Summary

Levelling and smoothing compounds are self-levelling or pump-applied cementitious products used to achieve a flat, smooth subfloor prior to installing resilient flooring (LVT, vinyl sheet, carpet tile, or resin coatings). They are not structural screeds and should not be confused with the primary screed layer in a floor build-up.

The most common site mistake is using the wrong product for the situation: applying a feather-edge compound over a surface that needs 15–20mm correction, or using a deep-fill compound for a thin skim when a smooth finish is needed. The result is either inadequate depth coverage (too little product, surface crumbles) or unnecessarily expensive material wastage and long drying times.

Key Facts

  • BS 8204-3:2004 — screeds, bases and in situ floorings: code of practice for polymer-modified cementitious screeds; relevant standard for levelling compounds
  • BS 8203:2017 — code of practice for installation of resilient floor coverings; references flatness tolerance (3mm in 1.8m) and subfloor requirements
  • PSA (Pressure Sensitive Adhesive) suitability — levelling compounds that pass the PSA suitability test are suitable for pressure-sensitive adhesive LVT; compounds without this suitability can cause adhesive failure
  • Feather-edge (smoothing compound) — applied at 0–5mm; flows freely and self-levels; achieves smooth surface
  • Deep-fill compound — applied at 5–50mm; coarser mix; multiple pours required above 30mm; does not self-level as effectively; surface may need a thin feather-edge compound over it
  • Pumpable levelling compound — used for large areas; specialist pump equipment required; bulk supply in silo or large bags
  • Calcium sulphate (anhydrite) screed — common in modern wet UFH installations; must be primed with a specific anhydrite primer before any levelling compound or adhesive
  • Drying time — approximately 1 hour to foot traffic for most feather-edge compounds; 24–48 hours before flooring installation; deep-fill requires longer — check datasheet
  • Minimum application temperature — 5°C substrate and air; do not apply in freezing conditions or when frost expected within 24 hours
  • Coverage rates — feather-edge: approximately 1.6–2.0 kg/m² per mm depth; deep-fill: approximately 1.6 kg/m² per mm; packaging states coverage — calculate from depth required
  • Mixing — add compound to water (not water to compound); use a slow-speed drill with bucket paddle; mix for 2–3 minutes to smooth creamy consistency; no lumps
  • Fibreglass mesh — used over movement joints or expansion cracks; embed into first coat of compound, allow to set, then apply levelling compound over; prevents cracking through

Quick Reference Table

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Compound Type Depth Range Drying (Walk) Drying (Floor) Best For
Feather-edge/smoothing 0–5mm 30–60 min 24h LVT, vinyl, carpet tile over minor undulations
Standard levelling 3–15mm 1h 24–48h Filling hollows, levelling raised tiles
Deep-fill 5–50mm 3–6h 48–72h Major depressions, damaged screeds
Pourable cementitious 5–25mm 1–2h 24h Large areas; pump application
Anhydrite-specific 3–25mm 1h 24h Over anhydrite/calcium sulphate screed

Detailed Guidance

Identifying the Subfloor Type

Before selecting a levelling compound:

  1. Cement/sand screed or concrete — use standard cement-based levelling compound; primer usually required (diluted PVA or specific manufacturer primer)
  2. Anhydrite (calcium sulphate) screed — identifiable by its very hard, smooth, slightly yellow surface; do not use standard cementitious compounds without an anhydrite-specific primer; adhesion will fail. Test: a drop of HCl (dilute) on the surface produces vigorous bubbling on anhydrite but little reaction on cement
  3. Existing ceramic/porcelain tile — check tiles are firmly bonded (tap test); apply levelling compound directly over clean, sound tiles; do not prime glazed surfaces with standard PVA — use a specialist tile primer or etch with dilute HCl
  4. Plywood — prime with a timber-specific primer; fill joints and screw heads with epoxy filler before levelling compound
  5. Existing adhesive — mechanical removal preferred; residual thin adhesive sometimes acceptable — check with compound manufacturer

Preparing the Surface Before Application

  1. Remove all loose material, debris, oil, and dust (vacuum thoroughly)
  2. Fill any open cracks wider than 1mm with epoxy crack filler first; allow to cure
  3. Apply primer as directed — brush, roller, or diluted flood application; allow full drying (check datasheet — typically 30 min for PVA-type, longer for specialist primers)
  4. If anhydrite screed: apply anhydrite primer and allow full cure; this seals the sulphate surface and prevents chemical incompatibility

Application Procedure

  1. Mix the compound by adding powder to clean water (ratio specified on bag — typically 5–6L water per 25kg bag); mix until smooth (no lumps)
  2. Pour in strips or areas of approximately 5–8m² — work quickly (pot life 15–25 minutes for most products)
  3. Use a spreader or gauging rod to level the compound; most feather-edge compounds will self-level but coarser deep-fill products need manual spreading
  4. Work from the furthest point back towards the door
  5. Do not walk on the compound during application; use wide boards or stilts for large areas
  6. Allow compound to reach full hardness before installation — typically 24 hours for most products; longer for deep fills or in cold conditions

Common Failures and How to Avoid Them

Crumbling or dusting surface: Incorrect water ratio (too much water weakens the mix), premature use before curing, or applying over unprimed absorbent substrate (too much water drawn out during curing). Solution: prime absorbent substrates; follow mix ratios precisely.

Delamination from substrate: Insufficient or wrong primer; applying over contaminated surface. Solution: mechanical preparation and correct primer for substrate type.

Shrinkage cracks: Deep application in single pour or rapid drying in hot conditions. Solution: apply in layers for depths >25mm; keep out of direct sunlight; maintain minimum 5°C during curing.

Adhesive failure when laying LVT: Compound not PSA-rated; or residual primer contamination on surface of compound. Solution: confirm PSA suitability before specifying; ensure surface is clean and fully cured.

Frequently Asked Questions

How long do I need to wait before laying LVT?

For most feather-edge compounds at 3–5mm depth: 24 hours minimum at 20°C with normal ventilation. For deep-fill at 30mm: 48–72 hours minimum. These times assume 20°C ambient; cold conditions (10°C) can triple drying times. Check the specific product datasheet — some rapid-set products allow flooring installation within 4–6 hours.

Can I use levelling compound on top of underfloor heating?

Yes, but do not apply compound over a live UFH circuit. UFH must be switched off and cooled before any levelling compound is applied, and must remain off for the full curing period (minimum 24 hours). Some products require the UFH to be commissioned and fully cycled before the compound is applied — check the screed/compound manufacturer's instruction. For anhydrite screeds over UFH, the screed commissioning cycle must be complete before any levelling compound.

My levelling compound application has dried uneven — how do I flatten it?

If the unevenness is within 2–3mm, a second very thin skim of feather-edge compound (correctly primed) can be applied over the first. For ridges, lightly grind or sand before the second coat. If a large area has failed (delamination, hollow sound when tapped), remove the failed compound completely back to the substrate, identify the cause, and start again.

Regulations & Standards