Minimum requirements before you start
All existing tiles must be solid, dry, clean, and free from movement before tiling can begin.
Stability
- Every tile must be firmly bonded. Tile adhesive cannot compensate for movement underneath.
Dryness
- There must be no moisture trapped behind the tiles. Damp substrates lead to debonding and long-term failure.
Surface condition
- Tiles must be free from grease, polish, wax, silicone residue, and surface contamination.
Quick checks that tell you what prep you need
Soundness check
- Tap every tile using a coin, screwdriver handle, or light mallet.
- A sharp, solid sound indicates good adhesion.
- A hollow or drummy sound means the tile is loose and must be removed.
You cannot tile over loose or hollow tiles. The entire system will fail if movement remains underneath. If you are unsure what the symptoms indicate, use our guide to diagnose tile failure quickly before proceeding with repairs.
Visual inspection
- Check for cracked or bulging tiles, missing or loose grout, signs of moisture damage, and silicone around edges or joints. Silicone must be removed completely as adhesive and slurry will not bond to it.
Preparation steps
Clean thoroughly
- Old tiles must be completely clean before priming. Wash the surface using a strong degreaser, scrub grout joints, rinse with clean water, and remove all soap residue. Allow the surface to dry fully.
- Any wax, grease, polish, oil, silicone residue, or dirt left behind will prevent proper bonding.
Abrade the surface
- Abrasion is strongly recommended to improve mechanical keying, especially on glossy, glazed, vitrified, or very smooth porcelain tiles. This can be done using abrasive pads, coarse sandpaper, or a diamond grinding cup wheel.
While slurry can bond chemically, abrasion always improves reliability and reduces risk.
Remove contaminants
- Plaster often gets contaminated on site (paint overspray, bonding liquids, wax or polish, hand oils, construction dirt). If anything is sitting on the surface, the adhesive cannot bond to the plaster, it bonds to the contaminant. Sand or abrade and clean until the surface is sound.
Priming existing tiles
Tile-on-tile installations always require slurry priming. Existing tiles are dense and non-absorbent, so tile adhesive alone cannot bond reliably.
- Prepare the surface using a Kwik-Key slurry to create a new cementitious bonding layer over the old tiles.
For the correct slurry mix and application method, refer to the article
Coverage rules (bond quality)
Good coverage is essential on tile-on-tile systems because the substrate is very rigid.
- Aim for at least 90 percent adhesive coverage in dry areas.
- Aim for about 95 percent coverage in bathrooms, showers, and wet areas.
- Use an appropriate trowel size and back-butter tiles larger than 600 mm. Lift tiles regularly to confirm full contact and collapsed ridges.
Movement and joints
Tile-on-tile systems are stiffer than standard installations, so movement joints are critical.
- Maintain all perimeter joints, structural joints, doorway transitions, and existing expansion joints. Fill these joints with a flexible sealant such as neutral-cure silicone or polyurethane. Do not fill movement joints with grout, as it will crack.
Never tile over these drywall conditions
- Stop and fix first if you find any of the following: hollow or loose tiles, moisture behind existing tiles, damp walls or floors, tiles contaminated with wax, oil, polish, or silicone, slurry not applied, tiles painted with enamel or epoxy coatings, structural cracks running through the original tiles, or surfaces that cannot be cleaned properly.