Interior and finishes · Flooring Materials
Carpet Underlay (Cushion Layer Beneath Carpet)
Explains the layer that most carpet warranties depend on, and why a cushion chosen for comfort alone shortens the life of the carpet stretched over it.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What carpet underlay is
Carpet underlay is a cushion laid loose beneath a stretched carpet, held at the room edges by gripper rod. Common types are polyurethane foam made from bonded offcuts of furniture foam, crumbed or sponge rubber, and needled felt or blends of felt with rubber. Each has its own density and recovery behaviour, and those two characteristics matter far more than which family it belongs to.
The layer it is confused with is floating floor underlay, and the two are chosen against opposite criteria. A floating board floor rests on a thin resilient sheet that must barely compress, because its locking joints work loose if the base keeps moving. 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. 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.
Because of that mechanical role, underlay density is a carpet decision rather than a comfort decision. 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. Where a heated floor is present the manufacturer's published thermal figures for the underlay and the carpet together are what the heating designer needs to see.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- carpet cushion
- carpet pad
- underfelt
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Beneath stretch-fitted broadloom carpet on gripper rod in living rooms, bedrooms and landings.
- On stairs, where the cushion is cut and fitted separately to each tread and riser.
- Over timber floorboards, where it also takes up minor variation between boards.
- Over concrete or screed, usually with a separate membrane where moisture is a consideration.
- In rooms where warmth and softness underfoot are part of the reason carpet was chosen at all.
- Beneath a stair runner, where the wear concentrates on the tread nosings.
Consideration
Where it is commonly considered
- Where broadloom carpet is being stretched over gripper and the manufacturer sets requirements for the cushion.
- Where an existing floor is uneven enough that a cushion helps the carpet sit without reading every board line.
- Where warmth and softness underfoot are a stated priority for the room in question.
- Where an old underlay has flattened and the carpet above it has started to show wear in traffic lanes.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Beneath a rigid floating floor, where a compressible cushion works the locking joints and is the wrong product entirely.
- Beneath carpet tiles or fully bonded carpet, which are normally laid direct and rely on the deck for support.
- Where heavy wheeled office chairs or trolleys are used, since a soft cushion lets the carpet deform under them.
- Where a heated floor is in place and the layers have not been reviewed by the heating designer.
- Where the deck's moisture condition is unknown, because a cushion is not a moisture control layer.
- Where the carpet manufacturer's stated requirement for the cushion has not been obtained and read.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Substrate dependency
- The cushion goes down on whatever the deck offers. Nail heads, splinters, loose boards and debris all work up through it into the carpet, so preparation and often a lining paper or board come before the underlay is unrolled.
- Surface wear
- Wear in a stretched carpet is largely a story about what is underneath it. 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 itself is worn out.
- Installation dependency
- Underlay is laid to fit inside the gripper, butted and taped at joins, and stapled or bonded down as the fitter judges the deck allows. On stairs it is cut and fixed piece by piece, which is slower and more skilled work than a flat floor.
- Thermal behaviour
- A cushion sits between a heated floor and the room and affects how heat reaches it. Where underfloor heating is present, the figures the manufacturers publish for the specific underlay and carpet are what the heating designer works from.
- Acoustic behaviour
- A soft layer under a textile floor changes how footfall sounds in the room. Transmission between dwellings belongs to the complete tested floor construction and to the ceiling below it, not to a cushion considered on its own.
- Documentation
- Carpet warranties routinely name what may be fitted beneath the product. Reading that document before ordering is what prevents an underlay chosen on feel from quietly invalidating the cover on the carpet above it.
- Maintenance
- Underlay is not cleaned or maintained; it is replaced. Because it is invisible, a flattened cushion is usually only discovered when the carpet is lifted, by which time the carpet above it has already taken the consequences.
Exposure
Exposure and environmental conditions
- Is the deck timber or concrete, and does moisture from below need addressing before anything is laid?
- Is there underfloor heating here, and have the layers been shown to whoever designed it?
- Will this room see wheeled chairs, walking frames or trolleys crossing it regularly?
