How Much Underlay Do I Need?

How We Calculated This

• Floor area: 4m × 3.5m = 14m²
• Add ~5% for overlaps: 14 × 1.05 = 14.7m²
• Standard roll: 15m² or 25m² — 1 roll of 15m²

Underlay Types by Flooring

• Laminate/engineered wood: 2–3mm foam or fibreboard underlay. Use one with built-in DPM on concrete subfloors
• Carpet: 8–11mm sponge rubber or PU foam. Minimum 80kg/m³ density for heavy traffic
• Vinyl/LVT: 1–1.5mm smooth, non-compressible underlay. Do not use carpet underlay
• Engineered wood (glued): Often no underlay needed — check manufacturer guidance

Common Roll Sizes

• Laminate underlay: 15m² or 25m² rolls (1m × 15m or 25m)
• Carpet underlay: 15m² rolls (1.37m × 11m) or 11m² (1.37m × 8m)
• Wood fibreboard: Sheets 590 × 790mm (packs of 7–15m²)

Do I Need a DPM?

On concrete subfloors, you need a damp-proof membrane (DPM) under the underlay. Many laminate underlays have a built-in DPM layer (silver or foil backing). If yours doesn’t, lay a separate 1200-gauge polythene sheet first with 200mm overlaps taped at the seams.

Tips

Lay underlay perpendicular to the flooring direction. Tape all joins with underlay tape to prevent moisture ingress and stop sheets moving. Never overlap underlay sheets by more than 50mm as this creates an uneven surface under the flooring.