The best underlay for vinyl flooring is very often none at all. If your planks have a pad attached to the back — and most rigid-core luxury vinyl plank now does — the manufacturer has already chosen the underlayment, and adding a second one usually voids the warranty and lets the click joints flex until they break. The only thing such a floor typically needs underneath is a 6-mil polyethylene vapor barrier when it goes over concrete.
When the planks have no pad, you do need something, and it must be thin and firm: roughly 1–1.5 mm, occasionally up to 2 mm, and dense enough not to compress under foot traffic. For that job our pick is MP Global QuietWalk LV, a 1.4 mm dense-fiber underlayment with an attached vapor barrier. For condos with strict sound rules, Roberts SoundBarricade rubber is the stronger choice. The soft 2–3 mm foam sold for laminate is the wrong product, even when it sits beside the vinyl in the store.
Our Top Picks
- BEST OVERALL: MP Global QuietWalk LV Underlayment
↓ Jump to Review - BEST FOR CONDOS & SOUND CONTROL: Roberts SoundBarricade Underlayment
↓ Jump to Review - BEST THIN UNDERLAYMENT: FloorMuffler Encore Underlayment
↓ Jump to Review - BEST FOR THICKER RIGID CORE: FloorMuffler UltraSeal Underlayment
↓ Jump to Review - BEST FOR PADDED PLANKS ON CONCRETE: MP Global MoistureBlock 6-mil Vapor Barrier
↓ Jump to Review
Do You Even Need an Underlayment?
Answer this before buying anything, because the wrong underlayment does more damage than none. Turn over a plank, or check the spec sheet, and find out three things: whether a pad is attached, what core it has, and whether it floats or is glued down.
Attached pad (IXPE foam, cork or similar). This describes most mid-range and premium click-lock LVP — COREtec, Pergo Extreme, LifeProof, many Shaw and Mannington collections. The pad is typically 1–1.5 mm and has been engineered with that plank's locking profile. Do not add a foam, felt or cork underlayment on top. A second soft layer lets the joint move up and down every time someone walks across it. A click joint is a small, precisely machined tongue of vinyl and mineral filler, and repeated vertical flex is what cracks it, peaks the seams and opens gaps. Almost every manufacturer states this in the installation instructions, and a warranty claim on a floor installed over a second pad is easy to refuse.
The narrow exception is a product the plank maker explicitly allows. Some acoustic underlayments are marketed as compatible with attached-pad planks — SoundBarricade's sheet permits attached pads of 1.5 mm or less, for instance — but the underlayment maker's permission is not the plank maker's. If the flooring warranty says no additional underlayment, that is the answer.
Rigid core, no pad. Budget SPC lines, some contractor-grade products and many thinner planks ship bare. These need an underlayment for sound, for a little comfort, and — over concrete — for moisture protection. The manufacturer will name a maximum thickness and often a minimum density or an approved product list.
Flexible glue-down vinyl (LVT), sheet vinyl and peel-and-stick tile. These bond to the subfloor and must not go over any cushioned underlayment. What they need is a smooth, flat, solid substrate — sometimes a plywood underlayment panel, which is a different thing entirely. More on that below.
The Picks at a Glance
| Product | Material | Thickness | Vapor barrier | Published sound ratings | Best for |
|---|---|---|---|---|---|
| MP Global QuietWalk LV | Dense recycled fiber | 1.4 mm | Attached film | IIC 70 / STC 62 / ΔIIC 28 | Unpadded rigid core, most homes |
| Roberts SoundBarricade | Rubber, 25 lb/ft³ | 1.5 mm | No — add poly on concrete if required | IIC 69 / STC 63 / ΔIIC 25 (LVP) | Condos, upper floors |
| FloorMuffler Encore | Closed-cell polypropylene foam | 1.1 mm | Yes, UltraSeal lip & tape | IIC up to 69 / STC up to 62 / ΔIIC 22 | Thinnest option, tight limits |
| FloorMuffler UltraSeal | Closed-cell polypropylene foam | 2 mm | Yes, UltraSeal lip & tape | IIC up to 74 / STC up to 73 / ΔIIC 25 | Thicker rigid core that allows 2 mm |
| MP Global MoistureBlock | 6-mil polyethylene film | 0.006 in | Yes — that is all it is | None (not acoustic) | Padded planks over concrete |
Sound ratings are the manufacturers' published figures for their tested assemblies, usually a concrete slab with a specific flooring on top. They are useful for comparing products but are not a promise of what your own floor will achieve — see the sound section for why.
