Materials guide
Underlay Guide: Grades, Tog Ratings & Why It Matters
Underlay is the layer nobody sees and everybody notices — it decides how a carpet feels underfoot on day one and how long it still looks good five years later.
What Underlay Actually Does
Underlay sits between the subfloor and the carpet, and it does three jobs at once: it absorbs impact so the carpet backing doesn’t wear through in high-traffic spots, it insulates against a cold subfloor, and it adds cushioning that makes a carpet feel far more comfortable underfoot than the same carpet laid straight onto bare boards or concrete. A good underlay under a modest carpet often feels better than a premium carpet over poor or worn-out underlay.
It also protects your investment in the carpet itself — without proper cushioning, the backing takes the full force of every footstep directly, which shortens the carpet’s working life regardless of how good the fibre on top is.
The Three Grades We Fit
Nearly every domestic job in the UK uses one of three underlay grades, each suited to a different combination of room, traffic and budget.
| Grade | Typical use | Guide price |
|---|---|---|
| PU foam | Entry-level, all-purpose — bedrooms, low-traffic rooms, rentals | £3–£5 per m² |
| Premium 10–11mm | Extra cushioning and warmth — living rooms, main bedrooms | £5–£8 per m² |
| Crumb rubber | Dense, heavy-duty — stairs, hallways, high-traffic family rooms | £6–£9 per m² |
Guide prices — your exact quote depends on room sizes and flooring choice.
PU foam
Polyurethane foam is the lightest and most affordable grade, and it’s perfectly adequate for low-to-medium traffic rooms. It’s the sensible default for rentals and bedrooms, where cost matters and the room isn’t taking the heaviest daily wear in the house.
Premium 10–11mm
Noticeably thicker and softer than PU foam, this grade adds real cushioning and warmth — the step up most people notice underfoot the moment they walk on it. It suits living rooms and bedrooms where comfort is a genuine priority rather than an afterthought.
Crumb rubber
Made from recycled rubber and genuinely dense, this is the grade that resists compression under sustained, repeated pressure — exactly what stairs and busy hallways need. It costs a little more than PU foam but pays for itself in how much longer the carpet above it keeps its shape.
Tog Rating & Comfort Underfoot
Tog measures thermal insulation — the same scale used for duvets — and higher-tog underlay keeps more warmth in the room rather than letting it escape through the subfloor. It’s particularly worth paying attention to over a concrete ground floor or a suspended timber floor with airflow underneath, where a low-tog underlay can leave a room feeling noticeably colder than the carpet on top would suggest.
Comfort and tog rating tend to rise together — a thicker, denser underlay generally scores higher on both — but they’re not identical. If warmth matters more to you than plush softness (or vice versa), it’s worth asking specifically about tog rating rather than assuming thickness alone tells the whole story.
Matching Underlay to the Room
Stairs take concentrated, repeated point-loading on the nose of every tread, which is why crumb rubber is the standard choice there regardless of what’s used elsewhere in the house — see our stair carpet fitting page for how the two come together. Bedrooms and lighter-traffic rooms can comfortably run PU foam without any real compromise, while living rooms and hallways in a family home usually justify the step up to premium or crumb rubber given how much daily use they see.
It’s entirely normal to mix grades across one job — crumb rubber on the stairs and landing, premium underlay in the bedrooms, PU foam in a spare room. Our underlay picker tool walks through this room by room if you’d rather answer a few quick questions than work it out from scratch.
Can You Reuse Old Underlay?
Almost never, and it’s one of the most common false economies we see. Underlay compresses permanently under years of footfall — once it’s flattened, it doesn’t spring back, no matter how good it looked when it was new. Fitting a brand-new carpet over worn-out underlay wastes a meaningful chunk of what you’re paying for in the new carpet’s comfort, warmth and lifespan.
The only real exception is underlay that’s genuinely recent, still springs back fully when pressed, and was fitted correctly to begin with — and even then, most fitters will recommend a fresh layer simply because it’s a relatively small cost against the overall job, for a carpet that’s meant to last a decade or more.
How Underlay Affects Carpet Lifespan
Every footstep’s impact has to go somewhere — with good underlay beneath it, much of that force is absorbed before it reaches the carpet backing; without it, the backing takes the full impact directly, which accelerates wear at exactly the point where a carpet is hardest and most expensive to repair. Over several years, the difference between a carpet fitted on a decent underlay and the same carpet fitted on none, or on worn-out underlay, is genuinely visible — not a marginal effect.
It’s the reason we always fit underlay and carpet together as part of a carpet fitting job rather than treating it as an optional extra — the two work as a system, and skimping on one undermines the other regardless of how much you spent on it.
Frequently Asked Questions
Is expensive underlay worth it under a budget carpet?
Often more than a premium underlay under a premium carpet — a budget carpet has less fibre and body of its own to fall back on, so a good underlay does proportionally more of the work supporting comfort and durability. It's one of the best-value upgrades on a budget-conscious job.
Why do stairs always get a denser underlay?
Stairs take repeated, concentrated point-loading on the nose of every tread, which compresses and flattens a soft foam underlay far faster than anywhere else in the house. Dense crumb rubber holds its shape under that kind of load, which is why fitters default to it on stairs regardless of the budget elsewhere in the job.
Can I fit new carpet without new underlay?
Technically yes, but it's rarely a good idea unless the existing underlay is genuinely recent and in excellent condition. Underlay compresses permanently over years of use, and fitting a new carpet over a flattened, worn-out underlay wastes much of what you're paying for in the new carpet's comfort and lifespan.
Does underlay affect how warm a room feels?
Yes, noticeably — underlay is a genuine layer of thermal insulation between your feet and a cold subfloor, particularly over concrete or a suspended timber floor with airflow underneath. A thicker, higher-tog underlay makes a real difference to how a room feels in winter, not just underfoot comfort.
More Guides & Tools
Ready to Sort Your Underlay?
Tell us the rooms and how they're used and we'll recommend a grade and give you a firm price — usually the same day.