Flooring Installation: Engineered Wood, Laminate, LVT & Solid — Subfloor Requirements

Summary

Flooring is a high-visibility trade. A floor that moves, creaks, delaminates, or gaps within a year is a reputation-destroying callback. Most flooring failures are caused by one of three issues: moisture in the subfloor, insufficient acclimatisation of the product, or inadequate subfloor preparation. Understanding the specific requirements of each product type is essential before pricing or starting any flooring job.

The UK market broadly splits into four product categories: solid hardwood, engineered wood, laminate, and luxury vinyl tile (LVT)/plank. Each has different tolerance to moisture, different installation methods, and different subfloor requirements. LVT is the most forgiving of all four — it can go over almost any level subfloor and is dimensionally stable. Solid hardwood is the most demanding — it is highly sensitive to moisture and temperature change, and needs to be used in controlled environments.

Building type matters too. Concrete ground floor slabs are common in UK properties and present ongoing moisture challenges. A new slab may take months to dry to acceptable levels; even an existing slab can show elevated moisture readings in wet weather. Always test before laying — a moisture-related flooring failure is rarely covered by product warranty.

Key Facts

• Subfloor moisture — engineered wood: Maximum 75% relative humidity (RH) or 17–19% WME using a wood moisture meter
• Subfloor moisture — solid hardwood: Maximum 65% RH or 14–16% WME — more sensitive than engineered
• Subfloor moisture — LVT/laminate: Typically 75–80% RH maximum — check manufacturer's specific data sheet
• Concrete slab test: Use a hygrometer in a sealed insulated box for minimum 72 hours (BS 8201) or digital probe method per ASTM F2170
• Flatness tolerance: 3mm deviation under a 1.8m straight edge for most products; 2mm for glue-down LVT
• Expansion gap: 10–15mm at all fixed perimeters (walls, door frames, pipes, steps)
• Floating floor max room size: Most manufacturers limit floating floors to 12–15m in any direction without a T-section break
• Acclimatisation — solid hardwood: 5–7 days minimum in the room at normal temperature and humidity; longer for wide boards
• Acclimatisation — engineered: 48 hours minimum recommended; less sensitive than solid
• Acclimatisation — laminate/LVT: 24–48 hours to reach room temperature; dimensional stability much better than solid wood
• Underlay: Required under laminate and floating engineered; not used under glue-down LVT; some LVT has pre-attached underlay
• DPM (damp proof membrane): Required over concrete subfloor if RH exceeds acceptable levels — liquid-applied or sheet DPM
• Underfloor heating: LVT and engineered preferred; floating method requires insulated underlay (10 tog limit); glue-down preferred for UFH response
• Hardwood grade: Prime (fewest features), Select, Natural, Rustic (most knots/character) — grade does not affect structural performance
• Species hardness: Janka scale — oak 1290 lbf (common), bamboo 1380, walnut 1010, pine 870 — relevant for high-traffic areas

Quick Reference Table

Detailed Guidance

Subfloor Assessment and Preparation

Subfloor preparation is not an add-on — it is the most critical part of any flooring installation. The subfloor must be assessed for three properties: flatness, moisture, and structural integrity.

For concrete subfloors: Check for hollows with a chain or scraper (hollow sound = delaminated screed), and measure moisture using a calibrated hygrometer or digital probe. Apply a suitable self-levelling compound to correct flatness issues — sand/cement screeds take approximately 1 day per mm to dry; anhydrite (liquid) screeds may take longer and require surface laitance removal before bonding anything.

For timber subfloors: Check for deflection and squeak by walking the floor. Re-fix any bouncy boards with screws (not nails). Check joist depth and condition — severely deflecting joists may need sistering. Plane or sand high spots; use flexible floor adhesive compounds (e.g. Mapei Ultrabond Eco) to fill low spots. Timber subfloors will inherently move with moisture and temperature — this is more compatible with floating installations than full-bond methods.

Engineered Wood Installation

Engineered wood is a sandwich: a decorative hardwood top layer (2–6mm) bonded to a stable core of plywood or HDF. The plywood cross-ply construction resists expansion and contraction much better than solid wood, making it suitable for underfloor heating and ground floor concrete applications.

Installation methods: floating (click or glue joint), secret nail over plywood subfloor, or full glue. For UFH, always glue — floating engineered boards create an air gap that acts as an insulator and slows UFH response. Use a suitable wood flooring adhesive recommended by the board manufacturer — not standard PVA or contact adhesive.

Board width and moisture: wider boards are more prone to cupping (edges rising) or gapping in dry conditions. Boards over 180mm wide on concrete subfloor should always be glued, not floated. Some manufacturers void warranty on wide boards installed floating over concrete.

Staggered joints: End joints between adjacent rows must be offset by minimum 300mm (typically half a board) to prevent visible stepped pattern and to maximise structural rigidity of the floor.

Laminate Installation

Laminate is a high-density fibreboard (HDF) core with a photographic wood or stone image under a hard resin wear layer. Modern AC4/AC5 laminate is highly durable but is not waterproof — the HDF core swells when wet. Not suitable for wetrooms, bathrooms with regular water splashing, or laundry rooms.

Wear layer rating (EN 13329): AC1 (residential light) through AC5 (commercial). For domestic kitchens and hallways, specify AC4 minimum.

