How do you prepare a subfloor for flooring installation?

Quick Answer: Subfloor preparation in the UK follows BS 8204-1 to BS 8204-7 depending on substrate type. Every subfloor must meet three criteria before any floor finish is laid: moisture ≤75% RH per BS 5325 hygrometer method (or equivalent CM value), surface regularity to SR1 (3mm gap under a 2m straight edge) for most resilient floorings, and full soundness with no hollow areas, loose patches, or contaminating residues.

Summary

Subfloor preparation is the most underpriced element of a flooring quote and the most common cause of premature failure. A floor finish is only as good as the substrate it is bonded to. The skin of a floor — what the customer sees — is laid in a day; the substrate that holds it flat, dry, and bonded for 20 years takes longer and matters more.

This guide covers the complete preparation cycle for the four substrates that account for 95% of UK flooring work: sand/cement screed, anhydrite (calcium sulphate) liquid screed, existing concrete slab, and timber boarding or chipboard. Each has distinct preparation requirements that determine the finished floor's longevity.

The principles are: verify, repair, regulate, prime, finish. Verify the substrate against the manufacturer's specification for the chosen finish. Repair any damage. Regulate the surface to the required tolerance. Prime to bond the regulation to the substrate. Apply the finish to a substrate that is now fit-for-purpose. Skipping any step risks the finish failing within the warranty period and a callback to lift, prepare, and relay.

Key Facts

• BS 8204-1 — Concrete bases and cementitious levelling screeds; the parent UK standard
• BS 8204-2 — Concrete wearing surfaces
• BS 8204-3 — Polymer-modified cementitious levelling screeds
• BS 8204-4 — Cementitious terrazzo wearing surfaces
• BS 8204-5 — Mastic asphalt underlays and wearing surfaces
• BS 8204-6 — Synthetic resin floorings
• BS 8204-7 — Pumpable self-smoothing screeds (anhydrite and modified cementitious)
• BS 5325 — Hygrometer moisture test method for subfloors before resilient flooring
• 75% RH — universal upper moisture limit for bonded floorings (LVT, vinyl, wood)
• 4% CM — equivalent moisture limit for sand/cement screed by carbide meter
• 0.5% CM — equivalent limit for anhydrite screed by carbide meter
• Surface regularity SR1 — 3mm maximum gap under a 2m straight edge
• Surface regularity SR2 — 5mm maximum gap under a 2m straight edge (acceptable for thicker resilient and tile)
• Surface regularity SR3 — 10mm maximum gap (only acceptable under screed regulation)
• Hollow area test — strike with metal rod; resonant ping indicates void, dull thud indicates bond
• Contaminants — paint, plaster, gypsum, oil, curing compound all prevent bond and must be removed
• DPM systems — surface-applied epoxy DPM allows flooring when substrate exceeds 75% RH but cannot be left to dry naturally
• Acoustic underlay — required under separating floors per Part E; chosen before substrate preparation
• Primer types — acrylic for cementitious substrates, specific anhydrite primer for liquid screed, epoxy for non-porous substrates
• Plywood overlay — 6mm WBP minimum on timber subfloors; 9-12mm where additional thickness needed
• Screw centres — 150mm grid into joists, 100mm at sheet edges for plywood overlay
• Smoothing compound depths — 3-5mm trowel-apply; 5-25mm pump-apply; 25-50mm with aggregate addition
• COSHH — cementitious dust, epoxy vapour, MDI in PU adhesives all require RPE and ventilation

Quick Reference Table

Detailed Guidance

Moisture testing — the most important step

No flooring should be laid until subfloor moisture has been verified against the relevant standard. The BS 5325 hygrometer method is the UK reference test:

1. Drill a hole into the substrate to half the substrate thickness (maximum 50mm)
2. Insert a hygrometer probe in an insulated test chamber (typically a sealed dome)
3. Leave the chamber sealed for 72 hours minimum
4. Read the relative humidity at the substrate temperature
5. Repeat at minimum 3 locations per 100m², including known wet zones (perimeters, plumbing penetrations)

The 75% RH limit is universal across resilient flooring, wood, and most adhesives. Some manufacturers specify 65% RH for sensitive products (engineered timber, hardwood block). Anhydrite screed must be ≤75% RH AND the surface must be free of laitance — moisture below the limit is necessary but not sufficient.

The carbide meter (CM) method is faster but more invasive (requires drilling and sample collection) and is referenced in BS 8204-1. Different substrates have different equivalent CM values:

• Sand/cement screed: 4% CM ≈ 75% RH
• Anhydrite screed: 0.5% CM ≈ 75% RH
• Concrete slab: 4% CM ≈ 75% RH (varies with aggregate)

A pin-type meter on timber should read ≤18% MC for chipboard or plywood, ≤14% MC for floorboards.