The Best Underlayments for Vinyl Plank
What We Like
- 1.4 mm suits typical rigid-core thickness limits
- Attached vapor barrier covers concrete in one layer
- Strong published Delta IIC for its thickness
- Approved for radiant heat
- Wide range of roll sizes
What We Don’t Like
- Costs more per square foot than basic foam
- Some plank makers cap underlayment below 1.4 mm
- Not a fix for an uneven subfloor
Product Specs
- Type: Dense Fiber / Attached Vapor Barrier
- Rank in this guide: #1
QuietWalk LV is the vinyl-specific version of MP Global's long-running QuietWalk line, and it is built around the constraint that matters most for rigid core: it is thin and it does not squash. The published construction is a 1.4 mm mat of dense recycled fibers with an attached vapor barrier film, rated at a water vapor transmission of 0.12 lb per 1,000 sq ft per 24 hours. MP Global lists it for floating and glue-down vinyl plank and tile, SPC, WPC and laminate, and approves it over in-floor radiant heat.
Fiber behaves differently from foam underfoot. It is dense enough to support a locking joint but deadens the hollow, clicky sound floating floors are known for, and the manufacturer's figures — IIC 70, STC 62 and a Delta IIC of 28 — are strong for something this thin. It also smooths very minor subfloor texture (MP Global quotes about 1 mm), which is not the same as levelling a dip. Rolls run from 50 to 360 sq ft, so a single room does not leave you with a surplus.
The caveat applies to everything on this list: MP Global says QuietWalk LV can go under planks with a pre-attached pad, but your plank maker may say otherwise. If the planks are padded, read their instructions first.
Roberts SoundBarricade Underlayment
See It on AmazonWhat We Like
- Dense rubber resists compression and protects joints
- Published LVP ratings above typical condo minimums
- Clear stated limits for core and pad thickness
- Also approved for double glue-down
What We Don’t Like
- No built-in vapor barrier
- Heavier rolls, higher cost than foam
- Not permitted under planks with cores below 4 mm
Product Specs
- Type: Rubber / Acoustic
- Rank in this guide: #2
Rubber is the material acoustic consultants reach for when impact noise is the problem, and SoundBarricade puts it in a format a homeowner can roll out. Roberts lists it at 1.5 mm and 25 lb per cubic foot, made from rubber blended with recycled tire crumb, in 200 sq ft rolls 36 inches wide. Under LVT and LVP, the published ratings are IIC 69, STC 63 and a Delta of 25.
What makes it relevant to vinyl rather than only laminate is that the product sheet spells out its own vinyl rules: approved under LVP, SPC, WPC and engineered vinyl plank, the flooring must have a core of 4 mm or more, and any attached pad must be 1.5 mm or thinner. That candour is useful, because it is exactly the kind of limit that keeps a thin plank's joints intact. Rubber's density also means it resists compression far better than foam over time, and Roberts publishes an indentation figure of 12% (ASTM F1914).
The trade-offs: it is heavier and costs more than foam, and it has no attached film, so over concrete you may still need a separate 6-mil poly depending on the plank maker. It also has a lower R-value than fiber, which matters little in practice but is worth knowing over radiant heat.
What We Like
- 1.1 mm fits tighter manufacturer limits
- Integrated vapor barrier with self-sealing seams
- Closed-cell foam does not absorb moisture
- Lightweight and easy to cut and lay
What We Don’t Like
- Lower published Delta IIC than fiber or rubber
- Less comfort underfoot than thicker options
- Ratings published as "up to" figures
Product Specs
- Type: Closed-Cell Foam / 1.1 Mm
- Rank in this guide: #3
Some rigid-core manufacturers set the ceiling at 1 mm or a little over, and a 1.4–1.5 mm product is then simply not allowed. Encore is FloorMuffler's answer: a 1.1 mm closed-cell polypropylene foam sold in 100 sq ft rolls and marketed specifically for vinyl plank and rigid-core floors. It carries FloorMuffler's UltraSeal system, an overlapping film lip with adhesive tape that turns the seams into a continuous moisture barrier.
