Going Over Existing Tiles With Microcement

Check one: is every tile genuinely bonded

Tap testing is unglamorous and non-negotiable. A tile that sounds hollow has a void beneath it, and a void under a three millimetre finish becomes a crack the first time somebody stands on it with a heel. On a large floor this survey takes an hour and saves an argument six months later.

Loose tiles come out, the bed is made good and the patch is levelled to match. If more than a modest proportion of the floor is drummy, the honest recommendation is to strip the lot, because a heavily patched substrate costs more to prepare than it does to remove.

Check two: what is the tile surface like

Highly polished porcelain gives a primer almost nothing to grip. It is not a stopper, but it does mean mechanical abrasion and a primer specified for non-absorbent surfaces rather than a general-purpose one. Textured or matt ceramic is far more cooperative.

Any residue matters too. Floors that have lived under a wax, a sealer or years of polish need degreasing and abrading before anything is applied. Primer bonds to the surface it can actually reach, not to the surface you assume is there.

• ✓Polished porcelain: abrade mechanically and use a bonding primer for non-absorbent substrates
• ✓Ceramic and quarry tile: usually straightforward after a thorough clean
• ✓Natural stone: check for sealers and previous treatments
• ✓Anything waxed or polished: degrease first, prime second

Check three: grout lines and profile

Grout joints do not disappear on their own. Deep or wide joints are filled and levelled during the base coat stage, and on a mosaic or a heavily profiled tile that filling is a real quantity of material and time. Where the joint grid is deep, reinforcing mesh across the whole area stops the pattern reappearing as a faint ghost later.

This is where quotes diverge. An applicator who has priced grout filling and full mesh will look more expensive than one who has priced a straight coating over the top. Six months on, only one of those floors will still look flat in raking light.

Ghosting of a grout grid through a finished floor is almost always a base coat and mesh shortfall, not a material fault.

Check four: height, thresholds and movement

An overlay adds the tile depth you kept plus a few millimetres of finish. In a bathroom that usually does not matter. Where a floor meets an external door, a lift threshold or an existing timber floor in the next room, the junction needs designing rather than discovering.

Movement joints in the tiled floor must be honoured through the new finish. Bridging an existing expansion joint with a continuous coating guarantees a crack in exactly the line you were trying to hide.

• ✓Confirm door undercuts and threshold levels before work starts
• ✓Carry existing movement joints through the finish
• ✓Check the junction with adjoining floor finishes in both directions
• ✓Allow for the finish depth at a shower drain flange

When stripping is the better commercial decision

If the tiles are widely hollow, if the floor is dished, if there is a suspected damp issue below, or if losing the tile height would let you resolve an awkward threshold, taking them up is usually the cheaper route once preparation time is counted. Removal is a day of noise and a skip. Preparing a bad substrate is several days of skilled labour.

The point of the overlay option is speed and disruption, particularly on occupied or trading premises. When those advantages disappear because the substrate is poor, so does the reason for keeping the tiles.