of that produces a layer that debonds later, which is worse than not levelling at all.
Can I install new flooring over the old floor instead?
Sometimes, and it is genuinely attractive because it avoids demolition and disposal. Whether it is appropriate depends on the existing floor's condition, whether it is bonded and stable, whether the combined assembly stays within the flatness tolerance, and what the added height does to door clearances, appliance heights, and transitions to adjoining rooms. Layering over an existing floor also means any problem beneath it — a squeak, a moisture source, a soft spot — is now permanently sealed in. We look at the specific situation rather than answering the question in the abstract, and we say plainly when removing the old floor is the better decision even though it costs more.
Planning new floors over a slab in Elk Grove, a raised foundation in Sacramento, or a foothill home in Auburn? We assess what is under the old floor before quoting what goes on top of it, right across Northern California. Request a free estimate
The following government agencies, industry organizations, and official resources provide additional information relevant to your remodeling project.