UK Moisture in floor advice...please

[sparkymarky78]

Hiya, concrete floor under the backer boards. Must be a few voids of tile adhesive under the boards. Couple of places you walk you feel the backer boards compress down a little over a small area. I've just drilled, rawl and screwed one defelction area using the large metal board washers to pull the areas down and that seems to work. Maybe I can do that in the remaining coupe of deflection areas, so that I end up with no defelection but obviously dips in the board from what would be level.

 

[sparkymarky78]

Self level over that (using moisture tolerant slc) to level it out, then ditra then tile I guess.
I wanted electric ufh for the tiles so was thinking electric ufh cables on top the ditra, but guess that could be a bad thing and make humidity worse as whilst warm air holds more water than cold air and so rh would decrease, but obviously warm air also expands and so may make things worse pushing the moisture upwards - but I could very well be wrong. I could only find a post or two on the internet whether ufh makes moisture better or worse, nothing conclusive.

 

[Tile Marble Granite]

I have done similar jobs in the past, brought in a dehumidifier, had it running for 2 weeks to get rid of the humidity, then installed Hardie-Backer on top, then tiled

 

[sparkymarky78]

Was that because you had a freshly laid screed or slab though which just needed to dry out ?
I think in this case if I did that it would only be a temporary fix - this slab was laid in the 1970s so it would have dried out a long time ago. As the moisture vapour in the floor is the same in every room (high) It seems to be an ongoing thing. Outside, perimeter drains may help I suppose but I think it's due lack of or failed dpc from the 1970 (dpc weren't regs until mid 1970's so it may or may not have one.
So dehumidifier would work initially but moisture would just build up again afterwards as it comes up through slab from the ground. Thanks for the suggestion though.

I put a piece of vapour barrier on top of the backer boards the other day sealed to the floor and puy the floor moisture meter on top of that - it read 80 percent without the polythene it and 70 percent with it. So that seems it would help, or the liquid dpm but I know you said that's bad practice on top of the backer boards.
I'm going round in circles haha.

 

[Tile Marble Granite]

The similar jobs I have done were flooded at some point and needed a dehumidifier, walls and floors. To attach the Hardie-Becker to the concrete Pros would use SBR, tile adhesive Rawl plugs and screws. The best bond is done on dry surface.

You know how to get on with the project now. Send us pictures during/after

 

[Dan]

Had this response on electricians forums. Thought I'd post it but I bet this is long sorted.

• Monday at 11:14 AM

Rather than covering the issue, find where the moisture is coming from and address from that end, is the outside ground level above the inside, is this earth or a hard standing, path or road, do your gutters overflow, is there a drain of some kind adjacent to the external wall or even under the slab, these and many others can cause moisture inside a building, if you are sure none of these issues exist then the moisture in the slab can be prevented from rising with an epoxy resin tile adhesive, but this may just move the problem into the walls if not laid correctly, better to find out why and address that.

Just looked on the Tilers forum at the pictures and would suggest that the shower or WHB drains may be the problem, check the shower tray for cracks, find the external connection for both and block the drain, fill the shower tray and WHB with water and see if the level drops.