In North America, “underlayment” usually means a smooth panel laid over the structural subfloor: plywood, hardboard or a poured levelling compound. It exists to give a thin resilient floor a clean, flat surface to bond to. It is a substrate.
In the UK, Ireland, Australia and New Zealand, “underlay” usually means a foam, rubber or felt cushion laid under a floating floor or a carpet. It exists to cushion and to absorb sound. It is a comfort layer.
Those are opposite products. One is rigid and gets glued to. The other is soft and gets laid on. Asking “does vinyl flooring need underlayment” without naming which one you mean is like asking whether a car needs tyres.
People are clearly confused by this, because they search for it directly. Queries like “is underlayment the same as subfloor” and “what is underlayment in flooring” are common enough to appear in autocomplete. The answer is that subfloor, underlayment and underlay are three different layers, and vinyl flooring treats each one differently.
| Layer | What it is | Who needs it |
|---|---|---|
| Subfloor | The structural floor: concrete slab, plywood, OSB or floorboards | Always, it is the building |
| Underlayment (US) | A smooth surfacing panel or levelling compound over the subfloor | Sheet vinyl and glue-down products, almost always |
| Underlay (UK/AU) | A soft cushion layer under a floating floor | Some click planks, and almost never sheet vinyl |
The short answer, by product type
| Product | Soft cushion underneath? | Smoothing layer underneath? |
|---|---|---|
| Sheet vinyl, glued down | No. Never | Yes, always |
| Felt-backed or foam-backed sheet | No. The backing is the cushion | Yes |
| Glue-down LVT or LVP | No | Yes |
| Click LVT or SPC with an attached pad | No, and adding one usually voids the warranty | Minimal, just flat and clean |
| Click SPC without an attached pad | Only if the flooring manufacturer names one | Minimal |
Read that table again with the two definitions in mind and the contradiction in the search results disappears. Sheet vinyl wants an underlayment. It does not want an underlay.
Sheet vinyl: no cushion, and that part is not negotiable
Roll flooring is glued to the substrate. Full stop. That is how the system is designed, and it is what makes a welded, continuous floor possible in a hospital corridor or a school.
Put a foam underlay under glued sheet vinyl and you break the system in four ways at once. The adhesive is bonding to a moving surface instead of a stable one. The foam compresses under footfall and furniture, so the sheet flexes where it was never meant to. The seams open because the two sheets are no longer sitting on the same plane. And every ridge and dip in the subfloor, which the foam was supposed to hide, shows through as soon as the foam compresses unevenly.
What sheet vinyl wants instead is boring and effective: a substrate that is flat, sound, dry and clean.
Over a concrete slab that generally means a levelling compound where the floor is out of tolerance, plus patching at cracks and slab joints. Over timber it usually means a plywood underlayment panel, typically around 6 mm and rated for the purpose, laid over the boards so the board joints do not telegraph upward. Hardboard was the traditional choice and still appears in older specifications, but it has no place anywhere that might see moisture.
Thin sheet is the least forgiving of all. A 0.35 mm to 0.8 mm roll has almost nothing to hide a defect behind, which is one reason thin PVC sheet belongs in rental rooms and light interiors rather than commercial corridors. If the substrate is not good enough for a thin sheet, the answer is either to fix the substrate or to move up to a commercial-grade roll, not to add a cushion and hope.
If you are working through the sequence, our guide to installing vinyl roll flooring covers the preparation steps in order.
The exception people quote, and what it actually says
You will find guidance that thin vinyl can be laid loose over an existing hard floor. That is true in a narrow set of conditions: a small, dry, domestic room, over a smooth hard surface, with a product the manufacturer has designed for it. It is not a statement that sheet vinyl wants a soft layer underneath. It is a statement that in a small bedroom, a loose-laid sheet over floorboards can work.
Nothing about that applies to a corridor, a kitchen, a clinic or anything with a trolley on it.
Felt and foam backings already have the cushion built in
This is where a lot of money gets wasted.
A felt-backed sheet arrives with a polyester felt layer laminated to the back, typically 2 mm to 5 mm. A foam-backed sheet arrives with a foam layer already bonded in. Both of those layers are the cushion. Both are engineered to sit directly on the prepared substrate and be glued to it.
