new plaster


Postby ianb5454 » Sat Sep 15, 2007 12:03 pm

hello just had bathroom re-plastered can i tile straight on to new plaster or does it have to be treated with anything first?
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Postby kbrownie » Tue Sep 25, 2007 11:35 am

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
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Postby rosebery » Fri Sep 28, 2007 6:54 am

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
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Postby kbrownie » Sat Sep 29, 2007 8:47 am

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
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Postby rosebery » Sun Sep 30, 2007 8:26 am

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
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Postby kbrownie » Sun Sep 30, 2007 11:07 am

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
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Postby rosebery » Mon Oct 01, 2007 7:22 am

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
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Postby PHIL7923 » Wed Oct 21, 2009 4:07 pm

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???
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Postby chris_on_tour2002 » Thu Jan 21, 2010 6:21 pm

"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???"

that's because the new layer of plaster is applied whilst the diluted PVA is still tacky so it bonds equally to both new layer plus layer beneath.

if a wall has been PVA'd for plastering and has dried out it must be wetted down with more diluted PVA to liven it up before plastering.

if wet plaster is applied on top of dry PVA same problem - might take 2 days, 2 weeks or 2 decades but sooner or later it's going to fail. especially with tiles on top of that.
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Postby chris_on_tour2002 » Wed Jan 27, 2010 6:47 pm

another question for rosebery; how do you feel about the use of waterproof PVA as a sealer prior to tiling? presumably this would not present the same issues of becoming 'live' when in contact with moisture? or is intended strictly as an adhesive?

just curious, i use an acrylic or latex primer for tiling and normal PVA for plastering.

i must admit i use waterproof PVA for priming exterior walls prior to rendering, rather than the usual PVA.
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Postby plumbbob » Wed Jan 27, 2010 11:59 pm

I am sorry, but I can't agree with Rosebery's fear of PVA. I and everyone else I know in the plastering, painting and tiling trade uses PVA all the time. In fact, the instructions on the product recommend it for these purposes.

Of course, the point of using a pva is to stabilise the surface prior to any finish, not to assist with the adhesion. Without it any porous surface will result in an inadequate bond and any tile WILL fall off.

Yes it can be better to use waterproof sealants, but to be honest if water gets behind tiles never mind any pva, all is lost anyway. It's just a matter of time as even waterproof tile adhesive will soften and fail if constantly wet let alone any plaster substrate.

As for the quote about tiles falling off plywood, I would simply say the wrong adhesive was used and that had nothing to do with the PVA. Sorry.
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Postby hewzey » Tue Mar 02, 2010 6:12 pm

I agree with plumbob, I am a general builder and qualified plasterer of 14 years and watered down pva (4:1) should always be used to seal new surfaces before tiling(or even sealing in bath prior to tiling and must be allowed to dry over night before silicone will bond properly) and almost neat pva(2:1) on low suction areas such as concrete. Of course the plaster must be dry first. 6 weeks for bonded out walls seems a bit overkill but once it has gone light coloured all over is ok. Tiling on timber of any sort requires special adhesive to ensure a longterm bond. Ignore at your own peril... :-(
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Postby alibhere » Wed Aug 04, 2010 10:07 am

Hi
The reason for bonding porous surfaces with a PVA solution is to stop the moisture being sucked out of the adhesive. If the surface is too porous the adhesive wont set properly and will dry too quickly. this may cause the tiles to fall off. Pva should be diluted to suit the surface also when dry I do not agree that it will return to a liquid state unless possibly it has been subjected to continued soaking (Like maybe underwater for many hours).
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