Skim before tiling?

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Hawkman

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I have asked a plasterer to quote on re-plastering a bathroom (existing plaster is damaged in some places when old tiles removed). He suggests just putting on a bonding coat, not skimming. Is this ok?

Also, he suggests using PVA before tiling, but I have read that is a no no, since it creates a surface barrier and that a primer like BAL Prime APD should be used. What do you think?
 
Ask the tiler what he wants to tile onto?I find they want it skimmed most of the time, as for pva before tiling.... Again, that's the tilers prep? It's his shout as to what to do before tiling
 
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Usually for tiling we skim it, flatten and run a brush over it to give a keyed surface.
 
its better not to skim plasterboard at-all ,as plasterboard can take a better weight of of tiles per m2 than skim !! and never use PVA it's crap and can re-liven if it comes into contact with any moisture , hope this helps .
 
sorry just re-read this !! you would be better fixing tile-backerboard over the existing surface then tile directly to this sub straight
 
dnt pva it doesnt fully permeate the surface so essentially you will have a laver of pva sitting ontop of what ever substrate you choose as soon as the tiler puts addy onto this it will reconstitute and it could all come crashing down plus the fact your tiles are just hanging on the pva rather than the addy biting into the substrate use something like you suggested or sbr
 
PVA - Why you shouldn't use it as a tiling primer

I have to give guarantees for my work (many of these projects are commercial such as sports centre showers and changing rooms). For me to be able to give guarantees I need to follow strictly the specification of the adhesive manufacturers.

Ardex, BAL and Nicobond are the three suppliers I use most. Their products are similar in many respects, sometimes one will make products the other don't, and I also find some of there products more useful in different applications. All three of them have one thing in common, they all specify that under no circumstances may PVA be used before using any of their adhesives. If you do all guarantees are void.

OK why then? Well I asked this question to Ardex when I once had problem, I'd tiled a bathroom that had been constructed in 25mm Marine ply. Thinking he was doing the right thing, the builder got his guys to seal the ply with unibond PVA...I wasn't aware of this.

I tiled it and 6 months later every single tile fell off the ply, the adhesive solidly stuck to the tile but came clean a whistle off the ply.

We had Ardex Technical down to the site to compile a report, the basis of which was it's the PVA that causes the problem.

When you treat a surface with PVA it partly soaks in and partly sits on the surface of the substrate much in the same way as wallpaper paste.

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. Basically your tiles, grout and adhesive are being held to the wall by a thin layer of PVA.

Most tile adhesive works by crystallising when it sets (some are slightly different such as epoxy based ones) but generally they all work the same way. Once the adhesive starts to set crystals from and 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.

Ok so what's the difference between this and Ardex or BAL primer, well basically the tile manufacturers primers soak right in to the substrate and stop the sponge like "draw "effect but they don't coat the surface in any way, they are an impregnator as opposed to a barrier. They also stop a chemical reaction occurring between the cement based adhesive and a plaster substrate, a known problem know as "Ettringite failure"

I hope this clears up any misunderstandings.
So only use PVA before tiling if the adhesive manufacturer specifies it in the instructions.
these are not my words ,but i have copy and pasted for you .
 
Update: plasterer says it is pointless to skim and that he can get the undercoat perfectly flat for tiling. Tiler says: make sure the walls are straight and level (no bumps) not just bonded.

Previous advice on this thread has been get it skimmed - is this because only skimming will give a straight and level surface?

Whose advice should I follow? Two professionals with opposing views.
 
what do YOU want and WHAT does the tiler want if u and he say skim it then thats what ya plasterer should do no arguing
personally i always skim it then lightly run me scratcher over itto give tiles a key etc
 
If the tiler wants it skimmed, then skim it. It's him that's puttin the tiles on so as said before it's the tilers call
 
Undercoat plaster is lightweight & not strong enough for tiling onto,
But once skimmed it will hold more tile weight.

But people still do it.

Sand & cement would be a better option for tiling on no need to skim it :RpS_wink:
 
You wee stirrer Lee :rolleyes)

The plasterer should prepare it to the tilers' requirements :RpS_thumbup:
 
Thanks for pushing the point about it being the tiler's call - I hadn't really thought that through properly before, but it now makes sense to me. So skimming it is.
 
Gooners info was brilliant.

