How do you choose the right floor levelling compound?

Quick Answer: Floor levelling compounds in the UK must comply with BS EN 13813 (binder classification) and be applied to BS 8204-3 (polymer-modified) or BS 8204-7 (pumpable self-smoothing) standards. Select by depth (1-5mm trowel-apply, 3-25mm pump-apply, 25-50mm deep-pour), by substrate (cementitious, anhydrite, timber, or non-absorbent), and by service (foot traffic, wheeled load, wet area, or external).

Summary

Floor levelling compounds are the bridge between a defective substrate and a sound bonded floor finish. They correct flatness, fill defects, and provide a uniform porosity for adhesive bonding. The wrong compound will fail in service — pulling the floor finish with it — and the right compound applied wrongly will fail just as quickly.

The product range available in the UK has expanded significantly. F. Ball, Mapei, ARDEX, Bostik, and Uzin between them offer 30+ levelling compounds covering every realistic substrate and service condition. Selection is driven by depth, substrate, end-use, and programme.

This guide walks through the selection decision systematically: what's the substrate, what depth needs filling, what's the floor finish, how soon does it need to be trafficable. Each answer narrows the product choice. The cost difference between the cheapest and most appropriate compound is rarely more than £2-3/m² — but using the wrong one risks the whole floor.

Key Facts

• BS EN 13813 — Classification standard for screed materials including levelling compounds
• BS 8204-3 — Code of practice for polymer-modified cementitious levelling screeds
• BS 8204-7 — Code of practice for pumpable self-smoothing screeds
• Trowel-apply (feather-finish) — 1-5mm depth, applied by hand, fast-set
• Pump-apply (self-smoothing) — 3-25mm depth, mixer-pump or hand-mixed
• Deep-pour (bulk-fill) — 25-50mm depth, often with aggregate addition
• Polymer modification — SBR or acrylic polymer added for flexibility and bond strength
• Working time — 15-40 minutes depending on product and ambient temperature
• Walk-on time — 2-6 hours typical
• Time to flooring — 12-48 hours for most products; longer for thick sections
• Substrate moisture — 75% RH maximum for cementitious; 0.5% CM for anhydrite
• Primer required — almost always; specific primer per substrate type
• Coverage — depends on depth; 1.5-1.8 kg/m² per mm of depth typical
• Mixing water — strictly per manufacturer instructions; too much water reduces strength
• Aggregate addition — clean 4-10mm stone for depths over 25mm; check product allows it
• Internal vs external — most compounds are internal only; specific external grades exist
• CE/UKCA marking — required under BS EN 13813
• Wheeled load tolerance — varies by product strength class (C20, C25, C30)
• Wet area tolerance — only compounds rated for permanent immersion in wet rooms

Quick Reference Table

Detailed Guidance

Categories of compound

Feather-finish compounds (trowel-apply) — Fine-particle cementitious or polymer-modified cement, designed for 1-5mm application by steel float or trowel. Sets fast (15-30 minutes), walk-on in 1-2 hours. Used for final regulation, filling shallow defects, and creating a smooth surface for thin resilient flooring. Not for deep fill or level correction over significant areas.

Standard self-levelling compounds — The workhorse of the category. Polymer-modified cementitious, mixed with water to a flowable consistency, poured and spread with a smoothing rake then back-rolled with a spiked roller. Self-levels under gravity. Suitable for 3-10mm depth in a single pour. The default choice for most flooring jobs.

High-flow / high-strength compounds — Premium pump-applied compounds with extended depth range (3-25mm) and higher compressive strength (C25-C30 to BS EN 13813). Used where the levelling compound takes some structural load — heavy traffic, wheeled loads, or where the finish floor is fragile.

Polymer-modified SBR compounds — Higher polymer content for flexibility. Tolerate small substrate movement, suitable for timber substrates (with ply overlay), and movement-prone substrates. More expensive but essential where flexibility matters.

Anhydrite-compatible compounds — Standard cementitious smoothing compounds can react with calcium sulphate (anhydrite) screed at the interface, causing ettringite formation and bond failure. Compounds rated as anhydrite-compatible are formulated to bond reliably over calcium sulphate without reaction.

