Ceramic on hardiebacker on ply

[Grant Faulks]

Hi all,
I'm new to the forum so please forgive any silly questions, I've taken on rather a big DIY job so some expert guidance would be appreciated on occasion.

I recently noticed our tiles cracking
in both our adjacent upstairs bathrooms. After taking these up I found that the original chipboard had been soaking up water, had expanded, rotted and gone mouldy.

After a sizeable quote and insurance excess i decided to tackle as much of the flooring and tiling as I could myself. 🙄

The job has typically ballooned from there, not least because I decided to take the opportunity to completely replace/ upgrade everything, move a toilet, basin and radiator, install a back to wall toilet and create some alcoves in the main bathroom (not done these last two yet).

I'm very careful to take on only what i realistically believe i can handle so after plenty of research and Youtubing I've gutted both rooms, ripped up the chipboard, reinforced the floor, replumbed, added insulation, cut and laid 18mm WBP ply (sealed with SBR under & sides), repaired the walls and am now at the point of laying 6mm hardiebacker.

I'm doing this today hence the urgent question(s), pretty certain I've done everything right but I want to make sure:

Q.1 - Do I use SBR on the ply to prime or bond?
I'm using Mapei Mapeker Rapid-Set for the Hardiebacker, which is ok for ply. It doesn't mention priming ply first and I've read the sealed ply shouldn't be primed on top just sponged with water before spreading adhesive. Is this correct or should I prime the ply with SBR before spreading the adhesive? (I realise it's only intended to fill gaps under the hardie in this instance so hoping not?)

Q.2 - What is the minimum depth of adhesive under Hardiebacker/floor tiles?
The 18mm ply plus 6mm hardie plus 10mm depth ceramic tiles plus adhesive throughout raises the floor height substantially so I'd like to minimise adhesive height where I can.

Q.3 - Do I use SBR to prime/ bond large ceramic tiles to the plasterboard walls?
I understand i should at least prime the walls before tiling which would be 1 part sbr to 3 parts water (like I used on the ply) but as I'm using large 500x300x10mm ceramic tiles I think bonding would be best, particularly as the walls are all dry wall, some partition dry walls with metal C studs which can flex a little, and are pretty patchy/had to be repaired/filled in parts..I'm worried about the flexibility and bonding, the Everbuild SBR instructions for bond mix indicates 1-2 parts fresh cement (mapeker for me) with 1 part SBR which has to spread and tiled on within 20mins/before dry. Should I use sbr to bond? If so is the best way to mix up the addy in one bucket and have a second for pre-bonding with an addy and sbr mix?

If someone can help in the next hour or two/sometime today that would be really appreciated.

Thanks,
GF

 

[Qwerty]

No need to prime the ply if sealed already. Wipe off will be fine

Min trowel size really is 3 or 4mm. Depends how flat your ply is

Prime the walls with SBR as a primer, don't go down the bond route. You can just wipe the plasterboard instead of priming it so long as it isn't too dusty or messy

 

[Tile seller]

Has the ply gone straight onto the joists?

 

[Grant Faulks]

Thanks Plan Tec,

No need to prime the ply if sealed already. Wipe off will be fine

Great, hoped that was the case, i'll lay the hardie with just a wet wipe down no primer.

Min trowel size really is 3 or 4mm. Depends how flat your ply is

Ply is very flat and no movement but minimum I have is a 6mm trowel. I thought I might try to angle the trowel down when spreading to reduce the ridge height but not sure how realistic that is?

Prime the walls with SBR as a primer, don't go down the bond route.

Ok, I can prime but won't it create a latex layer like PVA does or is SBR better? Either way, could you tell me why i shouldnt bond? I know it might be messy and use up addy but I thought it would help with flexibility and make a better bond than with priming?

You can just wipe the plasterboard instead of priming it so long as it isn't too dusty or messy

Ok, should I still do this before i prime/bond?

Cheers
GF

 

[Grant Faulks]

Has the ply gone straight onto the joists?

Yes, 18mm WBP plywood has gone straight onto the joists at 90 degrees to joists. new noggins along the ply joins, edges and under 2 partially 'floating' partition walls. 50mm Reisser Cutter screws into joists and noggins (missing pipes/electrics - all pre-marked) every 15-20mm.

Hope that helps.

Cheers,

GF

 

[Grant Faulks]

Yes, 18mm WBP plywood has gone straight onto the joists at 90 degrees to joists. new noggins along the ply joins, edges and under 2 partially 'floating' partition walls. 50mm Reisser Cutter screws into joists and noggins (missing pipes/electrics - all pre-marked) every 15-20mm.

Hope that helps.

Cheers,

GF

Sorry every 15-20cms😳

 

[Tile seller]

I was just thinking that a more flexible adhesive may be better however, because your over-boarding it with hardie backer mapeker will be fine.
I don't see that there's any need to bond it in honesty. I don't know much about SBR but alot of tilers use it successfully. Used watered down enough it should absorb into the substrate rather than sitting on the surface.
I always sell primer g for mapei adhesive as that's what Mapei recommends.

Sounds like your doing a great and very thorough job.

 

[Grant Faulks]

I was just thinking that a more flexible adhesive may be better however, because your over-boarding it with hardie backer mapeker will be fine.
I don't see that there's any need to bond it in honesty. I don't know much about SBR but alot of tilers use it successfully. Used watered down enough it should absorb into the substrate rather than sitting on the surface.
I always sell primer g for mapei adhesive as that's what Mapei recommends.

Sounds like your doing a great and very thorough job.

Thanks Tile Seller,
Bit concerned now, I'm intending to use the mapeker for the walls too, in your experience would you think it will be flexible enough for the c-stud partition walls or should I buy an alternative (has to be white) or is there anything that I can add to the mapeker mix to give it the required flexibility(the sbr?).
I've already bought 2x 25kg bags so prefer to make those work if possible but doing the job right is more important.
Cheers
GF

 

[Tile seller]

Keraquick would be better for both the floor and wall but, if you can't get your cash back on the mapeker or its a lot of hassle too then I'd stick with what you've got.
Mapei don't recommend mapeker for ply walls and the extra flexibility of keraquick wouldn't hurt the floor either.

 

[Tile seller]

Sorry, I meant to put that Mapei don't recommend mapeker for plasterboard walls, but that's still true for ply walls too although irrelevant here.