Published figures are IIC up to 69, STC up to 62 and Delta IIC up to 22 — respectable for its thickness, though below the denser fiber and rubber options. Closed-cell foam does not absorb water, which makes it a sensible choice over concrete in a basement where the barrier and the cushion arrive in one layer. FloorMuffler also positions Encore as usable with or without a pre-attached pad; as always, treat that as permission from the underlayment side only.
What We Like
- Highest published sound ratings in this list
- Self-sealing lip and tape moisture barrier
- Available in 100 and 600 sq ft rolls
What We Don’t Like
- 2 mm exceeds many rigid-core limits
- Primarily designed for laminate and wood
- Softer than rubber or dense fiber
Product Specs
- Type: Closed-Cell Foam / 2 Mm
- Rank in this guide: #4
UltraSeal is FloorMuffler's flagship 2 mm underlayment, and it is primarily a laminate and wood-floor product — which is exactly why it needs careful handling here. FloorMuffler's physical properties sheet lists closed-cell polypropylene foam, IIC up to 74, STC up to 73, Delta IIC up to 25 and a compressive strength of at least 11 psi at 25% compression. Retail listings show it as compatible with rigid-core floors of 5 mm and thicker.
That is the use case: a thicker, stiffer SPC or WPC plank with no attached pad, whose manufacturer permits a 2 mm underlayment, where you want the most sound reduction available in a foam with a built-in moisture barrier. On a thin plank, or one whose instructions stop at 1.5 mm, it is the wrong choice regardless of its ratings, because a 2 mm foam is the upper end of what any click-lock vinyl will tolerate.
What We Like
- Adds a vapor barrier without adding a cushion
- Self-sealing tape edge simplifies seams
- Narrow rolls are easier to handle than fold-out sheet
- Inexpensive per square foot
What We Don’t Like
- No acoustic or comfort benefit
- Thin film tears if dragged or walked on carelessly
- Generic 6-mil poly does the same job for less
Product Specs
- Type: 6-MIL Polyethylene Vapor Barrier
- Rank in this guide: #5
For the most common situation — padded vinyl plank going onto a concrete slab — this is the only "underlayment" you should buy. MoistureBlock is a plain 6-mil (0.006 inch) polyethylene film with a published perm rating of 0.09, sold in 100 and 175 sq ft rolls with a self-sealing tape strip along the edge so the seams can be joined without a separate roll of tape. MP Global specifically recommends it for floors with pre-attached padding over concrete.
It adds no cushion and no sound reduction, which is the point. It stops slab moisture reaching the plank's pad and the underside of the joints without adding any softness that could flex them. Construction-grade poly from a big fold-out sheet does the same job if it is genuinely 6 mil and you tape the seams well; what you are paying for here is a manageable roll width and the integrated tape.
Underlayment Types Compared
Most underlayment sold for floating floors falls into six families. They are not interchangeable, and the cheapest and most widely stocked type is the one least suited to vinyl.
| Type | Typical thickness | Firmness | Moisture | Sound | Rough cost / sq ft | Under rigid-core LVP? |
|---|---|---|---|---|---|---|
| IXPE / IXPP cross-linked foam | 1 – 2 mm | Firm | Closed cell; often filmed | Good | $0.20 – $0.50 | Yes, the common choice |
| Dense felt / recycled fiber | 1.4 – 3 mm | Firm (dense grades) | Needs attached film | Very good | $0.40 – $0.80 | Thin vinyl-rated grades only |
| Rubber | 1.5 – 3 mm | Very firm, heavy | Not a barrier by itself | Best for impact noise | $0.50 – $1.00 | Yes, where approved |
| Cork | 1.5 – 6 mm | Firm | Not a barrier; needs poly on concrete | Good | $0.60 – $1.50 | Usually only as an attached pad |
| Standard PE / EPE foam | 2 – 3 mm | Soft | Some filmed versions | Fair | $0.15 – $0.35 | No — laminate product |
| 6-mil polyethylene film | 0.15 mm | None | Vapor barrier only | None | $0.05 – $0.15 | Yes, as a barrier under padded planks |
IXPE and IXPP — irradiated cross-linked polyethylene and its polypropylene cousin — are what most manufacturers attach to the back of their planks, so a loose roll of the same material is the most natural match for an unpadded plank. Cross-linking makes the foam denser and more resilient than ordinary packing foam; it springs back rather than permanently flattening.