Deep-pour / bulk-fill compounds — Modified compounds tolerating 25-50mm depth in a single pour, often with aggregate addition (4-10mm clean stone). Used for substantial level corrections — for example, levelling a ramp transition or filling a step-down.

Rapid-set / early-traffic compounds — Fast-cure variants allowing walk-on in 1 hour and flooring within 4-12 hours. Premium-priced. Used on fast-track refurbishment where programme matters more than cost.

Wet area / external compounds — Compounds rated for permanent or frequent water exposure — wetroom floors, external balconies, plant rooms. Higher cement content, reduced shrinkage, water-resistant after cure.

Fibre-reinforced compounds — Standard compounds with polypropylene or alkali-resistant glass fibre dosing. Resist micro-cracking, particularly useful over timber overlays or substrates with crack potential.

Substrate-driven selection

The substrate often dictates compound choice more than the depth:

Sand/cement screed (BS 8204-1) — Most general-purpose compounds work. Verify moisture <75% RH, prime with diluted acrylic primer, apply compound to required depth.

Anhydrite (calcium sulphate) screed (BS 8204-7) — Surface laitance must be removed first (sand with 60-80 grit). Prime with anhydrite-specific primer. Use anhydrite-compatible smoothing compound only. Standard compounds will react with the sulphate and lose bond within months.

Concrete slab (new) — Verify moisture <75% RH (often takes months for new concrete to reach this). Grit-blast or shot-blast to remove curing compound and create a mechanical key. Prime with appropriate primer. Apply compound.

Concrete slab (existing) — Lift any existing covering, remove all adhesive residue with mechanical scraper or shot-blaster. Repair cracks. Prime, apply compound.

Timber subfloors (chipboard, plywood, floorboards) — Overlay with 6mm WBP plywood first (screwed at 150mm centres). Polymer-modified or fibre-reinforced compound mandatory. Standard compounds will crack and lose bond on timber movement.

Existing tile (sound) — Clean thoroughly, prime with non-porous substrate primer or epoxy primer. Apply compound at minimum 3mm depth (tile joints telegraph through thinner application). Wider tile joints (>4mm) need filling before levelling compound.

Existing vinyl (sound, bonded) — Test asbestos status if pre-2000. Clean, prime with non-porous primer. Apply compound. Loose or partially bonded vinyl must be removed first.

Existing resin floor (epoxy, polyurethane) — Mechanical etch (grind or grit-blast) to create a key. Prime with non-porous primer. Apply compound.

Mixing and application

Manufacturer mixing ratios are not suggestions. Too much water:

• Reduces compressive strength by up to 40%
• Causes surface dust and weakness
• Slows set and risks shrinkage cracking
• Bleeds water to surface, weakening the bond layer

Too little water:

• Reduces self-levelling action
• Leaves trowel marks
• Risks cold joints between bucket loads
• Can be unworkable

Use a high-speed mixer (Collomix or similar) at 600-800 rpm for 2-3 minutes per bucket. Mix until lump-free and smooth. Let rest 30-60 seconds, mix briefly again, then pour immediately.

For pump-applied compounds, the pump operator controls water content via the pump's water meter. Maintain consistent flow rate to give consistent water-to-powder ratio. Check water content at the outlet by a flow test (typically a Hägermann flow ring giving 200-240mm spread).

Priming — the most-skipped step

Every smoothing compound manufacturer specifies a primer for every substrate. Skipping the primer is the single most common cause of bond failure.

The primer:

• Seals porosity so the compound doesn't drag drying too fast
• Creates a chemical bond between substrate and compound
• Prevents pinholes from air rising through the compound during set
• Stabilises any dusty or chalky substrate surface

Common primer types:

• Diluted acrylic primer — 1:1 to 1:3 with water; for cementitious substrates
• Anhydrite-specific primer — water-resistant acrylic; for calcium sulphate screed
• Epoxy primer (single-coat) — for non-porous substrates (tile, existing vinyl)
• Two-coat epoxy DPM + sand broadcast — surface DPM where moisture exceeds 75% RH
• Multi-substrate primer — proprietary primer covering several substrate types

The primer must be fully dry (touch-dry, not just damp) before smoothing compound is applied. Typical drying time 30-60 minutes for acrylic, 2-12 hours for epoxy depending on temperature and humidity.

