Domakihub How to Fit Flooring Underlay: A Complete Step-by-Step Guide
Choose the right underlay, prepare your subfloor, and lay it correctly the first time.
Reviewed by Domakihub home & decor pros · 2026-06-23 · 5 min read
Underlay is the hidden layer that determines how your finished floor looks, sounds, and lasts — and it is one of the easiest parts of a flooring project to get wrong. The Quick-Step video above shows the physical process clearly; this article walks you through the full sequence with the technical context you need for Greek homes, holiday rentals, and hotel rooms. Get the underlay right and every plank or tile on top will perform better and last longer.
Why Underlay Matters More Than Most People Think
Underlay serves four distinct functions at once: it cushions foot traffic to reduce joint stress on click-lock planks, it absorbs impact noise (critical in apartment buildings and hotels with guests below), it smooths out minor subfloor irregularities, and — in the right specification — it acts as a vapour barrier against rising moisture from concrete slabs. Skip or skimp on any of these and you will see boards lifting, hear footstep transmission, or find buckling after the first wet winter.
In Greece, solid stone or tile subfloors are common in older buildings, and cast-concrete slabs without a damp-proof membrane are standard in island properties and many mainland apartments. Always check which situation you have before buying underlay — the right product varies significantly.
Choosing the Right Underlay for Your Floor Type
For laminate and engineered wood over concrete, choose a foam or felt underlay with an integrated polyethylene vapour barrier (marked ≥ 1 000 g/m²·day resistance). For timber joisted subfloors that already have a DPM below, a plain 3 mm foam or cork underlay is sufficient. Solid hardwood nailed directly to battens generally uses no roll underlay at all — the battens themselves provide the gap. For luxury vinyl plank (LVP) many products have underlay pre-attached; adding a second layer will void the warranty, so check the manufacturer spec first.
Acoustic ratings matter in multi-storey buildings. Greek building regulations reference EN ISO 717-2 for impact sound; an underlay with ΔLw ≥ 17 dB meaningfully reduces neighbour complaints. In hotel renovations, specifying acoustic underlay is effectively a guest-satisfaction and review-management decision, not just a technical one.
How to Fit Flooring Underlay: Step by Step
- 1
Prepare and check the subfloor
The subfloor must be clean, dry, and flat to within 3 mm over any 1.8 m span (most laminate and engineered-wood manufacturers specify this tolerance). Sweep or vacuum thoroughly, then scrape off any dried adhesive, plaster drops, or grout residue. Use a long straight-edge or spirit level to identify high spots and grind them down, and fill dips with a floor-levelling compound — let it cure fully before proceeding. On concrete, use a moisture meter: readings above 75 % relative humidity (or 2 % by mass) mean you need an epoxy DPM coat before any underlay goes down. This step cannot be skipped or corrected later.
- 2
Allow materials to acclimatise
Bring both the underlay rolls and the flooring planks into the room at least 48 hours before installation. In Greek summer heat or after a heating-off winter period, temperature swings can be significant. Underlay rolls stored in a cold warehouse and then unrolled immediately on a warm slab can trap condensation at the interface. Stack the planks flat in the room, keep the packaging intact, and maintain normal room temperature and humidity during this period.
- 3
Roll out and trim the underlay
Start along the longest straight wall, leaving a 10–15 mm gap at each wall (the expansion gap will be covered by skirting or beading). Unroll the first strip and trim cleanly to length with a sharp utility knife and a metal straight-edge — a ragged edge creates a ridge under your floor. Butt each subsequent strip edge-to-edge; never overlap, as even a 2 mm overlap creates a visible bump in the finished floor. For underlays with an integrated vapour barrier, overlap the silver or plastic membrane by 200 mm at each join and seal with moisture-resistant tape — this is the detail most DIYers miss. Tape along every join regardless of whether a DPM layer is specified; it prevents the sheets shifting during plank installation.
- 4
Navigate obstacles and door frames
At doorways, undercut the door casing with a flush-cut saw or an undercut saw so the underlay (and the flooring plank) slides beneath it cleanly rather than butting against it. Score and fold the underlay around radiator pipes; do not leave gaps around penetrations, as these become squeaky weak spots. At thresholds between rooms, plan your joining strip location before cutting the underlay — the underlay should stop at the centreline of the doorway where the threshold bar will sit. Mark this line on the subfloor with chalk before you start.
- 5
Secure and protect before laying the floor
Most roll underlays do not require adhesive or stapling on a solid subfloor — the weight of the flooring holds them flat. However, on wooden subfloors where the surface is not perfectly flat, use a low-tack self-adhesive tape at the wall edges to stop the underlay riding up as you work. Once all strips are laid and taped, do not walk on them with outdoor shoes or drag tools across the surface — grit caught under the underlay will telegraph through the finished floor as a permanent bump. Begin laying your flooring planks immediately; leaving underlay exposed for more than a day in a dusty renovation environment defeats the purpose of a clean installation.
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FAQ
- Can I lay new underlay over existing underlay to save time?
- No. Stacking two layers creates an overly compressible base that causes click-lock joints to flex and eventually fail. Remove all existing underlay completely, inspect the subfloor beneath it, and start fresh. The only exception is if you are laying a new floating floor over an old glued-down one that cannot be removed — in that case use a very thin 1.5 mm underlay to minimise stack height.
- Do I need a damp-proof membrane under underlay on a concrete floor in Greece?
- Almost always yes. Greek concrete slabs, particularly in ground-floor apartments, island villas, and older buildings, frequently have no damp-proof course. Test with a moisture meter or tape a sheet of polythene to the slab for 48 hours — condensation on the underside confirms moisture drive. Use an underlay with an integrated vapour barrier rated at ≥ 50 MN·s/g (check the data sheet), or apply an epoxy DPM coat first for high-moisture slabs. Skipping this on a coastal or basement-level property is the single most common cause of floor failure.
- How do I know if I need a professional rather than doing this myself?
- Underlay fitting itself is a DIY-friendly task for competent homeowners. However, call a professional if: your subfloor shows signs of structural movement or rot (bouncing, cracked screeds, hollow spots over large areas); moisture readings are persistently high and cannot be explained by recent cleaning — this may indicate a failed DPM or rising damp requiring a building specialist; or the project involves a hotel, commercial premises, or rental property where building regulations may require a certified installer for the finished floor warranty to be valid.