Felt and fiber underlayments vary enormously. Thick, fluffy 3 mm felt is a laminate and engineered-wood product. Thin, dense grades like QuietWalk LV are built for vinyl and are among the quietest options for their thickness.
Rubber is the material of choice for impact sound, which is why it shows up in condo specifications. Its weight and density mean it rarely compresses enough to stress a joint, but it costs more and has no vapor barrier of its own.
Cork is warm, naturally mould-resistant and firm, and it works very well as a factory-attached pad — COREtec's is the best-known example. As a loose roll, most cork sold is 3–6 mm and aimed at laminate, engineered wood or glue-down tile, and it is too thick for most floating rigid-core vinyl. Use loose cork under LVP only where the plank maker names it.
Thickness and Firmness Limits for Rigid Core
Rigid-core vinyl tolerates far less cushion than laminate, and the reason is the joint. A laminate board is thick fibreboard with a deep locking profile; a 4–6 mm SPC plank has a thin, precise lip of stone-plastic composite that carries every footstep across the seam. Put a soft layer beneath it and each step pushes the plank edge down a fraction of a millimetre, then lets it spring back. Over a few months that cycling fatigues the lock. The result is the classic failure: seams that peak, click when stepped on, or separate into gaps.
That is why manufacturers express their limits two ways. The first is maximum thickness, most commonly 1–1.5 mm and occasionally 2 mm for thicker planks. The second is firmness, stated as a minimum density, a minimum compressive strength, or simply a list of approved products. A 2 mm high-density underlayment can be acceptable where a 2 mm soft foam is not, which is why thickness alone does not settle the question.
Thinner planks are less forgiving. A 4 mm SPC plank with no pad is about the most demanding case, and several underlayment makers, Roberts among them, will not approve their products under cores below 4 mm. An 8 mm WPC plank is more tolerant, but it is also likely to have a pad already.
One thing no underlayment does is fix an uneven subfloor. Most LVP makers require flatness of around 3/16 inch in 10 feet or 1/8 inch in 6 feet. An underlayment smooths grit-level texture, not dips; using a thicker, softer one to "level" a floor just creates a soft spot where the joints will fail first. Grind high spots and fill low ones with a patching or self-levelling compound before anything else goes down.
Vapor Barriers Over Concrete
Concrete gives off water vapor for its whole life, more so in basements and on slabs poured without a membrane beneath them. Vinyl planks are waterproof, but the space under them is not: vapor that condenses below a floating floor can grow mould on the pad, encourage odour and, in bad cases, push moisture through the joints. Every LVP maker therefore requires a vapor barrier over concrete — the question is only whether the plank's own pad counts.
Some manufacturers accept their attached pad as the barrier; others require a separate 6-mil polyethylene film regardless. Underlayments with an attached film, like QuietWalk LV, Encore and UltraSeal, count as the barrier when installed with sealed seams. Plain cushion layers such as rubber or cork do not.
Lay poly with seams overlapped (6–8 inches is common unless the instructions say otherwise) and taped along their whole length, and run it 2–4 inches up the walls behind where the baseboard will sit, trimming it after the floor is down. Before any of that, test the slab: manufacturers set limits using either a calcium chloride test or an in-situ relative humidity probe, and a slab over those limits needs more than a sheet of plastic. A damp, musty basement is a room problem first — see our guides on fixing a damp basement and choosing the right basement dehumidifier before the floor goes in.
On a wood subfloor above grade, most LVP makers do not require a vapor barrier, and some advise against plastic over wood because it can trap moisture in the subfloor. The exception is a floor over a crawl space, where the right fix is a ground vapor barrier in the crawl space itself, plus whatever the plank maker specifies on top.
Sound Ratings: IIC, STC and Condo Rules
Three numbers appear on acoustic underlayment, and they measure different things.
- IIC (Impact Insulation Class) measures how well a floor-ceiling assembly blocks impact noise — footsteps, dropped objects, chairs scraping — to the room below. This is the one that matters for neighbours.
- STC (Sound Transmission Class) measures airborne noise, like voices and television, through the assembly.
- Delta IIC (ΔIIC) measures how much the underlayment itself improves impact insulation compared with a bare reference slab. It is the fairest way to compare one underlayment with another.