Most modern laminate is click-fit (Uniclic or similar) and installs entirely floating. The underlay provides sound reduction and slight cushioning — typically 3–5mm EVA foam or combination acoustic/thermal underlay. Check underlay TOG rating if laying over UFH.

Doorways and transitions: Laminate expands and contracts as a unit — a large open-plan floor will develop more cumulative expansion than a small room. Use T-section mouldings at doorways between separately installed areas. Threshold strips must allow for expansion on both sides.

LVT (Luxury Vinyl Tile/Plank) Installation

LVT is the fastest-growing flooring category in UK residential and light commercial projects. Key advantage: completely waterproof, dimensionally stable, warm underfoot, and compatible with UFH. Available in click-fit rigid core (SPC/WPC) or glue-down flexible formats.

Rigid core LVT: 4–8mm total thickness, typically with pre-attached IXPE foam underlay. Floats over subfloor. Suitable for most subfloors if flat within 3mm/1.8m. Very tolerant of subfloor moisture. Can be installed over existing tiles or hard flooring if flat and bonded.

Glue-down LVT: 2.5–3.5mm total thickness, installed with pressure-sensitive or hard-set adhesive. Requires flatter subfloor (2mm/2m) and lower moisture levels than rigid core. Best for UFH and commercial applications. No expansion gap required if properly glued — the adhesive absorbs dimensional movement.

One key issue with LVT: it is dimensionally stable but still affected by temperature. In conservatories or rooms with high solar gain, LVT can soften and deform. Keep room temperature below 27°C consistently. Do not lay in direct sunlight without solar control glazing.

Solid Hardwood Installation

Solid hardwood is the most premium product and the most demanding to install correctly. UK-grown and imported oak is the most common species. Solid boards (18–22mm typical) are secret-nailed through the tongue to a timber subfloor, or fully bonded to concrete.

Acclimatisation is non-negotiable for solid hardwood: the boards must be stored in the room at the intended living temperature and humidity for at least 5–7 days before installation. For wide boards (140mm+), some manufacturers recommend 14 days. Stack boards in the room with spacers to allow air circulation.

Concrete installation: Solid hardwood over concrete requires a plywood sub-base (minimum 18mm plywood, fixed to concrete with SDS masonry screws and adhesive) to allow secret nailing. Alternatively, full-bond with specialist wood flooring adhesive. The concrete must be below 65% RH — no exceptions.

Solid hardwood is not recommended over UFH — the wood will move significantly with the heating/cooling cycle and is likely to gap in winter. Some very thin solid boards (12mm) can go over UFH but with strict temperature controls; consult the specific manufacturer.

Frequently Asked Questions

How long does a concrete slab need to dry before I can lay flooring?

As a rule of thumb, sand/cement screed takes 1 day per mm of thickness. A 75mm slab takes approximately 75 days in normal conditions. Anhydrite (liquid) screed dries faster but varies by formulation. Always test with a calibrated hygrometer — do not rely on the builder's timeline. A new slab can test wet for 6–12 months if the property has not been heated.

Can I lay LVT or laminate directly over existing tiles?

Yes, provided the tiles are fully bonded, flat, and the added floor height is acceptable at thresholds. Any hollow or loose tiles must be removed and the floor made good — a hollow tile transmits movement into the new floor and can cause click joints to fail. Check door clearances — 8mm of LVT plus pre-attached underlay will close clearance gaps significantly.

What is the difference between AC3 and AC5 laminate?

The AC (Abrasion Class) rating is from EN 13329. AC3 is suitable for moderate residential use (bedrooms). AC4 is suitable for residential kitchens, hallways, and light commercial. AC5 is suitable for heavy commercial use. For most domestic jobs, AC4 is appropriate and represents good value.

Do I need underlay under LVT?

For rigid core LVT with pre-attached underlay — no additional underlay is needed. For rigid core without pre-attached underlay, a thin 1–1.5mm underlay may improve acoustic performance. For glue-down LVT — no underlay at all. Never double up underlays (e.g. pre-attached plus separate sheet) as this creates a too-thick and unstable base that causes click joint failure.

Can I tile over a floating floor?

No. Tiles require a rigid, bonded substrate. A floating floor will flex under tile weight and grout will crack. If tiling is required over an area that currently has floating flooring, the floating floor must be removed and a direct bond system used.

Regulations & Standards

• BS 8201 (Code of practice for installation of flooring of wood and wood-based panels) — the primary UK standard for wood flooring installation including moisture testing and acclimatisation

• BS EN 13329 (Laminate floor coverings — specifications) — performance requirements including AC wear ratings

• BS EN 16511 (Semi-rigid tiles including LVT — specifications) — covers LVT product requirements

• Approved Document C — moisture resistance requirements for ground floors

• BWFA (British Wood Flooring Association) — technical guidance and installation standards [verify specific document numbers]

• British Wood Flooring Association Technical Guides — installation standards and subfloor preparation guidance

• Kahrs Technical Installation Manual — detailed engineered wood installation instructions

• Amtico Technical Data Sheets — LVT subfloor preparation and moisture requirements

• Contract Flooring Association Technical Bulletins — industry guidance on subfloor preparation and adhesive selection

• BS 8201 Code of Practice — British Standard for wood flooring installation

• stud walls — stud wall construction relevant when installing flooring around new partitions

• underfloor heating tiles — UFH-compatible tiled floor systems

• floor screeds — subfloor screed preparation