The unfortunate thing with this question is there is no correct way to do it.
There are people (mainly tilers) who want it skimmed always to make their life easier, but the question is IF it is skimmed the plasterer will be bonding the skim with pva, and as Gooners already said pva sits on the wall, so if the pva fails through any possible future damp maybe caused by broken silicone or, god forbid bad tiling/grouting then the pva will re-emulsify and the tiles will drop off, taking the tile adhesive and the skim coat, and film of pva off with it.

The only way to do it properly is to do it to spec which more often than not means hack off and using proper cement tile board.

I would say hope this helps, but I know it doesn't... You are just going to have to opt for the option which you think is lesser of two evils... Or rip it off.
 
surely it all depends on the weight of the tiles? i think plasterboard can take around 33kg p/m2 and skim can only take around 20kg. thats what a tiler told me last week anyway
 
I would always prefer s/c in a bathroom, bonding has terrible problems with any moisture, some tilers ask for a skim coat pre tiling others dont, I do not know which is better myself
 
S & C is the best sub straight to tile onto every time !! which i do a lot as i render/screed my own swimming pools before appalling mosaic to them ,first of all there is a scale of sub straights and the correct tile weights each can take, ( i will post later ) cement based adhesive /grout is never waterproof and just for your information nether are most tiles/stone so water in very wet areas like showers/bathrooms /splash backs /wet-rooms are subject ,to prospective damage by water/steam/moister ,if you use PVA in any of these situations you are asking for trouble ! and when it comes if you have PVA'd it then the buck stops with you so as spreads
Never assume you know best !! as it's the tilers area of expertise ,and if you throw hat in to the ring then BEWARE ,if you are a Plasterer/tiler then you should get on the tilers forums two of them and educate yourselfes in the correct way of doing these jobs,like plastering in the last 10 year the ranges of materials and new thinking and systems have exploded ,i have to keep myself up-to date with the tiling more than the rendering,as it's moving so fast, and the reason i 'am on here is to try and get with it on the plastering scene ,from talking to you lot and the sponsors with is great and slowly working ,but i still have no idea about the machine's for plastering like some of you guys , i'am 53 so an old timer ,in that sense ,having said that ,i have probs forgotten more than some of the F on here LOL, my way of approaching other trades is to ask them ! they should know, and never get involved in there job's and liability !!........
 
I specialise in wet rooms and carry out the whole job from start to finish. If the walls are poo i normally level out with bonding coat let dry and then prime with bal primer and use bal grip adhesive. Been doing this years now and never had a problem. Also i have to add there are several ways to achieve what you want to. My way is only a suggestion. I think these days there are so many products out there it creates great confusion with what to use.
 
Mike814 said:
I specialise in wet rooms and carry out the whole job from start to finish. If the walls are poo i normally level out with bonding coat let dry and then prime with bal primer and use bal grip adhesive. Been doing this years now and never had a problem. Also i have to add there are several ways to achieve what you want to. My way is only a suggestion. I think these days there are so many products out there it creates great confusion with what to use.
Click to expand...

Do you ever use a tanking system between tile adhesive and walls/floor of any sort Mike on your wet-rooms ?.......
 
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Just done a bathroom and near the bath was boxed out and boarded with Gypoc MR board. Builder asks us to skim it but not brush it (we usually give it a coat of skim, flatten then run a brush over it to give a key) he asked us to trowel it flat so he could slide his tiles around if needs be. Other builders have specced not to skim on Gyproc MR as they say it doesn't need it. We just do as asked by what the tiler wants or what's in the specs.
 
If you are going to re line the walls, why not use a cement based tile backer board - Knauf Aquapanel Interior.....Knauf-Aquapanel-Interior-Brochure

This brochure also shows the Knauf Aquapanel 6mm tile floor underlay........ What gooner says above is right, water will go through grout lines and flooring for a wetroom floor is different!!

Gypsum Tile Boards have a tile weight limit of around 30 - 32 kg/m2 (check their data sheets and refer to the backing structure). Knauf Aquapanel Interior has a load limit of 50kg/m2........ (although this is overkill for most domestic applications I know...)

If you need more help - contact Knauf Drywall Technical - number found on the back page of the brochure....
 
gooner59 said:
Do you ever use a tanking system between tile adhesive and walls/floor of any sort Mike on your wet-rooms ?.......
Click to expand...

On a full wet room we often batten out the walls. Ply the area with marine ply and then use the mapegum system which seems to be pretty reliable. It entails placing mesh corners and joints in the first coat of mapegum and then a second coat approx 4 hours after although drying time is temp dependent. You can tile straight onto this with no other primer required.
 
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