Adding an underlay underneath them does not make the floor softer. It makes the floor unstable, and it doubles the thickness the adhesive has to bridge.
The reason to choose felt or foam backing is that the cushion is part of the product, laminated under factory conditions, and it comes with a warranty attached to that construction. Adding your own layer on site throws that away.
Click plank: adding underlay is often the mistake, not the fix
Now the other half of the question.
Most rigid SPC click flooring sold today arrives with an acoustic pad already attached to the back, usually IXPE or EVA. The plank is designed to sit directly on a flat, clean subfloor and float on its own attached pad.
Add an underlay on top of that and you have created a sandwich that is too soft. Under footfall the click joints flex. Flex them enough times and they unlock, or the locking tongue cracks. The floor starts to click, then to separate, then to look like a bad installation. It is a bad installation, but the cause was a well-intentioned extra layer.
There is a second reason to leave it alone. Most rigid click manufacturers state plainly that installing over an additional underlay voids the warranty. If you add one and the joints fail, you have bought the problem yourself.
Where underlay is legitimate for click flooring, it is because the flooring manufacturer names a specific product or type. That is usually a thin acoustic sheet, often no more than about 1 mm to 3 mm, and usually for a product that does not have its own attached pad. If a manufacturer gives you a maximum thickness, treat it as a maximum rather than a starting point for negotiation.
Products sold as “integrated underlay” are simply planks with the pad already on. There is nothing to add.
When underlay earns its money
Four things underlay is genuinely bought for. Only some of them apply to vinyl.
Acoustic performance. In multi-unit residential buildings, impact sound is regulated. Approved Document E in the UK, DIN 4109 in Germany, and ratings expressed to EN ISO 717-2 in much of Europe all put a number on how much impact noise a floor assembly must reduce. Underlay contributes to that number on floating floors.
A vapour barrier. On a ground-floor concrete slab, a combined underlay and moisture barrier is a normal part of a floating floor build-up.
Thermal separation. Underfloor heating and underlay have a difficult relationship, which is covered below.
Evening out minor irregularity. This is the one most people buy underlay for, and the one it is worst at. Underlay hides small variations for a few months and then follows them.
Acoustic requirements: where the answer changes by building type
If you are specifying flooring for apartments, student housing or any multi-storey residential building, acoustic performance is not optional and it is not a comfort feature. It is a condition of the building being signed off.
For a floating click floor, the acoustic solution is an underlay that the flooring manufacturer has tested with the product, plus the plank’s own attached pad. The two are specified together because the tested assembly is what carries the rating.
For a sheet vinyl floor, the acoustic solution does not involve underlay at all. You choose a product with acoustic backing built in. A felt-backed sheet or a foam-backed sheet is the sheet-vinyl answer to impact noise, and the performance comes from the laminated backing rather than anything added on site. This is also why felt-backed and foam-backed products dominate apartment renovation supply in Europe.
Moisture barriers: helpful in one direction, destructive in the other
This is the part that catches out competent installers.
On a suspended timber floor, a vapour barrier underlay is doing useful work. It stops moisture from below reaching the flooring.
On a concrete slab with no working damp-proof membrane, a vapour barrier underlay does the opposite. It seals moisture under the floor where it cannot escape, and it will find somewhere to go. Usually that means into the flooring, into the skirtings or into the room as a smell nobody can locate.
The only way to know which situation you are in is to test the slab before you buy anything. Our guide to choosing vinyl flooring adhesive sets out what those tests are and why the limits belong to the adhesive manufacturer rather than to a rule of thumb.
If a slab is damp, the fix is a surface damp-proof membrane or a different floor build-up. An underlay with a foil backing is not a fix.
Underlay and underfloor heating
Underlay resists heat transfer. That is what insulation does. On a floor with underfloor heating, a thick underlay means the heating system has to run hotter to reach the same surface temperature, which costs money every day the building is occupied.
If underlay is being used over underfloor heating, it needs to be specified with a low thermal resistance, usually quoted as a tog value, and it needs to be approved for use over a heated slab. Many cushion underlays are not.
