Do You Need to Replace Underlay When Replacing Carpet?

"Can't I just reuse the existing underlay to save money?" It is the single most common question homeowners ask before fitting new carpets. Here, Paul explains why a new underlay is crucial, how it impacts your warranty, and how to choose the right thickness.

While it is highly tempting to reuse your old underlay to cut down on costs, doing so is almost always a false economy. Underlay behaves exactly like the shock absorber in your car. By the time your old carpet is worn out, the underlay beneath it has already lost its elasticity, flattened out, and accumulated years of dust, dirt, and pet dander.

How Long Does Carpet Underlay Last?

A good quality PU foam underlay lasts roughly 10 to 15 years, which is about the same as a standard domestic carpet. Stairs and hallways take the heaviest traffic, so underlay there tends to wear out nearer the 10-year mark, while a spare bedroom can outlast the average. Cheaper thin foam or rubber crumb underlay often gives up sooner. The practical rule is simple: underlay should be replaced every time the carpet is, because it's buried under the carpet and can't be inspected or swapped afterwards without lifting everything.

Having underfloor heating? Thick foam can over-insulate, so see carpet and underlay on underfloor heating before choosing. Whether it's still fine to keep is a separate question, so use the checklist below. And if you're wondering what new underlay costs, see the carpet fitting cost guide: our own supplied-and-fitted underlay runs £6.50/m² (8mm PU) up to £9.50/m² (11mm PU), including grippers.

How to Tell if YOUR Underlay Needs Replacing (A Quick Checklist)

If you are still wondering whether your specific underlay can survive another round, try this quick self-diagnosis:

• The Tread Test: Walk across the highest-traffic area (like the centre of the lounge or the hallway) and then walk into a corner. Does the corner feel noticeably bouncier? If yes, the main underlay has collapsed.
• The Crumb Check: Lift a corner of your old carpet. If the underlay is disintegrating, tearing easily, or leaving a dusty, sandy residue on the floorboards, it is completely spent.
• The Odour Trap: Old underlay acts like a giant sponge. If you have had pets or spills over the last decade, those odours are locked in the foam, not just the carpet pile.

(Need an expert opinion? With my mobile showroom, I bring the samples to your door and can check your existing underlay for free while I measure up. Call Paul on 07903 320 414 to book a home visit across South London, Surrey, Kent, or Sussex.)

Why Old Underlay Cannot Be Reused:

• Uneven Wear: Old underlay flattens heavily in high-traffic pathways. Reusing it will cause your brand-new carpet to wear unevenly, making it look prematurely aged within months.
• Voiding Your Warranty: Most premium carpet manufacturers explicitly state that their wear warranties are only valid if the carpet is fitted over a brand-new, suitable underlay.
• Dust and Hygiene: Over 10 to 15 years, underlay crumbles and collects dust mites and moisture. Removing it gives your subfloor a clean, hygienic slate. Travelling across South London and Surrey to fit carpets, I frequently pull up 15-year-old underlay that has completely perished into dust—fitting a fresh £40/m² carpet over that is a recipe for disaster.

The 4 Main Types of Carpet Underlay

Before we get into the specific grades we fit, it helps to know what is actually out there on the market. Underlay generally falls into four categories, and each one trades off comfort, insulation, noise reduction, and price slightly differently.

PU foam is the modern mainstream choice for most UK homes: it's comfortable underfoot, insulates well, and sits at a reasonable price point. Rubber is the densest and most hard-wearing option, and it tends to outperform foam on raw impact-noise isolation, though it's heavier and firmer to walk on. Felt is a traditional, natural-fibre option that insulates well but isn't the strongest choice for impact sound on its own. Hybrid underlay, which layers felt and rubber together, is generally the best performer for blocking footfall noise, but it's also the heaviest and most expensive category.

Why We Fit PU Foam Exclusively

After years of fitting carpets across South London, Surrey, and Kent, PU foam is the one I carry as standard, and it's the only underlay I stock. I only supply and fit two grades directly: 8mm standard PU foam and our 10mm premium "Cloud Nine" foam. For the vast majority of the homes I work in, foam hits the right balance of comfort, warmth, and cost. It doesn't need justifying with a niche spec sheet. It just works, room after room, for ordinary families who want a carpet that feels good underfoot without paying for performance they'll never notice.

That said, I know PU foam isn't the answer for every situation. If you have a specific reason to need a denser rubber or an acoustic hybrid underlay, most commonly a lease or management company insisting on a certain impact-noise rating, I won't try to talk you out of it. Source the specialist underlay yourself and I'm happy to come and fit your carpet over it as a labour-only job, the same way I do for customers who've bought their carpet elsewhere.

Living in a flat? Read our full guide to underlay, leasehold rules and stopping footfall noise before you buy.

Choosing the Thickness: 8mm vs. 10mm

When I visit customers with my mobile showroom, I usually carry two primary grades of underlay. Here is how they compare:

My Honest Fitter's Caveat:

While 10mm offers a beautiful, luxurious bounce in a bedroom, thicker is not always better for stairs. For winder staircases or steps with a sharp bullnose, a 10mm underlay is often too thick to bend tightly over the nose of the stair. This can cause the carpet to wear out faster on the edge. For staircases, I always recommend sticking to a high-density 8mm underlay for the perfect balance of grip and padding.

Frequently Asked Questions

Areas I Cover

Based in South London, covering Surrey, Kent and Sussex with the mobile showroom: