Who this guide is for
- Owners choosing an underlay after the covering has already been selected
- Anyone in a flat or upper storey where footfall to the space below is a concern
- People laying a rigid floating floor over an uneven or unknown deck
- Readers who have been offered a thicker underlay as an upgrade
- Anyone with a heated floor who has been told an underlay may or may not be used
Four jobs, not one product family
Comfort under a textile floor, an even bearing under a rigid one, a flat plane under a thin resilient one, and a deliberate break between a deck and the structure are four separate problems. A product made for one of them is usually actively wrong for another, and that is why the products look interchangeable and are not.
There is a fifth item that gets filed here by mistake: a grooved heating overlay board is not an underlay at all. It is a component of a heating installation designed and commissioned by other people, and it happens to occupy the same position in the build-up.
- Carpet cushion: compresses underfoot, chosen on density and recovery
- Floating floor sheet: must barely compress, so the locking joints have even bearing
- Lining board: fixed down across its face to flatten a deck for a thin covering
- Isolated deck: a constructed break on resilient battens, cradles or bearers
- Heating overlay board: part of a heating installation, not a resilient layer
Compression is the axis that divides them
A carpet is a flexible textile meant to compress. The cushion beneath deflects underfoot, and the resistance it offers stops the backing bending sharply in the same place, which is why traffic lanes appear in the pile long before the fibre is worn out. Underlay density is a carpet decision rather than a comfort decision.
A rigid floating floor works the other way round. Its locking joints rely on even support that does not give way, so resistance to taking a permanent set matters more than softness ever does. Put a carpet cushion under a rigid floating floor and the joints suffer; put a thin rigid sheet under a carpet and it loses the support it was designed around.
What the documentation usually already says
Most carpet manufacturers state what characteristics the cushion beneath their product must have, and fitting something softer or thicker than that is one of the more common reasons a claim is refused. Covering manufacturers for floating floors commonly state what underlay is required and what happens to the guarantee if a different one is used.
Those statements exist for the actual products being supplied, and they are worth having before delivery rather than after a problem. The underlay is often the last item priced and the first one substituted, even when the covering guarantee names it.
Levelling is the job none of the thin ones do
A resilient sheet bridges small irregularity and does nothing about a genuinely uneven or hollow deck. A hollow beneath the underlay stays a hollow after it is laid, and it works against the joints above for as long as that floor exists. A carpet cushion is worse in this respect: nail heads, splinters, loose boards and debris all work up through it into the carpet.
The product that does flatten is a thin lining board, fixed down across the whole face rather than only at the perimeter, and it too has limits — it lies on the high points and fills nothing. Real hollows are a levelling question that belongs to the base, not to any of these layers.
Stacking: why two layers rarely give two benefits
Adding a soft layer beneath a resilient sheet to get comfort as well as isolation gives the rigid covering above exactly the yielding base its joints were designed to avoid. Adding a lining board over a cushion pins the cushion. Each layer changes what the layer above it bears on, and the combination is what the manufacturers have not tested.
An isolated deck is the case where this matters most, because the whole value of the assembly lies in the break being continuous. A single rigid fixing through the deck into the structure can undo the intent, and skirtings, door frames and thresholds are where a carefully built break is quietly bridged.
Heat, moisture and the two things people assume
Where a floor is heated, a layer between it and the room affects how heat reaches the room, and the heating designer works from the figures the manufacturers publish for the specific underlay and covering together. Whether an underlay may be used over a heating installation at all is a matter for the heating and covering documentation.
The second assumption concerns moisture. Some floating underlays arrive with an integral film or overlap flap, and a film on the roll is not automatically a moisture strategy for that substrate. Whether it is sufficient alone, and how laps and edges are treated, are questions for the covering maker and the responsible designer.
It is the layer nobody can reach again
Once the covering is locked together or stretched into gripper, everything below it is effectively inaccessible for the life of that floor. A sheet that has taken a permanent set, an underlay left bunched at a wall, or a cushion already flattened in the old traffic lanes is discovered only when the floor is lifted, by which time the covering above has taken the consequences.
Before an underlay is ordered
- 1Decide which of the four jobs this layer is actually being asked to do
- 2Obtain the covering manufacturer's statement on what must go beneath it
- 3For a carpet, read what the manufacturer requires of the cushion before ordering
- 4Establish whether the deck is flat and sound, or whether it needs levelling first
- 5Ask whether any hollow or rocking board has been dealt with before laying
- 6Confirm whether the covering is to be floated or bonded, since that changes everything
- 7Where a floor is heated, ask the heating designer what the layers do to the design
- 8Ask whether any integral film is being relied on as a moisture strategy, and by whom
- 9Confirm the laying direction, joint treatment and perimeter gap from the documentation
- 10Ask who checks the underlay has not bunched at walls before covering starts
- 11On stairs, allow for the extra time and skill that cut and fixed pieces need
- 12Record what was laid, since nobody will see it again until the floor comes up
Common mistakes to avoid
- Assuming thicker and softer is better, which is a common reason a carpet claim is refused
- Putting a compressible cushion under a rigid floating floor and working its locking joints
- Expecting an underlay to level a deck that has real hollows in it
- Stacking two layers to get two benefits and giving the covering an unsuitable base
- Reading the word acoustic in a product name as a measured result
- Reusing old underlay under a new carpet, flattened in the old traffic lanes
- Treating an integral film on a roll as the moisture strategy for the substrate
- Bridging the perimeter break of an isolated deck with a skirting or a threshold fixing
When to involve a professional
- Acoustic behaviour is a property of the complete tested assembly, the structure it sits on and the rooms concerned, and is established by a qualified acoustic professional
- The covering manufacturer states what underlay is required and what happens to its guarantee if a different one is used
- Whether any layer may be used over a heating installation is governed by the heating and covering documentation together
- Whether substrate moisture has been adequately addressed is a question for the responsible designer, not for a film on a roll
- Test evidence belongs to a specific assembly, and a floor differing from it in one layer is a different floor
Frequently asked questions
Questions readers ask about this topic
Is a thicker underlay a better underlay?
Thicker and softer is not automatically better, and under a carpet it is the usual reason a claim is turned down. A cushion that compresses too far lets the backing flex repeatedly at the same points, and traffic lanes appear in the pile long before the fibre is worn out.
Can I use carpet underlay under a laminate floor?
The two are chosen against opposite criteria. A rigid floating floor needs a base that barely compresses, because its locking joints work loose if the base keeps moving. A carpet cushion is designed to compress, which is exactly what the joints above cannot tolerate.
Will an underlay stop footfall reaching the room below?
That belongs to the complete floor construction, the ceiling below and the junctions around them, not to a single layer. The word acoustic in a product name describes a family of goods and settles nothing, and a qualified acoustic professional is the source for what an assembly achieves.
Does an underlay level an uneven floor?
It bridges small irregularity and does nothing about a genuinely uneven or hollow deck. A hollow beneath the underlay is still a hollow afterwards, working against the joints above. Real levelling is a question for the base, through a compound, a screed or a board floor.
Can I put an underlay over underfloor heating?
That depends on the heating documentation and the covering documentation together, and on the figures the manufacturers publish for the specific products. The heating designer works from those figures, so the question goes to them before the underlay is ordered rather than after.
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