new plaster

[ianb5454]

hello just had bathroom re-plastered can i tile straight on to new plaster or does it have to be treated with anything first?

[kbrownie]

HI ianb5454,
First let the plaster dry completly then treat the area you are tiling with PVA before tiling and you should be okay.
Regards
KB

[rosebery]

Bonding and finishing coat or plasterboard and skim?

If the former it'll need at least 6 weeks to dry out enough to tile. If the latter a couple of days is sufficient.

PVA it first and they'll fall off in about 6 months! No I'm serious! If PVA gets wet it becomes slightly live again, it doesn't completely return to it's liquid state but it becomes sticky. When you spread tile adhesive onto the wall, the water in the adhesive makes the PVA live and stops the adhesive from penetrating the substrate and providing a mechanical grip.

Most tile adhesive works by crystalising when it sets. The crystals expand into any imperfections in the substrate surface (at a microscopic level) to create a grip. PVA stops this process by creating a barrier between the substrate and the tile adhesive. Basically a moist layer of PVA is then holding the enitre weight of the tiles, grout and adhesive and stopping a great deal of the mechanical grip occuring.

You can tile direct onto the plaster in dry areas, a coat of APD primer around the bath area is a good idea and tank the shower area. Use a decent tile adhesive not the cheapo rubbish from the sheds.

Cheers

[kbrownie]

Hi, using this post as vechile to asked rosebery Question.
Is that true? about PVA. I have used it a lot, in all sorts of applications. Tiling, plastering, dot and dag, concrete repairs, etc..
To my knowledge I have yet to have a tile, plaster or plasterboard fall off of anything that I have used PVA on!
I am not saying that your comments are unfounded, far from it. I may have been very lucky in the past and find your comments very interesting and scary.
Regards
KB

[rosebery]

Hi K

Yes - scary isn't it but it has happened. Heres a real life example. Builder sticks up a load of ply and being helpful PVAs it. Tiler turns up a week or so later doesn't know it's been PVAd and tiles it. A few months later tiles all over the floor and tiler gets a kicking for something that wasn't really his fault.

I'm just finishing a bathroom where the water had got behind the original tiles. They fell off virtually just by touch. The wal behind was soaking wet and had clearly had stacks of PVA put on it before the last effort.

From a DIY point of view it probably doesn't matter however when you're doing it for a living and there's such a thing as "latent defects" to be considered then I'd rather be safe than sorry.

Oh - don't get me wrong I'm not TOTALLY anti PVA - it has it's uses iand I would admit to frequently using it where it's appropriate. I just don't think it's appropriate in a tiling environment and I really can't understand the national obsession with the stuff.

Cheers

[kbrownie]

Hi Rosebery,
It's me again!
Good example, but still a little bit confused.
Is problem when the PVA has set and is re-moisterised (sounds a bit girly, I was never confused!) or does this problem still occur when, say you prime the wall and apply tiles for example within the hour when pva is still tacky and what would be the outcome if I plastered wall primed with pva then tiled on plaster without primer? Can I assume that the tiles are going to pull the plastered wall off!

Info is greatly appreciated
KB

[rosebery]

I think of it (practically) in terms of the tiling substrate. If it's shiny (like new finishing plaster or ply) then you're struggling for good mechanical grip before you start. If its a rough surface that's a different question.

Look at it this way you can hang a certain weight per m2 of tiles, adhesive and grout on plasterboard. The weight is quite a bit less per m2 for plaster skim over board.

Yes I would PVA (but well diluted) the brick wall before plastering and would not expect the plaster to fall off unless I really ...... it up!

Cheers

[PHIL7923]

little confused with the pva glue n water thing... as plasterers use it all the time and before rendering. yet the plaster has water but yet that stays on the walls???