The catch with IIC and STC is the word "assembly." A published IIC of 70 was measured on a specific slab, often 6 or 8 inches of concrete, sometimes with a suspended ceiling below, with a specific flooring on top. Your building's slab, ceiling and flooring are different, so your result will be different. The International Building Code sets a minimum of 50 for both (45 when field-tested), and many condo associations require more — 55, 60 or higher — sometimes measured in the finished unit rather than taken from a brochure.
So in a condo, start with the association's rules, not the product. Find out the required rating, whether it is lab or field-tested, and whether the board pre-approves specific underlayments. Then confirm the plank maker allows that underlayment. Where the plank has an attached pad, the plank maker may publish its own ratings for the plank alone, or approve a specific acoustic product to use with it. If the association demands a field test, a dense rubber or fiber product with a high Delta IIC gives you the most margin.
Hard flooring also changes the noise you hear in your own room — the hollow, clicky sound of a floating floor. A good attached pad or dense underlayment reduces it noticeably, and so does a heavier, thicker plank.
Why Laminate Underlayment Is Not the Same
Walk the underlayment aisle and most rolls are sized and priced for laminate: 2–3 mm of soft foam or felt, often with "3-in-1" or "universal" on the label. Products like Roberts Black Jack (2.5 mm closed-cell foam), Roberts Super Felt (3 mm felt with film) and QuietWalk Plus (3 mm fiber) are good underlayments for the floors they are designed for, which are laminate and engineered wood, not most vinyl plank.
The difference comes down to what each floor needs. A laminate floor is a thick, stiff board of high-density fibreboard that needs cushion and a little give to bridge small subfloor flaws and quieten its harder, louder surface. Vinyl plank is denser, heavier and thinner at the joint, and needs support rather than cushion. Putting a 3 mm laminate pad under a 5 mm SPC plank is one of the most common DIY mistakes, and the forum posts about clicking, separating vinyl floors trace back to it again and again.
If a roll says it is for laminate and does not specifically list luxury vinyl or rigid core, assume it is not for your floor.
Glue-Down, Sheet and Peel-and-Stick Vinyl
The word "underlayment" also means something entirely different in the vinyl world. For glue-down LVT, sheet vinyl and peel-and-stick tile, an underlayment is a thin, smooth panel — typically 1/4-inch plywood underlayment, fastened on a tight nailing schedule — that gives the adhesive a flat, clean, void-free surface. Our guide to types of plywood covers which panel grades suit. Flexible vinyl telegraphs every nail head, seam and grain pattern underneath, so the panel's surface quality matters more than its thickness.
Never put foam, felt or cork under a glued vinyl floor. The adhesive needs a rigid base, and a cushion lets tiles shift and seams open. Where a glued floor needs acoustic improvement, the solution is a dedicated product approved for double glue-down — a rubber or cork sheet bonded to the subfloor and the vinyl bonded to it — specified by the flooring maker. SoundBarricade is one such product. That is a commercial-style installation and worth having a flooring contractor price.
Installing It Without Causing Problems
Underlayment installation is simple, which is why its mistakes are so common. A few rules cover nearly all of them.
Prepare the subfloor first. Sweep and vacuum thoroughly; a pebble under a thin underlayment becomes a bump in the plank above it. Fix flatness with compound, not underlayment.
Lay it one layer thick. Butt the edges of cushion underlayment together; do not overlap the cushion itself, which creates a ridge of double thickness. On products with a film lip, overlap only the film, as designed, and tape the full seam.
Run it perpendicular to the planks where the instructions allow, so underlayment seams never sit directly under plank seams.
Film side as instructed. On filmed underlayments the barrier usually faces up toward the flooring or down toward the slab depending on design. The roll's instructions say which; getting it wrong can trap moisture on the wrong side.
Keep the expansion gap clear. Trim the underlayment at the wall so it does not ride up into the 1/4-inch gap the floor needs for movement. Poly barriers are the exception, turned up the wall and trimmed afterwards.
Keep the paperwork. Save the underlayment's spec sheet and the receipt with the flooring documents. If a warranty question ever arises, being able to show the product was approved and within the thickness limit settles it quickly.
In kitchens, plan around the appliances and cabinetry before you buy: most makers want cabinets installed on the subfloor rather than on top of a floating floor, which changes how much underlayment and barrier you need. Our guide to the best vinyl flooring for kitchens covers layout and water protection in detail. In homes with large dogs, a firm underlayment matters even more, as claws and heavy paws concentrate load on the joints — see the best flooring for dogs.
