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Surface Preparation

Concrete & Screeds 

Concrete and cement screeds are strong, durable substrates and are widely used for floors and external areas. However, they are also responsible for more tile failures than almost any other surface when preparation is rushed or incomplete. The most common problems are excess moisture, poor curing, surface laitance, contamination, and overly dense finishes. This guide gives clear, correct preparation steps so tiling over concrete or screeds is done properly the first time.

Minimum requirements before you start

Concrete and screeds must be cured, dry, sound, and flat before any priming or tiling.

Curing

  • New concrete and sand cement screeds must cure for at least 28 days unless a rapid system is specifically designed for earlier tiling. Tiling too early traps moisture and leads to debonding and efflorescence.

Dryness

  • Only tile once moisture levels are below 5 percent. Damp concrete is one of the most common causes of hollow tiles in South African conditions.

Stability

  • The surface must be solid and well bonded. Any loose, hollow, or crumbling areas must be repaired or replaced before tiling.

Quick checks that tell you what prep you need

Moisture check

  • Moisture can be checked using a moisture meter or a simple plastic sheet test. Tape a plastic sheet to the surface and leave it for 24 hours. If condensation forms underneath, the concrete is still too wet to tile.
  • Do not tile if the slab is younger than 28 days, has cold or damp patches, or shows moisture readings above 5 percent.

Surface strength check

  • Scratch the surface firmly with a key or screwdriver. A hard, scratch-resistant surface is suitable. If the surface powders or chalks easily, it is weak and must be treated or removed.

Soundness check

  • Tap the surface lightly. A solid sound indicates good bonding. A hollow or drummy sound means the screed is loose and must be removed and repaired.

Laitance check

  • Most screeds develop a soft, dusty top layer known as laitance. If present, it must be removed before priming and tiling.

Preparation steps

Remove laitance and weak layers

  • Laitance and weak surface layers must be removed. This is typically done by light grinding with a diamond cup wheel or aggressive brushing, followed by thorough vacuuming. Weak screeds that continue to powder must be stabilised or replaced.

Remove contaminants

  • Concrete and screeds are often contaminated with plaster splashes, paint, oils, curing compounds, bitumen, or general construction dirt. Tile adhesive will not bond to contamination. These materials must be removed by grinding, scraping, or washing until the surface is clean and sound.

Repair cracks and defects

  • Hairline cracks should be cleaned and filled with a suitable cementitious repair mortar. Wider or structural cracks should be opened slightly, filled deeply with a high-strength mortar, and allowed to cure. Do not tile over cracks that are still moving.
  • Any hollow or loose screed sections must be removed and re-screeded before tiling.

Priming the surface

The correct primer depends on how dense or absorbent the surface is.

Normal concrete and standard screeds

  • Prime with Kwik-Key diluted 1:1 with water. One uniform coat is usually sufficient. This controls suction and improves adhesive bonding.

Highly porous screeds

  • Apply two coats of neat Kwik-Key, allowing the first coat to dry before applying the second. This prevents flash drying and adhesive starvation.

Dense, smooth, or power-floated concrete

  • Very smooth or power-floated concrete always requires slurry priming. These surfaces are too dense for tile adhesive to bond reliably without it.

For the exact slurry method, refer to the article: 

Kwik-Key Slurry Guide

Painted or sealed concrete

  • Painted, sealed, or coated concrete must be mechanically abraded until most of the coating is removed. Clean thoroughly, then slurry prime. Never tile over glossy or sealed concrete without proper preparation.

Levelling requirements

  • Tile adhesive is not a levelling compound. Floors must be flat before tiling.
  • Concrete and screeds should be flat within 3 mm per metre and free from dips, humps, or ridges. If the floor is uneven, correct it using a suitable levelling compound and allow it to cure fully before tiling.

Coverage rules (bond quality)

  • Concrete and screeds require good adhesive contact to prevent hollow spots. Aim for full ridge collapse and check coverage regularly by lifting tiles during installation.

Movement and joints

  • Movement joints are essential on concrete floors.
  • Always allow joints at room perimeters, around columns, structural breaks, and where new concrete meets old concrete. As a general guide, joints are required every 5 metres indoors and every 3 metres outdoors. Fill joints with a flexible sealant such as silicone or polyurethane, never grout.

Never tile over these concrete or screed conditions

  • Stop and fix first if you see any of the following: damp or curing concrete, screeds that powder when scratched, contaminated surfaces, power-floated concrete that has not been slurry primed, hollow or loose screed sections, unrepaired cracks, or surfaces that have not been primed correctly.