The Night Tripper wrote:
Hi All
We had our bathroom tiled a few years ago. Recently I have
noticed that some of the bottom row of wall tiles - the ones butting
up to the bath - have rung 'hollow' when tapped. Even more recently I
noticed that the grouting between these was failing. So since I have
a few days free (ha!), I have had a closer look. Ended up by pulling
of two or three off this bottom row. They came off pretty easily ;-(.

Underneath there seems to be a thick layer of pink-ish plaster (I
don't know my plaster types very well, sorry) slapped onto the
(solid) wall behind. This plaster is very friable, I easily scratched
deep hatch marks into it with a 50p piece. Images he

http://www.nicorp.co.uk/download/DSCF1856-1.JPG
http://www.nicorp.co.uk/download/DSCF1857-1.JPG

Now I don't know if the plaster is friable because water has got at
it, or it was friable for some reason anyway and that has allowed the
tiles to loosen. It looks like the pink plaster was put on in the
first place to level the wall up - though even with that, the tile
adhesive was only touching it in a few places on each tile, as I
think you can see from the pictures.

I have some spare replacement tiles, and obviously I'm now keen to
make this all good. I'm presuming it would be best to chisel off
this rubbish plaster and replace it (with what?), put on to the right
depth so that the replacement tiles will adhere across their whole
surface.

Any other ideas or suggestions?

The pink plaster is skimming and it looks water damaged from those pics.
Either the grout has failed and allowed water ingress or it's an external
downstairs wall?
Possibly penetrating damp if the latter.
Don't try and remove or plaster this patch as you will not be able to get
the tiles right afterwards, simply find the cause of water damage (probably
higher up) and then re-tile and grout