- Is spillage likely, and what would reach the cushion if liquid passed through the carpet?
- Does anyone in the household react to dust, and does that affect the choice of cushion type?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The deck has to be clean, sound and free of proud fixings before the cushion goes down over it.
- Gripper rod is fixed around the perimeter first, and the underlay is cut to sit inside it rather than over it.
- Over old boards a lining is often laid first so that gaps and board edges do not read through the carpet.
- Where a membrane is needed over concrete, it is a separate layer with its own requirements and laps.
- Doors, thresholds and transitions to hard floors are set out around the finished height of carpet and cushion together.
Appearance
Appearance and finish considerations
- The cushion is never seen, but it decides how the carpet feels and how quickly traffic lanes become visible.
- Too soft a cushion makes seams and joins in the carpet above more likely to show over time.
- Debris left on the deck telegraphs through as small lumps that are obvious under raking light.
- The finished floor height rises, which shows at doors and where the carpet meets a hard floor.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- What does the carpet manufacturer state about the cushion required beneath this particular product?
- How busy is this room, and does the traffic follow the same route across it every day?
- Is the existing underlay being reused, and if so, has anyone assessed whether it has flattened?
- What happens on the stairs, where wear concentrates on a narrow band at each nosing?
- Are heavy items going to stand in fixed positions and compress the cushion permanently?
Upkeep
Maintenance and repair considerations
- If the carpet is lifted for any reason, is the cushion expected to be replaced at the same time?
- What does the fitter say about the condition of the existing underlay before recarpeting?
- How would a spillage that reached the cushion be dealt with, and by whom?
- Is a note kept of what underlay was fitted, in case a warranty question arises later?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Gripper, underlay and carpet are one continuous operation, and the fitter's preparation of the deck governs the result.
- Joins in the cushion are butted and taped so they do not coincide with seams in the carpet above.
- Stairs take considerably longer than flat areas because each piece is cut and fixed individually.
- Furniture has to be cleared, and doors may need easing once the new build-up height is known.
- Any lining, membrane or board layer goes down before the cushion, not alongside it.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The carpet manufacturer's stated requirement for the cushion beneath its product.
- The underlay manufacturer's data sheet, including its density and intended use.
- Written confirmation that the chosen combination keeps the carpet warranty intact.
- The published thermal figures for the layers, where a heated floor is involved.
- A note of what was actually fitted, kept with the property records.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What cushion does the carpet manufacturer require or permit under this specific carpet?
- What condition is the deck in, and does it need lining before the underlay is laid?
- Is the existing underlay sound enough to keep, or is it being replaced as a matter of course?
- How will the stairs be treated differently from the flat areas?
- Is a membrane needed over this deck, and is that separate from the cushion?
- How much does the floor level rise, and what does that mean for doors and thresholds?
- If there is underfloor heating, has the full layer build-up been shown to the heating designer?
Blind spots
Commonly overlooked points
- Thicker and softer is not automatically better, and it is the usual reason a carpet claim is turned down.
- The cushion outlasts nothing; it flattens quietly and takes the carpet's appearance down with it.
- Stairs are where underlay choice shows first, because wear is concentrated into a narrow band.
- Anything left on the deck under the cushion will be felt underfoot for as long as the carpet lasts.
- Reusing old underlay to save on a recarpet leaves a cushion already flattened in the old traffic lanes beneath a new carpet, and it reports itself through the surface.
What this page does not do
- This page does not state what underlay any particular carpet needs; that comes from the carpet manufacturer's own documentation.
- A cushion is not a moisture control layer, and any moisture in the deck is a separate matter for assessment.
- Where underfloor heating is present, the whole layer build-up must be reviewed by the heating designer against the specific products.
- Build Design Hub publishes general reference material and does not test, approve or recommend any underlay or carpet.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Flooring Materials
Reference entries for floor coverings and floor surface systems — wood and engineered wood, laminate, vinyl and LVT, linoleum, rubber, cork, bamboo, carpet, terrazzo and resin.
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