Hi,

I'm about to replace my floorboards with 17mm Marine Ply and then 6mm of Hardibacker on top before laying tiles.

I thought I just screwed the Hardibacker to the Ply, but apparently I need a layer of tile adhesive in between. That's news to me, is that right?

If so, I assume I need to seal the Ply first too, I've had a look at the Mapei range but none explicitly say for this use, what would you guys use there instead?

Cheers!

What tiles will you be fixing.

Sorry, didn't think that'd be important!

It's these: Arteak Black | Topps Tiles

Ply primed on the back and sides. Then it is screwed to the floor.
The Hardibacker is glued with single part flexible adhesive and screwed in accordance with Hardibackers instructions. Some adhesive manufacturers say to prime the ply 1st and some say not.

I'm using Bal's Single Part Flexible for the tiling, so figured I'd use it for the ply/hardibacker. The datasheet here:

BAL Adhesives - Products - single part flexible

Says to just "In wet conditions tank out with the BAL WP1 Tanking System.". This is a bathroom, but not a wetroom, so BAL WP1 just to play safe? But no other sealant for the ply?

Sorry, I just re-read that and it says "Tanking System", that seems like massive overkill for the floor next to a bath. So, that's no sealant at all then?

Sealant or primer ?

Sorry, you've outed that this is the first time I've tiled...

It's primer I should be asking about isn't it? I'm after ensuring a good bond between the layers, not waterproofing...

Oh, I'm priming the adjacent plaster board with Bal APD but it looks like that's no good for Ply either. I can't find anything that says it should prime Ply OK... this is probably where you tell me I don't need to...

Prime back and sides with SBR not the face unless stated by BAL. Having not used BAL products for a while I am not sure what they say.
BAL Adhesives - Media Centre - What's the difference between ... priming and not priming?

Thanks for that, that's what I'll do.

Out of curiosity though, why prime the back and sides - which nothing till be bonded to - but ignore the front why there'll be adhesive?

Not doubting your (or Bal's) advice, I'm just interested in the "why" behind it

Moisture ingress mainly.

Helps control the expansion of the wood due to moisture