Self-Levelling Compound: Subfloor Preparation, Primer Selection and BS 8204 Flatness Tolerances

Quick Answer: Self-levelling (self-smoothing) compound is a pumpable or pourable cementitious or anhydrite screed applied to uneven subfloors before laying tiles, LVT, or other floor coverings. The subfloor must be primed to control porosity and adhesion. Minimum thickness is typically 3–6mm for self-levelling compounds; maximum depends on product. BS 8204-7 flatness tolerance is ±3mm under a 3m straightedge for most floor coverings.

Summary

Self-levelling compound (SLC) is the standard method for correcting uneven concrete and timber subfloors before floor coverings. It is not a structural repair material — it is designed to create a smooth, flat surface on an essentially sound substrate. Using SLC on a structural problem (a failing screed, weak or hollow concrete, unsecured timber boards) will result in cracking and failure.

The most common failure mode is inadequate priming. Without primer, SLC either dries too fast (draws moisture into the substrate), loses adhesion, or crazes (surface cracks due to differential drying). A second common failure is pouring on an unstable substrate — SLC applied to springy timber will crack with movement.

For most tradespeople (tilers, floor layers, kitchen/bathroom fitters), self-levelling is a preparation step that forms part of a larger job. Getting it right prevents call-backs and rework on the finished floor covering.

Key Facts

• BS 8204-7 — The UK standard for calcium sulphate (anhydrite) screeds, which includes guidance on SLC flatness tolerances. The general reference for floor flatness.
• Flatness tolerance — For most resilient and tile floor coverings: ±3mm under a 3m straightedge (SR2 level). For thin floor coverings (LVT <2mm): ±2mm under a 2m straightedge (SR1 level).
• Minimum depth — Typically 3mm minimum for most self-levelling compounds. Check product TDS (technical data sheet) — some products require 6mm minimum.
• Maximum depth — Single-pour maximum typically 10–25mm. Use in multiple pours with appropriate drying time, or switch to a floor-levelling mortar for depths >25mm.
• Primer is mandatory — Always use the manufacturer's designated primer. Never use a competitor's primer with another brand's SLC — adhesion is not guaranteed.
• Moisture content before flooring — After SLC, substrate must reach ≤75% relative humidity (RH) or ≤3% MC before resilient flooring. ≤65% RH before wood flooring. Test with hygrometer (insitu probe, not surface) to BS 8201.
• Timber subfloor requirements — All boards must be securely fixed (nail/screw every 150–200mm). Deflection must be <L/300. Movement in the boards will crack the SLC.
• Feather-edge products — Some products can feather to zero edge (for transitions). Most standard SLCs cannot — check TDS for minimum edge thickness.
• Mixing ratios — Always follow manufacturer's water:powder ratio exactly. Too much water weakens the compound; too little creates lumps and poor flow.
• Working time — Typically 15–25 minutes. Do not over-work after gelling begins.
• Foot traffic — Most compounds allow light foot traffic after 2–4 hours. Normal loads and floor covering application after 24 hours (check TDS).

Quick Reference Table

Detailed Guidance

Subfloor Assessment

Before applying any SLC, assess the subfloor condition:

Concrete/screed:

• Test with screwdriver: sound screed resists; hollow or crumbly screed must be cut out and repaired before SLC
• Check for contamination: oil, paint, adhesive residue — these prevent primer and SLC adhesion. Mechanically remove or degrease.
• Existing adhesive: pull-off test strength should be >1.5N/mm² (tape test or pull-off meter). Below this, remove the adhesive.
• Check moisture: if substrate RH is >80% or rising damp present, SLC will not bond. Resolve moisture source first.

Timber floors:

• Check for movement: walk the floor, feeling for springiness and creaking boards
• Punch down any raised nails; add screws at 150–200mm intervals to secure loose boards
• Check for rot or damage: probe with a screwdriver. Replace damaged sections.
• Add noggins under floor perimeter if boards are unsupported at edges.
• Floor conditioning: engineered and solid boards must acclimatise before SLC is applied over them.

