Subfloor Preparation for Flooring: Concrete DPM, Suspended Timber Conditioning and Ply Overlay

Summary

Most floor covering failures trace back to poor subfloor preparation. The floor covering industry estimates that 80% of all callbacks from flooring contractors relate to subfloor issues: inadequate flatness, damp problems, movement, or substrate contamination. Addressing the subfloor correctly at the start is always cheaper than lifting and re-laying a failed floor covering.

There are two primary subfloor types in UK housing:

• Ground floor concrete slabs — most properties built post-1960, often with a built-in DPM but increasingly showing moisture issues as membranes age
• Suspended timber floors — dominant in pre-1960 housing, still common in upper floors of all housing

Understanding what floor covering is being laid dictates what the subfloor needs to achieve. LVT and ceramic tiles are highly sensitive to movement and moisture. Carpet is forgiving. Solid hardwood needs moisture-controlled, stable substrates.

Key Facts

• Moisture testing — Surface moisture meters are unreliable for floor decisions. Use a calibrated hygrometer with insitu probes (BS 8201, method B) or a calcium carbide test (BS 8203 Annex A). Allow 72 hours for equilibration.
• RH limit for resilient flooring — ≤75% RH (or ≤0.5% moisture by calcium carbide — equivalent to ≤75% RH). Above this, most adhesives and floor coverings fail.
• RH limit for wood flooring — ≤65% RH (≤0.3% CC). Wood is more sensitive to moisture than resilient materials.
• DPM over concrete — Epoxy DPM or polyurethane-based DPM systems for residual moisture above 75% RH. Apply in 2–3 coats, allowing full cure between coats.
• Polythene DPM — Standard 1200-gauge polythene under new concrete slabs during construction. If not present or damaged, must be replaced with surface-applied DPM.
• Plywood overlay minimum — 9mm for light residential use; 12mm for heavy use or when bridging uneven boards. BS EN 636 Class 2 (humid conditions) minimum.
• Fixings for ply overlay — Screw at 150mm centres throughout and 100mm at perimeter. Countersink screws flush or just below surface.
• Timber moisture content — Solid hardwood: lay at 8–12% MC. The subfloor should be within 2% MC of the floor covering. Acclimatise timber flooring in the room for minimum 48–72 hours.
• Sub-floor ventilation — Suspended timber ground floors need cross-ventilation via airbricks. Blocked airbricks cause rot and damp. Replace with equivalent ventilation if upgrading to foam insulation.
• Existing resilient flooring as substrate — Only if fully bonded, no asbestos, not contaminated. Scuff surface to key for new adhesive.
• Adhesive residue — Old solvent-based adhesives (black bitumen, yellow contact adhesive) often contain asbestos. Test before grinding or sanding. Encapsulate or remove as licensed waste if positive.

Quick Reference Table

Detailed Guidance

Concrete Subfloor Assessment

Step 1: Sound test Walk the floor listening for hollow areas. Tap with a rubber mallet. Hollow spots indicate delamination or failure of screed — these must be cut out and repaired before any moisture testing or floor covering.

Step 2: Contamination check

• Oil or grease: use degreaser (Mapei Primer G or similar); mechanically remove heavy deposits
• Adhesive residue: test black or yellow residues for asbestos (licensed bulk sampling contractor if in doubt). If asbestos-free, skim SLC over to encapsulate. If contaminated, follow ACM procedures.
• Paint: coatings prevent primer and adhesive bond. Grind off or use acid wash before SLC.

Step 3: Flatness check Using a 3m straight edge and feeler gauges:

• Check in multiple directions (perpendicular strips)
• Mark all areas requiring levelling with chalk
• Tolerance ±3mm under 3m (SR2) for most coverings; ±2mm for thin LVT

Step 4: Moisture testing Insitu hygrometer test (preferred for final pre-lay test):

1. Core drill 3 holes per 50m² (minimum 3 holes)
2. Insert sleeves; seal with tape
3. Allow 72 hours equilibration at measured room temperature
4. Read with calibrated hygrometer
5. Repeat in suspicious areas (corners, near external walls)

Damp Proof Membrane (DPM) Options

When DPM is required:

• RH >75% (resilient covering) or >65% (wood)
• No existing DPM in slab
• Residual construction moisture in new concrete (<2 years old)
• Ground floor with suspected rising damp

Surface DPM (epoxy or polyurethane):

