friable plaster under bathroom tiles - advice on making good sought

[The Night Tripper]

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 here:

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?

Thanks a lot Jon N

[Phil L]

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

[The Night Tripper]

It's an upstairs internal wall, so signs of any damp problems with it.

The tiles were only sticking in a few places, as I mentioned - ie. there was a bigger gap than desired between the back of the positioned tile, and the skimming. So perhaps the initial poor mounting allowed the graout to fail, then allowing water behind?

So from what you say, although the skimming coat is water damaged, it would be OK for this to stay (perhaps, put a further thin layer on top to allow the tile tiles to properly stick to it) and then retile?

I'm curious why you say that it would be a lot harder to position the tiles if I were to remove and replace the skimming. I don't think just retiling and grouting will work without doing something to ensure wider adhesion.

Thanks again Jon n

[Dave Liquorice]

Looks more like a base coat to me, that is very soft. If it's sound I'd not worry too much about it.

TBH it looks a bit of gash tiling job. Why is there a 1/2 strip of tile above the bath rim? You have two joints just where you don't want any...

There is signs of water pentration along the bottom of the missing tiles. Either the silicone has failed or not applied properly or that grout like above the 1/2" strip has. This would allow damp behind and would weaken the bond between the tile adhesive and plaster. I expect all the missing adhesive on the back of the removed tiles.

Think I'd remove the entire bottom row and the silly little strip. Clean up the tiles and bath rim and refix the tiles with a waterproof adhesive and water proof grout. I'd also look at removing the grout between the other tiles at least for the row above inc the horizontal joint and regrouting with the waterproof grout. When fixing the tiles back leave a 4mm or so gap above the bath edge(*) and keep that clear of grout back to the plaster. Once the grout is dry fill the bath to the overflow with water and fill the gap, full depth, with silicone sealer. Leave the water in the bath for 24hrs after finishing the sealing.

The tricky bit will be how far up the adhesive/plaster bond has failed one may end up with rather more tiles off the wall than on...

(*) If that means having to trim a few mm of that silly strip see if you can find a decorative strip in a proper tile shop of suitable width and fix that below the run of tiles that are still on the wall so the tiles you have off are down next to the bath rim. Depending on the rest of the tiling and how that relates to other fixtures in the room doing the above might look naff...

[Tim Watts]

I know it's kind of my default answer, but soak the plaster in 1:2 diluted SBR a couple of times. It will stabilise it to a few mm depth. The reattach tiles with fresh adhesive.

I had some polished plaster which BAL instructed (for BAL Green anyway) to both scrach (wire brush) prime with dilute SBR to stabilse and achieve a better bond. I had a couple of walls done in one-coat which is softer than regular gypsum so I primed that too. The result was quite significant - a mauch harder finish afterwards.

[The Night Tripper]

It's 'sound' to the extent that it's not falling off the wall. I like Tim Watts' suggestion of soaking in dilute SBR to stabilise things. If SBR available in smaller quantities than 5 Litres, does anyone know?

Ah - it's not quite as bad as you think. That's not a thin strip of tiles, it's a mark on the other tiles an inch or two above the bath, from a silicon tape sealing strip that I have removed. I would have been *exceedingly* annoyed if he'd plastered with 1/2 a tile like that!

It is true that there seems to be water penetration at the bottom edge as well (I saw that the grouting was failing at the top endge; the lower edge was hidden by the aforementioned strip). But most of the adhesive on the back of the removed tiles has never made contact with the plaster underneath; the ridges wre 'unsquished'. This is why I think the original bond between the tile and skimming or base coat was not solid, perhaps being the original cause of these problems.

Thanks for the advice. I am pretty sure (until I look next ;-( ) that he row of tiles above is solid. I did the original silicon sealing myself IIRC, with the full bath trick ... but I don't recall the gap being as much as

4mm. I can see how having a decent gap width would be a good idea.

I do actually have a tile cutting machine (bought to do a floor tile job, will do it myself this time!) so I s'pose I could trim a few mm off the tiles I'm replacing, to ensure the gap is good, at least in that area. It is at the point which gets most rundown from the sohwer, which doesn't help.

[some stuff deleted]

Thanks a lot Jon N

[stuart noble]

The substrate looks fine to me. I'd just retile using a pre-mixed tub adhesive (the standard Topps version has worked well for me in similar circumstances). If you use the supplied spreader and can move the tile after 5 minutes, then I was wrong :)

[The Night Tripper]

Hi All A followup...

I've put a couple of coats of diluted SBR Bond on the underlying plaster as per Tim Watts' suggestion, and it certainly seems to have firmed things up a bit. But I think I've also seen what the actual problem is:

It looks like this coat of plaster is sloping away from the wall as oyu go down this last row of tiles. So I'm guessing that, when the tiles were affixed vertically, the adhesive on the bottom half (say) of the tile wasn't touching the plaster at the rear.

Thus the tiles would have been mechanically weak at the lower edge, and over time would have let in water, which would have further loosened thigsm so they also failed at the top edge, etc.

As far as I can see this leaves me with two options:

A) add a further thin tapered coat of plaster to bring the surface vertical across the whole height of the tiles of this row. Phil L. said that this would be a problem ("Don't try and remove or plaster this patch as you will not be able to get the tiles right afterwards"). Would this really be a problem?

B) leave the plaster alone, but when I retile this row, add extra 'height' to the adhesive on the area which will become the lower edge of the tile when mounted. Is this practial? I'm not sure if, and how, this can be done.

Thanks for your thoughts.

Cheers Jon N