XPS tile backer boards on blocks

[john2k]

On the current external side of my bathroom wall there is currently green plasterboard that was dot and dabbed. The thickness of the plasterboards look approximately 20mm. I’m ripping the plasterboards out from the blocks completely to replace it with 20mm xps backer boards. I’ve attached a photo below of how the blocks look and the current plasterboard looks.

What I noticed is that there is a considerable amount of gap between the block and the plasterboard itself. And whatever adhesive was used to bond it is super strong but what they did was had big thick layers of that adhesive which created quite a big gap where there is no adhesive.

My question is, when I now come to stick the XPS boards, I was planning to use rapid set tile adhesive and stick the boards on and after the rapid set has cured drill some holes through and use washers and screws for extra mechanical fixing. Will this work? Should I aim to not have the sort of gap that’s currently there?

If the block isn’t 100% straight could I just pack it with extra tile adhesive in problematic areas?

 

[phatboy]

The gap will hopefully vary across the wall, as it should be to level up a wonky wall.

The gap doesn't need to be big otherwise. Tile adhesive spread everywhere will only work on a perfectly flat wall.

Yes fixings with wedi washers would be a good idea afterwards. I can't tell if your wall is lightweight block. If it is, you may be fine just going in direct with woodscrews and they will thread into the wall... just don't overtighten them or drive them fast.

 

[john2k]

The gap will hopefully vary across the wall, as it should be to level up a wonky wall.

The gap doesn't need to be big otherwise. Tile adhesive spread everywhere will only work on a perfectly flat wall.

Yes fixings with wedi washers would be a good idea afterwards. I can't tell if your wall is lightweight block. If it is, you may be fine just going in direct with woodscrews and they will thread into the wall... just don't overtighten them or drive them fast.

Whatever adhesive was used by the original builders is rock solid. It literally takes chunks of the block out to come out. But they’ve only dabbed big thick blobs everywhere like some are 50cm apart. There’s no pattern that I can see. What kind of adhesive did they use? It’s rock hard not sure if it’s tile adhesive or something else.

Should I apply tile adhesive in dot and dab style like they did or use a notch trowel and trowel it in?

 

[^woody^]

You would use dabs to allow the new boards to be plumbed and aligned. You can't do that with a solid bed.

You should not use tile adhesive. Use proper board adhesive or foam adhesive. You would not normally require additional mechanical fixings.

Finished cavity would be between 10 and 25mm, so dabs would be about 50mm to compress. You need a continuous band of adhesive around the whole board perimeter, and any service penetrations.

Get hold of the board manufacturer's fixing guidance