For sheet vinyl, the question mostly does not arise, because there is no cushion layer to specify. The relevant numbers are the surface temperature limit and the commissioning schedule, both of which are covered in the adhesive guide, and neither of which changes because of underlay.
The subfloor decides what you actually need
| Existing floor | What vinyl actually needs |
|---|---|
| Concrete slab, flat and dry | Levelling compound where out of tolerance. Adhesive for sheet, floating or glue-down for plank |
| Concrete slab, uneven | Levelling compound first. Underlay will not substitute |
| Plywood or OSB | Screws set, joints filled and sanded. Often good enough as is |
| Suspended floorboards | Plywood underlayment panel to bridge the joints, otherwise every board edge shows |
| Existing vinyl or tile | Flat, sound and clean. Loose tiles come out. Deep grout lines need filling |
| Old adhesive residue | Removed or smoothed. Check compatibility before covering |
Three failures, one cause
Click joints that open after a few months. Almost always too much soft material under the plank. Either an underlay added to a plank that already had a pad, or an underlay thicker than the manufacturer allowed.
Mould appearing from nowhere. A vapour barrier underlay on a slab that was already damp. The barrier did its job, which was the problem.
Sheet vinyl that will not sit flat. Adhesive bonding to a moving cushion instead of a stable substrate. It shows up as lifting, seam failure or ripples, and it is usually reported as a defective roll. Our guides to vinyl flooring roll installation problems and why vinyl flooring rolls curl, shrink or lift cover the diagnosis, and in most of those cases the subfloor and the build-up are the place to start.
What to specify instead
- Sheet vinyl, including felt-backed and foam-backed: substrate preparation only. No cushion layer
- Glue-down LVT: substrate preparation only
- Click plank with an attached pad: nothing, unless the flooring manufacturer names a product
- Click plank without a pad: only the underlay named in the specification, at or below the stated maximum thickness
- Multi-unit residential: the tested acoustic assembly, whether that is underlay plus plank or a backing built into the sheet
- Any floor over underfloor heating: low tog, approved for heated slabs
FAQ
Does vinyl flooring need underlayment?
It depends which product you mean by underlayment. Sheet vinyl needs a smooth substrate, usually a levelling compound or a plywood underlayment panel, and it does not need a soft cushion. Some click planks want a thin acoustic underlay, and planks with an attached pad usually should not have one.
Can you put underlay under sheet vinyl?
Not a foam or felt cushion. Sheet vinyl is designed to be glued directly to a prepared substrate. A cushion layer under a glued sheet leads to seam failure, movement and telegraphing.
Does vinyl plank need underlay if it already has a pad on the back?
No. The attached pad is the underlay. Adding a second layer usually voids the warranty and makes the click joints flex, which is how they fail.
What thickness of underlay for vinyl plank?
Whatever the flooring manufacturer states, and no more. Where underlay is permitted at all, it is usually thin, commonly around 1 mm to 3 mm. Thicker is not better here.
Do you need a moisture barrier under vinyl plank on concrete?
On a suspended or upper floor, a combined underlay and vapour barrier is common and useful. On a ground-floor slab with no damp-proof membrane, sealing moisture in can cause more damage than it prevents, so test the slab first.
Is underlayment the same as subfloor?
No. The subfloor is the structural floor of the building. Underlayment is a smooth layer added on top of it to give a thin floor something flat to bond to. They do different jobs and are made of different things.
Can underlay be used over underfloor heating?
Only if it is approved for that use and has a low thermal resistance. Most cushion underlays are not, and a thick underlay over a heated slab pushes running costs up.
Matching the build-up to the floor, not the other way round
The underlayment question is really a question about which flooring system you have chosen, asked in the wrong order. Decide the flooring first, then the substrate it needs, then whether anything else belongs in the build-up. Almost every failure in this article comes from doing it backwards.
If the floor is sheet vinyl, the build-up is simple and the work is all in the substrate. If the floor is a rigid click plank, the build-up is usually nothing at all, and the discipline is in resisting the urge to add something.
For a project where the backing, the subfloor condition and the acoustic requirement all need to line up at once, our flooring specification checklist walks through the decisions in the order they should be made. If you tell us the room, the existing floor and the traffic, we can tell you which construction fits and what belongs underneath it. Send us the details.