Priming

Priming is not optional. Primers serve two functions:

1. Reduce porosity — Prevents SLC from drying too rapidly and losing strength
2. Improve adhesion — Chemical bond between primer and SLC

Primer types:

• SBR (styrene-butadiene rubber) diluted — General purpose. 1:3 to 1:5 with water. Applied by brush or roller, left to go tacky before pouring.
• Acrylic primer — Lower odour, often used in occupied buildings. Apply neat or diluted per manufacturer.
• Epoxy primer — For very smooth or low-porosity surfaces (polished concrete, existing resin). Used undiluted.

How to apply:

1. Ensure substrate is dry and free of dust.
2. Dilute primer per instructions — measure accurately.
3. Apply by brush, roller or squeegee; ensure full coverage.
4. Allow to become tacky before applying SLC — typically 30–60 minutes (don't leave overnight).

For high-porosity substrates (porous concrete, old sand/cement): apply two coats of primer, allowing first to dry before second.

Mixing and Pouring

1. Gauge water first — Add measured water to bucket before powder. Prevents lumps.
2. Mix — Slow paddle mixer at 400–600 rpm for 2–3 minutes until smooth (no lumps).
3. Rest — Allow 30-60 seconds, then mix again for 30 seconds to break up any residual lumps.
4. Consistency check — Product should flow to ~200mm diameter spread test.
5. Pour quickly — Start at furthest point from door; pour continuously. Do not pause.
6. Spread — Use gauging trowel or notched spreader bar to guide flow; compound should self-level to smooth surface.
7. Spike roller — Run over wet compound within 5 minutes to release trapped air. Use long-handle spiked roller included with most pump systems.
8. Do not over-work — After initial spreading, leave to self-level. Working it when it starts to gel creates ridges.
9. Protect — Seal room, prevent draughts and direct sunlight. Rapid drying causes crazing.

Depths and Build-Up

For depths greater than 25mm single pour:

• First pour: maximum depth per TDS; allow full cure time (24–48 hours).
• Prime second pour: re-prime over the cured compound before subsequent pours.
• Alternative for deep fills: use a levelling mortar (Damp Proof Membrane-backed screed-type product) for the bulk of the depth, then SLC as a finishing layer.

Special Substrate Situations

Over underfloor heating:

• Commission UFH at least 21 days before screeding
• Turn heating off 48 hours before applying SLC
• Restart at 5°C/day increments after 7 days minimum

Over existing vinyl/linoleum:

• Only if fully adhered and not contaminated with asbestos
• Scuff with coarse abrasive to key surface; wipe clean
• Prime with flexible acrylic primer
• Use flexible SLC with S1 classification (BS EN 12004 deformation class)

Frequently Asked Questions

Can I apply SLC in one pour over a large area?

Most standard SLCs have a single-pour maximum depth of 10–25mm. For areas requiring uniform depth throughout, the limiting factor is working time (~20 minutes) and the need to pour before the compound begins to gel. Pump application systems allow much faster coverage (one operative pumps while another guides the compound). For hand-mixing and pouring, keep pours to manageable areas.

How do I avoid cracks in SLC?

The main causes of cracks: insufficient primer, substrate movement (especially timber), low ambient temperature, direct sunlight or draughts during drying, or excessive water in the mix. Tick off all four: double-prime porous substrates, fix all board movement, maintain above 10°C, and seal the room.

Can self-levelling compound be used under tiles?

Yes — it is a standard substrate preparation for large-format tiles. After SLC has cured and reached correct moisture levels, prime with SBR before tiling. This creates a secure adhesion layer. For tiles ≥600mm, the SR2 flatness tolerance is especially important — check with a 3m straightedge before tiling.

Regulations & Standards

• BS 8204-7:2003+A1:2009 — Screeds, bases and in-situ floorings: calcium sulphate levelling screeds

• BS 8201:2011 — Code of practice for installation of flooring

• BS EN 13813:2002 — Screed material and floor screeds: properties and requirements

• COSHH Regulations 2002 — Silica in cementitious products: FFP2 mask if dust generated

• Ardex Technical Support — Levelling compounds

• Mapei Technical Data — Ultraplan product range

• Contract Flooring Association — Floor preparation guide

• subfloor preparation — Full subfloor assessment and preparation guide

• lvt installation — Flatness and moisture requirements for LVT

• large format tiles — SR2 flatness requirement for large tiles

• screed types — Types of screed and their properties