Repair before regulation

Before any smoothing or levelling compound, repair the substrate:

• Cracks — open out with grinder, vacuum, fill with flexible epoxy or rapid-set repair compound
• Hollow areas — tap-test the substrate; mark any resonant patches; inject low-viscosity epoxy or remove and patch
• Spalled edges — at thresholds and junctions; rebuild with structural mortar
• Loose screed — if more than 10% of the area is loose, the screed is failing and should be removed
• Pipe penetrations — pack around with non-shrink grout, seal with sealant after primer
• Joints — bay joints in screed should continue through the smoothing compound; mark and re-form after laying

For anhydrite screed: sand the entire surface with a 60-80 grit floor sander to remove laitance. The chalky surface film is weak and will fail under load if not removed.

Surface regularity and smoothing compounds

The choice of smoothing compound depends on the depth needed:

Trowel-apply (feather-finish) — 1-5mm depth, applied by hand with steel float, for fine surface regulation and skim coats. Fast-setting (15-20 minutes working time), walk-on within 2 hours.

Pump-apply (self-smoothing) — 3-25mm depth, applied via mixer-pump or hand-mixed in buckets, self-levelling action. Walk-on within 3-4 hours, ready for flooring 24-48 hours.

Deep-pour (bulk-fill) — 25-50mm depth, often with aggregate addition (10mm clean stone), for substantial level changes. Walk-on within 4-6 hours, ready for flooring 48-72 hours.

Polymer-modified (SBR or acrylic latex) — for substrates with movement potential (timber overlays, areas with thermal cycling). More expensive but tolerates substrate flexure.

All smoothing compounds must be applied over a properly primed substrate. The primer:

• Seals porosity to prevent the smoothing compound from drying too fast
• Creates a chemical bond between substrate and compound
• Prevents air bubbles rising through the compound during set

Acrylic primer (typically diluted 1:3 with water for absorbent substrates, 1:1 for dense) is the default. Anhydrite primer is a specific water-resistant acrylic that prevents the gypsum substrate from breaking down on contact with water-based cementitious compound. Epoxy primer is required where moisture is at or above 75% RH and cannot be left to dry further.

Surface DPM systems

When a substrate exceeds 75% RH and the programme cannot allow further drying time, a surface DPM is bonded to the substrate. The DPM seals the substrate moisture from the floor finish above.

Types:

• Two-part epoxy — two coats applied 12-24 hours apart, broadcast with sand for mechanical key, working tolerance 95% RH
• Single-part moisture-cured polyurethane — single thicker coat, faster, working tolerance 90% RH
• Resin and primer combinations — proprietary systems specific to flooring manufacturers

After the DPM has cured, prime and apply smoothing compound, then lay the finish as normal. Surface DPM is more expensive than waiting for natural drying but maintains the programme.

Surface DPM is not a fix for active water ingress. If water is rising through the substrate from below (rising damp, leaking pipe, defective DPM in slab), surface DPM will be undermined and fail. Identify and fix the source first.

Timber subfloor preparation

Timber substrates need different treatment to cementitious:

Floorboards — original Victorian/Edwardian floorboards are rarely flat enough for direct flooring. Options:

• Overlay with 6mm WBP plywood, screwed at 150mm centres into the boards
• Sand and refinish if exposed timber is the desired finish
• Replace damaged boards in matching size before overlay

Chipboard P5 — engineered floorboard, typically 22mm tongue-and-groove, in 600x2400mm sheets. Check for:

• Sag between joists (joists at 400mm centres maximum for chipboard)
• Loose or squeaking boards — screw down at 150mm centres into joists
• Water damage at perimeters and around plumbing — replace affected sheets
• Tongue-and-groove engagement — gaps suggest movement or installation defects

Plywood overlay — applied over either floorboards or chipboard for additional stiffness and surface flatness. WBP grade minimum, 6mm for thin resilient, 9-12mm under tile or stone. Screw at 150mm grid into joists and at 100mm at sheet edges. Stagger joints so no four corners meet.

