Underlay

Underlay is the single most under-appreciated component of a good floor. It insulates against cold subfloors, absorbs footfall sound (a legal requirement in flats), evens minor subfloor variance, and — for click floors — protects the click joints from failing early. Spending well on underlay adds 30% to floor lifespan.

Pros

• +Massive acoustic improvement — impact sound drops 18–22 dB
• +Thermal barrier against cold concrete or ventilated timber
• +Extends click floor life by protecting the joints from micro-movement
• +Levels minor subfloor variation (up to 2 mm) without self-levelling
• +Legal requirement in most UK flats (Building Regs Part E)

Trade-offs

• −Wrong underlay voids most floor warranties — match to the floor type exactly
• −Never stack two underlays; bounce causes click joints to fail
• −Over UFH, tog value must be under 1.0 combined with the floor above

FAQs

• Do I really need underlay?

  Almost always yes. The exceptions: SPC with attached underlay, and any glue-down floor. For click floors without underlay, warranty is voided and the click joints fail within 2–3 years.

• Can I use two layers of underlay for extra sound?

  No — the bounce causes click joints to fail. Use one thicker, higher-spec underlay instead.

• What's the best underlay for underfloor heating?

  A UFH-approved thin foam (1.5–2 mm) with a stated tog value under 1.0. Combined with the floor above, the total tog should be 1.5 or less.

• Do I need a damp-proof membrane?

  Yes for any floor over concrete or a concrete screed. DPM-integrated underlays combine both in one product and save an install step.