Subfloor Preparation Guide

Quick Answer: Every hard floor finish has a subfloor flatness and moisture tolerance: LVT requires ±2mm per 1m for glue-down; ceramic tile requires ±3mm per 1.8m (BS 5385); engineered wood requires ≤75% RH on concrete and ≤14% MC on timber. Assess subfloor type (concrete, screed, timber, or existing floor finish), test moisture, identify high spots and hollows, and correct before laying. An hour spent on preparation prevents days of remediation.

Summary

Subfloor preparation is the invisible foundation of every flooring job. A floor finish laid over a poorly prepared subfloor will fail — whether that is LVT lifting at the edges, grout cracking in tile joints, hardwood springing and gapping, or laminate joints opening. The failure is always attributed to the floor finish or the installer, when the real cause was the substrate beneath.

Professional flooring installers spend a significant proportion of their time on preparation — often as much or more than on the actual laying. This reflects the reality that fixing a bad subfloor before laying is always cheaper and faster than stripping and relaying a floor finish after it has failed.

UK subfloors fall into five broad categories: ground-bearing concrete slab; suspended concrete (beam and block, or hollow core); sand/cement screed over concrete; timber subfloor (joisted with boards or ply); and existing floor finishes (tile, vinyl, or hardwood). Each category has different preparation requirements. This guide covers assessment and preparation for each.

Key Facts

• BS 8203 — Relevant moisture testing and substrate requirements for resilient floor coverings (LVT, vinyl)
• BS 5385 — Substrate requirements for ceramic tile installation; Part 1 covers internal walls and floors
• BS 8201 — Subfloor requirements for timber and engineered wood flooring
• Hygrometer test — the correct method for measuring subfloor moisture in concrete/screed; surface moisture meters read only surface, not depth; calibrated hygrometer tests at depth for 72 hours minimum
• Relative humidity limits — ≤75% RH for most floor finishes; some engineered timber manufacturers specify ≤65%; some natural stone adhesives require ≤75%
• Timber subfloor MC limit — ≤14% MC measured with a pin moisture meter (for most engineered wood); ≤12% for solid hardwood
• Flatness for LVT (glue-down) — 2mm per 1m
• Flatness for LVT (click/rigid core) — 3mm per 1.8m
• Flatness for ceramic/porcelain tile — 3mm per 1.8m (BS 5385)
• Flatness for engineered/solid wood — 3mm per 1.8m (BS 8201)
• Grading aggregate — levelling compounds are flow-grade (self-levelling) for concrete/screed substrates, and fibre-reinforced for timber; check the product data sheet for substrate compatibility before ordering
• New concrete cure time — minimum 6 weeks before resilient floor covering; minimum 28 days before tiling (with hygrometer confirmation)
• Anhydrite screed — must have laitance removed (light sand or grit blast) and be primed before any adhesive; calcium sulphate reacts with cementitious adhesives if the surface is not prepared

Quick Reference Table

Detailed Guidance

Concrete and Screed Subfloors

Step 1 — Visual and physical assessment

Before any moisture testing, carry out a visual assessment:

• Look for visible cracks — hairline cracks (under 0.5mm) in concrete are typically dormant; larger cracks or cracks with differential height across them indicate structural movement and require investigation
• Check for soft or sandy surface (laitance) — scratch with a key; if the surface crumbles, it must be strengthened or removed
• Check level across the floor with a long straightedge (1.8–2.4m spirit level or laser level and datum stick)
• Look for evidence of moisture — efflorescence (white salt deposits), dark patches, or active condensation

Step 2 — Moisture testing

Place calibrated hygrometers (or Tramex Concrete Moisture Encounter calibrated probes) at a minimum of 3 locations per 100m², with the sensor at the depth specified for the floor finish. Leave for at least 72 hours. Record the readings.

Surface moisture meters give a surface reading only and are not a reliable indicator of moisture content at depth. Do not use them as the sole basis for proceeding to lay.

Step 3 — DPM assessment

On ground-bearing concrete, check whether a DPM is present and continuous. Evidence of prior moisture damage (vinyl lifting, efflorescence, damp patches on walls at floor level) suggests a deficient DPM. If the DPM is absent or breached, apply a surface DPM:

• Epoxy DPM (two-coat): effective for RH up to approximately 97%; most robust option
• Polyurethane DPM: similar performance; faster drying
• Apply before levelling compound; surface DPM must be bonded to the structural slab, not just to the levelling compound

Step 4 — Grinding and filling

High spots must be ground down — a 5-inch angle grinder with a diamond cup wheel or a floor grinding machine for large areas. Wear RPE (respiratory protective equipment) rated for concrete dust (silica) — RPF3 minimum; silica dust causes silicosis (an irreversible lung disease), and grinding generates very high concentrations. Secure dust suppression (vacuum attachment or wet grinding) is mandatory.

Hollows, cracks, and low areas are filled with levelling compound. For depths under 5mm, use a flow-grade self-levelling compound. For deeper fills (up to 50mm), use a smoothing compound or fibre-reinforced mortar, then a skim of self-levelling compound.

Prime the concrete before levelling compound — a PVA primer or bonding agent prevents the concrete from absorbing water from the compound too rapidly, which causes surface crazing. Screed primers and SBR bonding agents are the standard options.

