Sealing Joins On Backer Boards

[MrMiller]

Hi Folks

I'm about to start fixing backer boards to my solid brick bathroom walls. They're Marmox boards and my plan is to both fully tile adhesive the backs and use fixing cob dowels.

Once they're up, I also plan using some self adhesive butyl tape along the joins and over the dowel washers. What I'm not sure about is what to do between the board joins as I'm adding them in place:-

I've seen some videos where the guy has just put some additional tile adhesive along those edges before pressing up against the board already in place and I've seen another where they ran a bead of separate polymer based adhesive along the joins. Having said that - I think that second one may have been on a stud wall where they didn't have any tile adhesive being used anyway...

So which approach is correct? Or does it make no difference anyway, given the butyl tape will run over the join anyway and then the lot will have tile adhesive over that before the actual tiles themselves...

 

[James Carmichael]

Faced with this situation, I used GripSeal HydroBond 360, which is a specialised joint compound designed for this sort of board on solid substrates. It’s a hybrid adhesive that bonds the edges and creates a micro-seal that chemically reacts with cementitious tile adhesive; kind of double bubble waterproofing.

I ran a 5 mm bead along every board edge before pressing them together. You need to hold them in place for about 5 minutes because the Hydrobond expands slightly during curing (it's supposed to be self gap sealing, a bit like PE wood glue). Make sure it doesn't push the boards apart while it is doing this. I didn't try letting the tile cement holding the board to the bricks dry first then applying it down the gap; I assumed that wouldn't guarantee a good fill.

Once cured (about 45 minutes), you can apply your butyl tape and proceed with tiling as normal.