Tiling strawboard (stramit) type wall surface in bathroom??

[GtechMech]

Evening all,
Could really do with some help with the above- my problems are:

I am replacing bathroom suite. I have decided to remove and renew all tiles but when I removed old tiles on stramit walls (tap wall and bath side wall) the plaster skim was coming away so I chipped it all back to the cardboard backing - the problem is that the tiles just above the bath have had some water damage behind and rotted the cardboard backing in a few places and there is straw poking through. I cant fit aquapanel board as there is no wooden battens to screw onto in this type of partition wall and would be too heavy when taking panel , tiles and adhesive into account. What I need to know is:

1--- Whats the best way to water proof this setup as it is - what do I use to prime - I was told NOT to pva by a guy in tile shed?

2--- How do I deal with the corner gap of about 10mm between both strawboard panels before applying primer then tile adhesive? can i use silicone or waterproofing tape?

3--- The other "non wet" walls are block work with what looks like cement render then few mm of smooth skim plaster which has flaked off in many places when tiles were removed from these walls. How should I deal with these. I was going to chipoff skim as it comes of quite easy with wall
paper scraper and apply good quality tile adhesive straight onto render?

Sorry if this is long winded - its alot to put into words I know

Many thanks in advance

 

[Richard C]

1--- Whats the best way to water proof this setup as it is - what do I use to prime - I was told NOT to pva by a guy in tile shed?

The only waterproofing system is tanking either a physical or (paintable) liquid membrane, primer will not waterproof. For once your tile shed advice was good; never use standard PVA on any tile surface, it remains water soluble & will reactivate with damp/water; tiles fall off. Neither will it help tile adhesion in other areas. You should, however, never use a primer unless your tile adhesive or tile manufacturer recommends one.

2--- How do I deal with the corner gap of about 10mm between both strawboard panels before applying primer then tile adhesive? can i use silicone or waterproofing tape?

I have no experience with Stramit panel walls, do you know how are they fixed & what are they fixed to? Is there not a central frame/stud of some sort. Can panels not be completely replaced with something more suitable? I’ve looked at the product sheets but they are not exactly informative so if you can provide any further info/answers to the above it may help. I think it very unlikely they are suitable for tiling direct.

One method of repair perhaps would be to fill out any holes & gaps you have but I’m not entirely sure what will stick to straw (do not use silicone) & then apply a physical tanking membrane over both walls before you tile. Another may be to repair the holes & re-skim before tiling but because it’s around the bath, you’re going to have to tank it or you will have the same problem again but maybe worse. The walls must be in sound condition, any sign of breakdown & the tiling will most likely fail. What size/weight tiles are you thinking of using?

3--- The other "non wet" walls are block work with what looks like cement render then few mm of smooth skim plaster which has flaked off in many places when tiles were removed from these walls. How should I deal with these. I was going to chipoff skim as it comes of quite easy with wall paper scraper and apply good quality tile adhesive straight onto render?

If the plaster skim is not bonded to the render then removing it & tiling straight onto the render is probably your best option but it must be reasonable flat (critical with large format tiles) in sound condition, not powdery sounding hollow when tapped.

 

[GtechMech]

Hi richard and thanks for replying.
The walls are fixed together with small metal brackets. It looks like there are battens in the loftspace which sandwich the panels and I assume they are slotted into floorboard. There are definitlely no timber supports vertically.
I do not want to have to renew these walls due to the amount of work/time and costs involved. The original tiles had bonded well to the skim so I must be able to tile to these type of walls.
Swmbo has chosen some satin white ceramic tile 250x400x8mm.
What I dont fully understand is that if a good waterproof adhesive is used and applied properly as well as good quality grout and silicone round the edge of bath,then surely no damp should get past the adhesive?
Thanks again

 

[Richard C]

The walls are fixed together with small metal brackets. It looks like there are battens in the loftspace which sandwich the panels and I assume they are slotted into floorboard. There are definitlely no timber supports vertically.
I do not want to have to renew these walls due to the amount of work/time and costs involved. The original tiles had bonded well to the skim so I must be able to tile to these type of walls.