Anhydrite Screed Preparation

Anhydrite (calcium sulphate) screed requires specific treatment before any floor finish:

1. Remove laitance — using a belt sander, floor grinder, or grit blaster. The laitance is a weak surface layer that will delaminate if adhesive is applied to it. Test by scratching — if powder comes off, there is still laitance
2. Prime — apply a specialist anhydrite primer (e.g. Mapei Primer T, BAL Bond SBR) to seal the surface and prevent chemical reaction between the calcium sulphate and cementitious adhesive above
3. Do not use cementitious adhesive directly — without primer, the sulphate in the anhydrite reacts with aluminate compounds in cement adhesives, causing expansion and delamination

Timber Subfloor Preparation

Fixing spring in joisted floors

Walk the floor to locate springy boards. Spring is caused by: loose nails or screws; inadequate noggins (strutting) between joists; joists with too much deflection; or board joints not over joists.

• Re-fix loose boards with 75mm screws (not nails)
• Add intermediate joist supports if deflection is the cause
• If the boards have significant undulation or gaps, overlay with 18mm or 22mm plywood

Plywood overlay

This is the preferred preparation for timber subfloors receiving hard finishes (tile, LVT, hardwood):

• Use C4 exterior-grade plywood (moisture resistant); standard interior ply delaminates with adhesive contact
• Minimum 18mm for most hard floor finishes; 22mm if the board below is springy or has wide spans between joists
• Stagger the plywood joints; joints should not coincide with the underlying board joints
• Screw at 150mm centres across the sheet and 100mm centres at edges
• Fill and sand all ply joints to achieve flatness

For tiles over timber, a minimum 2-layer system is required — the existing floor structure plus ply overlay, or ply over boards — to achieve the required deflection limit (span/250 for ceramic tile per BS 5385).

Punching nail heads

Any protruding nail heads will telegraph through resilient floor finishes. Use a nail punch to set all nail heads 2–3mm below the surface, then fill with a hard-setting compound before levelling.

Existing Floor Finishes as Subfloors

Existing ceramic or porcelain tiles:

• Tap-test every tile — any hollow sound indicates a debonded tile that must be removed and replaced. Laying over hollow tiles will result in cracking above the debonded area
• Ensure grout joints are not proud of the tile surface — grind flush if necessary
• Degrease the tile surface with a solvent cleaner before applying any adhesive or levelling compound
• If laying new ceramic over existing: use a flexible adhesive (C2TE or better per EN 12004) and provide movement joints in the same positions

Existing vinyl or LVT:

• Embossed vinyl must be skim-coated — the emboss pattern will telegraph through new LVT within months
• Apply a levelling compound rated for non-porous substrates (not all products bond to vinyl)
• If the vinyl is lifting at edges or seams, re-bond with vinyl seam adhesive before skim-coating

Frequently Asked Questions

Do I need to remove old vinyl tiles before laying new LVT?

Not always. If the tiles are well-bonded, smooth, and RH is acceptable, you can overlay. However, if the tiles are bitumen-bonded (common with pre-1980s vinyl tiles), be aware that bitumen plasticiser migrates into new LVT over time and causes delamination. Either remove or encapsulate with a specialist skim product. If any tiles are loose, they must be removed individually and the area filled.

Can I use sand and cement to level a floor instead of levelling compound?

Sand/cement mortar is not suitable for thin beds (under 30–40mm) used to achieve floor flatness — it shrinks during curing, cracks, and does not bond well to concrete without reinforcement. Self-levelling compound is the correct product for preparing a flat, bondable surface for floor finishes.

How long after levelling compound can I lay the floor?

Most self-levelling compounds are walkable in 2–4 hours and ready for floor finish after 24 hours at 20°C and 65% RH. Check the product data sheet — drying time increases significantly in cold or humid conditions. Test the surface hardness before laying: it should resist a fingernail indent.

What is the deflection limit for tile over timber?

BS 5385-1 requires the subfloor to have a deflection no greater than span/250 under a load of 1.5 kN applied at the worst-case point. In practice, this means a minimum two-layer timber system: existing structural floor plus screwed ply overlay of at least 18mm. Thin (under 15mm) ply overlays frequently fail this criterion.

Regulations & Standards

• BS 8203 — Code of practice for installation of resilient floor coverings; Annex A covers substrate preparation

• BS 5385-1 — Wall and floor tiling; Part 1: design and installation of ceramic, mosaic, and stone tile on floors in internal areas

• BS 8201 — Code of practice for flooring of timber, timber products, and wood-based panels

• Control of Substances Hazardous to Health (COSHH) Regulations 2002 — silica dust from concrete grinding; RPE requirements

• Health & Safety Executive (HSE) EH40 — Workplace exposure limits; silica dust WEL is 0.1 mg/m³ (8-hour TWA)

• Contract Flooring Association Technical Documentation — substrate and moisture assessment guidance

• HSE: Control of Silica Dust — occupational exposure controls for concrete grinding

• BAL Technical Guidance: Substrates — tile adhesive and substrate preparation specifications

• floor levelling compounds: which product to choose and how to apply — the levelling stage in detail

• screed types and drying rates — when you are starting with a screed substrate

• damp floor diagnosis — investigating and resolving DPM failure before laying

• LVT installation: subfloor requirements and adhesive — LVT-specific requirements