Preparing surfaces for ceramic wall tiles.

To satisfactorily fix ceramic tiles, walls and other surfaces need to be 'clean', sound, flat and dry.

Checking should be carried out in the following order:

Prepare any existing surface finish,
Check that the surface is sound,
Check that the surface is flat.

Existing wall finish for tiles

The degree of work necessary will depend upon the existing finish:

Brickwork: render the wall to give a smooth finish and allow to thoroughly dry out.
Wall paper: remove all wall paper back to a solid surface. Wash down to remove any surface grease and dust etc.
Paint:
- Gloss and semi-gloss - remove any flaking paint and rub down using wetted medium grade wet-and-dry abrasive paper to take off the gloss and to provide a key for the tile adhesive.
- Emulsion: remove any flaking paint and wash down to remove any surface grease and dust etc.
- Distemper/whitewash: It is best to remove all distemper and whitewash from plaster (they are water soluble and removal is straight forward though messy). Alternatively, after removing any flaking areas, the surface can be treated with a stabilising primer before fixing tiles.
Existing ceramic tiles: New tiles can be fixed on top of existing tiles providing that they are firmly attached to the wall, tap each existing tile and if it sounds hollow, it needs to be chipped out using a hammer and cold chisel (always wear safety goggles and gloves) - level up the surface of the original tiles using a plaster filler.
Problems can arise where the new finish line of tiling will be different from the old, in this case, removing the old tiles is probably the most practical course of action.
To remove old tiles, use a hammer and bolster chisel to lift off the old tiles - wear safety goggles and gloves. A consequence of removing old ceramic tiles is that a lot of the wall will tend to come away with the old tile so having removed the old tiles, the wall will need to be replastered or re-lined to give a suitable surface for the new tiles.
Timber: If painted/varnished treat as above for Paint.
If bare timber, rub down to give a smooth finish for the tiles and apply a coat of wood paint primer.
Plasterboard: Tiles are heavy so should only be mounted on plasterboard at least 12.5mm (half inch) thick that is securely mounted. Total weight of the tiling should be no more than 20kg/sq m - this is roughly equivalent to ceramic tiles 8mm thick.
Special waterproof plasterboard is available for tiling in 'wet areas' (around a shower, alternatively, if using standard plasterboard, plaster skim it to seal it before tiling.
In 'non-wet areas', seal the paper covering of the plasterboard with a coat of emulsion paint.

Where a wall has been replastered, it should be left to dry out for about a month.

All bare plaster work (including repaired areas) should be sealed using a plaster primer before fixing tiles.

Check that the surface is sound

Any loose or flaking plaster should be removed and filled with a plaster filler. Cracks should similarly be filled in.

A problem particularly associated with older properties is very porous and/or dusty plaster walls. The alternatives here depend on how bad the wall condition is:

If the wall is in general good condition, a stabilising primer can be applied to seal the wall.
If the walls is in really bad condition, the walls can be relined or all the plaster removed and then the walls replastered (this may seem a bit 'over the top', but if a whole/room property is being renovated, it is well worth considering and, when using a professional plasterer, is usually cost and time effective).

Check that the surface is flat

Use a length of straight timber to check the surface to be tiled for dips and bumps - the flatter the area, the better. Hold the timber against the wall horizontally, vertically and diagonally to gauge the wall in all directions. Small dips can be filled in using a suitable filler, bumps can be rubbed down. If the surface is really bumpy, the whole wall can be skimmed, replastered or relined to give a suitable surface.