The bond test that decides everything
Before anything else, every existing tile gets tapped and listened to. A solid, sharp sound means the tile is well bonded. A dull, hollow sound means it has already debonded from the substrate, even if it looks fine, and tiling over it just adds weight on top of a failure waiting to happen.
We expect to find at least a few hollow tiles on any older installation, but if more than around 10-15% of the surface sounds hollow, tiling over the top is not a sound decision regardless of how convenient it would be.
We also check for cracked tiles, movement at grout lines and any sign of dampness behind the tiling, none of which improve by being covered over.
Surface preparation: roughening, cleaning, priming
A glazed tile surface is smooth and non-porous, which is a poor bonding surface for new adhesive on its own. It needs mechanically abrading, usually with a diamond or carbide abrasive pad, to key the glaze before anything else goes on.
The surface then needs degreasing thoroughly, since years of shower gel, limescale remover and cleaning product residue sit in the glaze and will sabotage adhesive bond if left in place.
A specialist bonding primer or bridging primer, formulated for tile-on-tile applications, is then applied and left to cure fully before new adhesive goes on. Skipping the primer step is the most common reason a tile-on-tile job fails early.
Thickness build-up and where it causes problems
Every layer adds height: the old tile, the primer, the new adhesive bed and the new tile itself typically add somewhere between 10mm and 20mm to the finished surface.
On floors that build-up matters against door clearances, the transition height into other rooms, and the level a shower tray or bath panel needs to sit at. We measure this at survey rather than assuming it will be fine.
On walls the main issue is around fittings: a shower valve, mixer tap or niche recessed to suit the old tile depth may end up sitting slightly proud or requiring an extension kit once the new surface build-up is added.
Cost comparison: over-tiling vs strip-out
Tiling over an existing surface saves the labour and skip cost of removal, but it is not a like-for-like saving once primer, extra adhesive depth and any trim adjustments are factored in.
| Approach | Typical cost per m2 | Time saved | Risk level |
|---|---|---|---|
| Full strip-out and retile | £45-£90/m2 | None, baseline | Low, if substrate checked properly |
| Tile over sound existing tile | £35-£70/m2 | 1-2 days on a bathroom | Low to moderate |
| Tile over marginal existing tile | £35-£70/m2 | 1-2 days | High, not recommended |
When we say no
We do not tile over existing tile where there is any sign of damp behind the surface, since covering it does nothing to solve the underlying moisture and simply delays discovery.
We do not tile over a significant number of hollow-sounding tiles, over cracked tiles that suggest substrate movement, or over a wall that is being altered to move a shower or basin, since new pipework needs access to the wall behind anyway.
Where a strip-out is the right call, we say so plainly rather than taking a quicker job that risks our own workmanship guarantee on a substrate we know is compromised.
Where to read next
Questions we get asked most
How do you know if my existing tiles are safe to tile over?
We tap every tile and listen for a hollow sound, which indicates debonding, and check for cracks, movement and damp. If more than roughly 10-15% sounds hollow, we recommend a strip-out instead.
Does tiling over old tiles save a lot of money?
It saves the labour and skip cost of removal, but that saving is partly offset by primer, extra adhesive and possible trim adjustments, so the overall saving is usually modest rather than dramatic.
Will the new tiles stick to a glossy glazed surface?
Only if the glaze is mechanically abraded and a proper bonding primer is used first. Applying standard adhesive directly to an untouched glaze is a common cause of early failure.
Will tiling over old tiles make my bathroom floor too high?
It typically adds 10-20mm, which can affect door clearance and the height of a shower tray or bath. We measure this at survey before recommending the approach.
What if you find damp or hollow tiles once you start?
We flag it before quoting rather than after starting, because a full strip-out is the only sensible fix once damp or significant debonding is present.