The Short Version
Padded plank: no extra underlayment. Over concrete, a 6-mil poly film if the plank maker requires one, such as MP Global MoistureBlock.
Unpadded rigid core: a thin, dense, vinyl-rated underlayment within the manufacturer's limit. QuietWalk LV for most homes, FloorMuffler Encore where the limit is around 1 mm, UltraSeal only for thicker planks that permit 2 mm.
Condos and upper floors: check the association's IIC/STC rule first, then choose an approved acoustic product. Roberts SoundBarricade gives the most impact-noise margin of the products here.
Glue-down, sheet or peel-and-stick: no cushion at all — a smooth, flat substrate, or a plywood underlayment panel if the subfloor needs one.
And whichever applies, read the plank maker's installation guide before you spend anything. It is the document that decides the warranty, and it is usually more specific than any underlayment box.
Frequently Asked Questions
Only if the planks have no pad attached. Most mid-range and premium rigid-core vinyl now has a 1–1.5 mm IXPE foam or cork pad on the back, and adding another underlayment lets the click joints flex until they crack — most manufacturers void the warranty if you do it. A padded floor typically needs only a 6-mil polyethylene vapor barrier over concrete, where the maker requires one. Unpadded planks do need an underlayment: thin, dense, approved by the plank maker, usually around 1–1.5 mm.
Usually not. The attached pad was designed with the plank's locking system, and a second cushion doubles the softness the joints must cope with, so seams eventually peak, click or separate. Most installation guides forbid it, and warranty claims on floors laid over an extra pad are routinely refused. Some acoustic underlayments say they are compatible with padded planks, but that is the underlayment maker's permission, not the flooring maker's. A 6-mil poly film over concrete is different — it adds no cushion and is often required.
Typically 1–1.5 mm, occasionally up to 2 mm for thicker rigid-core planks whose makers allow it — far thinner than the 2–3 mm common for laminate. Manufacturers often also set a minimum density or compressive strength, because a dense 1.5 mm product supports the joints better than a soft one of the same thickness. Within the limit, choose the denser product rather than the thicker one. An unpadded 4 mm plank is the least forgiving case.
Yes. Every vinyl plank maker requires vapor protection over concrete; the question is whether the plank's attached pad counts. Some accept it, others require a separate 6-mil polyethylene film regardless. Underlayments with an attached film, such as QuietWalk LV or FloorMuffler Encore, serve as the barrier when seams are sealed; plain rubber or cork does not. Test the slab first — one above the manufacturer's moisture limit needs mitigation, not just plastic.
Generally not. Most laminate underlayment is 2–3 mm of relatively soft foam or felt, made to cushion a thick fibreboard board. Rigid-core vinyl needs firm support at a thinner joint, and a soft 3 mm pad lets the lock flex until it fails. Products such as Roberts Black Jack, Roberts Super Felt and QuietWalk Plus are good for laminate and engineered wood, but unless a roll lists luxury vinyl or rigid core and fits your plank's thickness limit, assume it is a laminate product.
Whichever one your association and your plank maker both approve, so start with the condo rules: the required IIC and STC ratings (often 50–60 or higher) and whether they are lab or field tested. Then choose a dense acoustic product the plank maker permits. Roberts SoundBarricade publishes IIC 69, STC 63 and a Delta IIC of 25 under LVP; QuietWalk LV publishes a Delta IIC of 28. Published figures describe a specific test assembly, so treat them as a comparison, not a guarantee.
Cork makes an excellent attached pad — firm, warm, naturally mould-resistant and good for sound. As a separate roll it is less often suitable: most loose cork is 3–6 mm and aimed at laminate, engineered wood or glue-down work, well above the 1–1.5 mm typical for floating rigid-core vinyl. It is also not a vapor barrier. Use loose cork under floating vinyl plank only where the plank maker names it.
No. Underlayment smooths surface texture of roughly a millimetre at best; it does not fix dips or humps. Most vinyl plank makers want the subfloor flat to around 3/16 inch in 10 feet. A thicker, softer underlayment used to compensate creates a soft spot where the joints fail first. Grind high spots and fill low ones with patching or self-levelling compound before anything goes down.