• Epoxy 2-coat system: primer coat at ~0.15kg/m², then 2× surface DPM coats at 0.3kg/m² each
• Allow full cure between coats (typically 12–24 hours at 20°C)
• Abrade between coats to improve intercoat adhesion
• Total dry film thickness: typically 250–500 microns
• Products: Arditex NA, Ardex P51, Mapei Primer T, F. Ball Stopgap F78

Polythene membrane (self-adhesive):

• For constructing a new floor: 1200-gauge (300 micron) polythene; lap joints by 300mm and tape with self-adhesive DPM tape
• Turn up walls by 150mm
• Seal penetrations
• Cannot be used as a retrofit surface DPM over contaminated or uneven substrates

Suspended Timber Floor Preparation

Assessment:

• Deflection test: walk and feel for springy boards. Maximum permitted deflection for resilient flooring: span/300 (typically <3mm)
• Board condition: probe suspect boards with screwdriver — end rot and wet rot are common near external walls
• Existing nail heads: drive down with nail punch to 2mm below surface

Securing loose boards:

1. Screw every board at every joist crossing (typically 400mm joist centres → screw every 400mm along each board)
2. Use 40–50mm countersunk screws (Type 17 or similar self-countersinking)
3. At board edges: screw 100mm from end of boards
4. Add timber blocking where boards are unsupported at edges

Ply overlay:

1. Acclimatise ply in room for minimum 48 hours before fixing
2. Use BS EN 636 Class 2 (C2) plywood — moisture-resistant bonded
3. Thickness: 6mm for light residential over stable boards; 9mm for standard; 12mm for heavy use or spanning weak areas
4. Cut sheets to fit; stagger joints (no four corners meeting at one point)
5. Leave 1–2mm gap between sheets and at walls (expansion)
6. Fix with 30mm (6mm ply) or 45mm (9mm+) countersunk screws at 150mm in field, 100mm at perimeter
7. Punch screw heads just below surface

Tile Backer Boards as Alternative to Ply

For tiled areas (bathroom floor, utility room), tile backer board (Wedi, Hardie Backer, Schlüter Ditra) is preferable to ply. These products:

• Are dimensionally stable under moisture cycling
• Are compatible with tile adhesive (ply is not consistently compatible with all C2 adhesives)
• Typically 12.5mm thickness provides equivalent strength to 9mm ply with better moisture resistance
• Screw at 150–200mm centres using the manufacturer's specified screws

Frequently Asked Questions

My concrete floor tests at 80% RH. Can I lay LVT straight over it?

No. You must apply a suitable surface DPM first. An epoxy or polyurethane 2-coat system will reduce the effective RH below 75% and provide the adhesive bond for the floor covering. Apply the DPM, allow full cure (typically 24–48 hours), then lay the floor covering direct over the DPM surface.

Can I tile directly onto suspended timber floors without ply overlay?

Not recommended. Timber floors deflect under load. Deflection causes tiles to crack and adhesive to debond. The standard practice is: screw all boards (add screws if needed), apply 12–15mm cement tile backer board screwed at 150mm centres, then tile on the backer board. This provides a stable, moisture-resistant, tile-compatible substrate.

Does the plywood overlay void the floor covering warranty?

Not if the correct grade and thickness is used. Most floor covering manufacturers specify or accept ply overlay as a suitable substrate. Use the grade specified (typically BS EN 636 C2 for humid environments, C3 for wet areas). Check the product warranty document for specific substrate requirements.

Regulations & Standards

• BS 8201:2011 — Code of practice for installation of flooring of wood and wood-based panels

• BS 8203:2017 — Code of practice for installation of resilient floor coverings

• BS 8204-1:2003 — Screeds, bases and in-situ floorings: concrete bases and cementitious levelling screeds

• BS EN 636:2012 — Plywood: specifications

• COSHH Regulations 2002 — Adhesive residues containing asbestos; epoxy systems require gloves and eye protection

• British Adhesives and Sealants Association (BASA) — Moisture testing guidance

• Contract Flooring Association — Substrate guide

• F. Ball & Co — Subfloor preparation guide

• Ardex Technical Documentation — DPM systems

• floor levelling — Self-levelling compound selection and application

• lvt installation — LVT moisture and flatness requirements

• rising damp — Rising damp diagnosis before flooring

• screed depth — Screed depth calculations