I can understand it could involve a lot of work & why you’ve no desire for that. You can re-tile over it provided what’s left is in good condition & you prep it correctly. To ensure a flat tile base, I would advise you apply a tile bonding primer (not PVA), fill any holes & bring it out level where the plaster skim has come away using a flexible cement powder based tile adhesive or possibly quickset render & then apply a tanking membrane before tiling. Or you could get the damaged part of the wall re-skimmed but you will have to wait aeound 10 days for the plaster to dry out before tiling.

The problem you may encounter with tiling over plaster is tile weight (see below).

Swmbo has chosen some satin white ceramic tile 250x400x8mm.

The problem you may have is with those 8mm thick tiles which (with adhesive/grout) will probably exceed the maximum weight of 20 kg/sqm (including up to 4kg/sqm for adhesive & grout) you can hang off plaster & that’s if the plaster is in good condition, do you know the weight of the tiles? If you want to stick with the walls you have, SWMBO may have to select a lighter tile for the plastered walls (max 15-16 kg/sqm); unless she wants the possibility of them falling on her while she’s in the bath.

Render will take around 40kg/sqm so once you remove the skim from the block walls you should be OK, but again, providing it’s in good condition & not blown.

What I dont fully understand is that if a good waterproof adhesive is used and applied properly as well as good quality grout and silicone round the edge of bath,then surely no damp should get past the adhesive?

Commonly misunderstood is that except for very expensive epoxy products, waterproof adhesive & grout is only waterproof in the sense it will not break down when its wet, it is not impervious to water. Additionally, you only need the smallest crack in the grout to get wicking into the tile base & if that isn’t waterproof, there go your tiles & possibly your tile base as you’ve seen.

Use only powder cement adhesive on tiles larger than around 250mm not tub ready mix; any gypsum plaster must be acrylic primed. Use quality trade adhesive & grout only, the cheap DIY products are mostly crap.

 

[GtechMech]

I have no intention of re-plastering the strawboard or render for that matter. If I apply the primer and waterproof brush on liquid membrane such as bal kit using tape over corner gaps and slightly damaged parts, will the board be able to take more weight than it originally was able to? (with skim)
The original tiles were 250x300 and they were stuck on soundly only a tad smaller.
The stramit panels have a thick paper backing so would I be correct by using procedures that apply to plasterboard when following manufacturers instructions - there does not seem to be any mention of this kind of substrate when looking at various products

TIA
Graeme

 

[Richard C]

You need to fill any gaps & get the tile base flat before applying the membrane, just taping over wont give a good result; it’s also important to have a flat tile base with larger format tiles. BAL WP1 will support a max tile weight of 32 kg/sqm but it’s important to remember that the tile base will only be as strong as its weakest bond, in this case the plaster skim; unfortunately tanking a plaster skim will not increase the maximum tile weight you can hang. The original tiles may have only been slightly smaller but the tile thickness also dictates the weight. As a general rule, tiles over 8mm thick should not be laid over a plaster skim; the weight including adhesive would be 18-22 kg/sqm, so your sort of on the limit really. There is likely to be a small safety margin in there so you maybe OK or maybe not, it’s important you establish the dry weight of your tiles before proceeding. Remember you will need a powder cement adhesive, do not attempt to fix tiles that size with tub ready mix & you must acrylic prime the plaster to avoid the possibility of reaction between the cement & gypsum in the plaster.

Re prep for tiling directly over the Stramit panels; it’s not something I’ve ever done & short of searching the manufacturers spec sheets or contacting their technical dept, I’ve no idea. If it looks similar to the paper base over plasterboard then chances are it's very similar & should be able to treat it the same. It may be a good idea to apply an acrylic bonding primer first but you should be filling out flush before tiling as I described above if you want the finished result to look good