Existing finish removal

When relaying flooring, the existing finish often must be removed:

Carpet and underlay — straightforward lift; check for staples and gripper rods to remove Sheet vinyl — pre-2000 may contain asbestos backing; test before disturbing under CAR 2012 Glue-down LVT — usually requires mechanical scraper or shot-blaster to remove adhesive residue Ceramic tile — break out with SDS hammer or grinder-and-chisel; may need new substrate after removal Engineered wood, click LVT — lift dry; underlay may be reusable if clean and undamaged Old adhesive (bitumen) — sand back or remove with proprietary adhesive remover; never use sanders that generate sparks near solvent-based removers

Asbestos risk is highest in:

• Pre-1980 textured coatings and floor tiles (Marley tile, etc.)
• Pre-2000 sheet vinyl backings
• Pre-1990 bitumen-based adhesives

If unsure, get a sample tested by a UKAS-accredited lab (cost £25-50, 24-48 hours) before disturbing.

Acoustic considerations

For separating floors in flats, HMOs, and conversions, Building Regulations Part E requires acoustic performance to specific ΔLw values. The acoustic underlay must be chosen and incorporated into the substrate preparation:

• Pre-completion testing — separating floors in new build or conversion must be tested in accordance with ADE
• Acoustic mat — typically 5-10mm rubber/cork/foam composite, laid on the substrate before screeding or before flooring
• Resilient underlay — beneath floating floor systems
• Floating floor build-up — concrete topping or screed over acoustic layer, with isolation from walls

Plan the build-up before preparation begins — fitting acoustic mat after smoothing compound is fitted is rework.

Frequently Asked Questions

What if the subfloor is wetter than 75% RH and I can't wait?

Either install a surface DPM system (epoxy or PU resin) or stop the job. There is no other compliant route. Laying over a wet substrate guarantees failure within months. The customer needs to understand the moisture limit is set by the floor manufacturer, not by you.

Can I use a self-levelling compound over an old vinyl floor?

Only if the vinyl is fully bonded with no lifting, and after applying a primer specified by the smoothing compound manufacturer for non-porous substrates. Loose vinyl will lift the smoothing compound with it. Asbestos-backed vinyl must be tested and removed by a licensed contractor before any further work.

How do I check if a floor is asbestos-containing?

Take a 10x10mm sample, double-bag it in resealable plastic, send to a UKAS-accredited lab. Cost is £25-50, results 24-48 hours. Do not sample dusty or friable material yourself — engage a licensed asbestos surveyor.

What's the difference between BS 8204-1 and BS 8204-7?

BS 8204-1 covers traditional sand/cement screeds and concrete bases. BS 8204-7 covers pumpable self-smoothing screeds, primarily anhydrite (calcium sulphate) liquid screed and modified cementitious flowing screeds.

Do I need to prime over an existing power-trowelled concrete slab?

Yes. The power-floated finish is too smooth for a mechanical key, and may have curing compound on the surface that prevents adhesion. Grit-blast or grind the surface to remove the polish, prime with epoxy or acrylic primer per the smoothing compound manufacturer, then apply smoothing compound.

Regulations & Standards

• BS 8204-1:2003+A2:2011 — Code of practice for concrete bases and cementitious levelling screeds. Parent standard.

• BS 8204-3:2004+A2:2011 — Code of practice for polymer-modified cementitious levelling screeds and wearing screeds.

• BS 8204-7:2003+A1:2008 — Code of practice for pumpable self-smoothing screeds.

• BS 5325:2001 — Installation of textile floor coverings. References hygrometer moisture test.

• BS 8203:2017 — Installation of resilient floor coverings. Cross-reference for substrate requirements.

• BS 8201:2011 — Installation of flooring of wood and wood-based panels.

• BS EN 14041:2018 — Resilient, textile and laminate floor coverings. Essential characteristics.

• Building Regulations Approved Document E — Resistance to passage of sound.

• Building Regulations Approved Document L — Conservation of fuel and power; insulation under heated floors.

• CAR 2012 — Control of Asbestos Regulations; mandatory before lifting pre-2000 floor finishes.

• COSHH 2002 — Hazardous substances; cement burns, epoxy sensitisation, MDI in adhesives.

• CDM 2015 — Construction Design and Management; principal contractor responsibilities.

• BSI Standards Catalogue — BS 8204 series — Parent UK standards for screeds and floor preparation

• Contract Flooring Association — Industry guidance on substrate preparation

• HSE — Asbestos in Buildings — Required reading for any pre-2000 floor lifting

• HSE — Cement and Skin Disease — PPE requirements when handling cementitious materials

• Concrete Society Technical Reports — Slab and screed preparation guidance

• subfloor preparation — Overview of subfloor preparation

• floor levelling compounds — Selecting the right compound for depth and substrate

• floor levelling — BS 8204-7 levelling requirements

• levelling compounds — Compound types in detail

• screed types — Sand/cement, anhydrite screed characteristics

• underfloor heating screed — UFH screed specific preparation

• lvt installation — Resilient floor over prepared substrate