— Underlay · 6 min read · Updated June 2026
Underlay guide — the piece most people get wrong
Underlay decides how a floor feels, how it sounds, and how long it lasts. Every category — carpet, laminate, LVT, engineered wood — takes a different underlay, and specifying the wrong one is the fastest way to ruin a £3,000 install.
Carpet: 10mm PU foam for living areas, 8mm on stairs
PU (polyurethane) foam beats crumb rubber for compression recovery. 10mm gives that hotel-quality underfoot feel; 8mm is the maximum most stair regs allow. Never fit 6mm 'builders' underlay on a family carpet — it'll flatten in a year.
Laminate: 3mm combined underlay with vapour barrier
Combined = foam + integrated DPM. Over concrete you must have a vapour barrier or the HDF core will draw moisture and swell. Over timber, plain 3mm foam is fine. For quiet, spec a 5mm fibre-board acoustic underlay (Timbermate Excel or similar).
LVT: 1.5mm rigid underlay only
Never fit LVT on thick foam — the click joints will fail. If your LVT needs an underlay at all, use a purpose-designed 1.5mm rigid (Quickstep Basic, Karndean KA underlay). Many rigid-core SPC products need no underlay whatsoever — check the manufacturer spec.
Engineered wood: 3mm combined with DPM over concrete
Same rules as laminate — DPM mandatory over concrete. For glue-down engineered wood, no underlay is used; the adhesive bonds direct to the primed subfloor.
Underfloor heating: low tog only
Total floor build-up (underlay + floor) must be under 1.5 tog for UFH to work. That rules out thick foam or PU. Use 1.5-2mm rigid underlays or bond direct.
Frequently asked
Can I re-use my old underlay?
Only if it's under 5 years old, compression tests OK (bounces back when pressed), and matches the new floor type. Otherwise, fit new.
Do I need a DPM over concrete?
Yes, always, unless the subfloor has a factory-installed DPC and you have documentation to prove it. Damp is the number-one cause of laminate floor failure.