Depth and pour planning

Single-pour depth limits are absolute. Exceeding them causes shrinkage cracks, central depression, or surface defects.

For deep fills:

• Pour to maximum single-pour depth (e.g. 15mm for standard compound)
• Allow to cure 24 hours
• Re-prime and pour again to required depth
• Or use a deep-pour compound rated for the full depth in single application

Watch for "ramp" applications — where compound depth varies from 0 at one edge to 20mm at the other. Use a compound rated for the maximum depth and accept higher material cost rather than using two products on one pour.

For pump-applied compounds, plan the pour sequence:

• Start at the far end of the area
• Work backwards toward the exit
• Maintain wet edge — each new pour into the wet edge of the previous
• Allow continuous flow; do not pause and restart in the same area
• Back-roll with spiked roller within 5 minutes of pour to release air

Quality checks

After compound has set:

• Pinhole test — small holes from trapped air; acceptable in light traffic, fill with feather-finish if smooth resilient flooring is going down
• Trowel mark / lap mark — indicates wet-edge breakdown or wrong consistency; sand and re-skim
• Crack — indicates over-watered mix, fast cure, or substrate movement; investigate and re-pour affected area
• Powder / dust — indicates excessive water in mix or premature traffic; sand and prime before flooring
• Hollow area — tap-test the cured compound; voids indicate substrate not primed, no bond achieved; remove and re-do

Final surface should be smooth, uniform, and free of trowel marks. Smoothing compound is the substrate the customer's floor will bond to — get it right or get it out.

Frequently Asked Questions

Can I use one compound across my whole job regardless of substrate?

No. Anhydrite screed requires anhydrite-compatible compound. Timber substrates require polymer-modified compound. Non-porous substrates (existing tile, vinyl) require non-porous primer. Wet areas require water-resistant compound. Buying one product for the whole job risks bond failure on any non-matching substrate.

How do I know if my compound has set enough to walk on?

Press a clean finger firmly into the surface. If no mark is left, it's walk-on ready. If a mark is visible, wait longer. Walk-on time is typically 1-4 hours but is temperature-dependent — cold conditions extend, warm conditions accelerate.

Why does my smoothing compound crack as it dries?

Three common causes: too much water in the mix (reduce by 5-10%); applied too thick in a single pour (split into two pours with full cure between); substrate moving (timber substrate without ply overlay, or substrate not primed). Diagnose before re-pouring.

What's the difference between "self-smoothing" and "self-levelling"?

In manufacturer literature they are usually used interchangeably. Both describe a compound that flows under gravity to a flat surface with minimal trowelling. Self-smoothing emphasises the finished surface quality, self-levelling emphasises the substrate flatness correction. The product chemistry is the same.

Can I add aggregate to make a smoothing compound go deeper?

Only if the manufacturer permits it. Standard smoothing compounds are not designed for aggregate addition — adding stone disrupts the polymer matrix and weakens the cured material. Deep-pour compounds are formulated to accept clean 4-10mm aggregate at specified ratios (typically 1:1 by volume) for fills over 25mm depth.

Regulations & Standards

• BS EN 13813:2002 — Screed material and floor screeds. Material requirements. Classification basis for smoothing compounds.

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

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

• BS 8204-1:2003+A2:2011 — Concrete bases and cementitious levelling screeds for receiving flooring.

• BS 5325:2001 — Hygrometer method for moisture testing.

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

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

• Construction Products Regulations 2013 — UKCA/CE marking under BS EN 13813.

• COSHH 2002 — Cement burns risk, polymer dust, ventilation requirements.

• CDM 2015 — Construction Design and Management; PPE and risk assessment requirements.

• BSI Standards Catalogue — BS EN 13813 — Classification standard

• Contract Flooring Association — Best-practice guidance on compound selection

• F. Ball Technical Service — Product datasheets and substrate guides

• HSE — Construction Skin Risk — Cement burns and PPE requirements

• Federation of Resin Flooring Specifiers — Guidance on resin and modified cementitious systems

• floor levelling — BS 8204-7 levelling principles and SBR priming

• levelling compounds — Compound types overview

• subfloor preparation guide — Complete subfloor preparation workflow

• screed types — Underlying screed types and behaviour

• lvt installation — Resilient flooring over prepared substrate

• underfloor heating screed — Compound over UFH screed considerations