Bonded screeds on Hardy Ceramic Tile underlay?

[TryingMyBest]

I am trying to retrofit an open-plan bathroom into an old timber floor house. I need to create falls but want to minimise screed thicknesses. Can I bond Australian Builder's Tile and Paver Screed to Hardy Ceramic Tile underlay to create a "bonded screed"? What method or product would I use?

Can I add in either a Dunlop or Bastion waterproof membrane in between the Hardy Ceramic Tile underlay and the screed, and still achieve the same level of bond?

[EricL]

Hello @TryingMyBest 

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about bonding tiles to screed and tile underlay.

I'm currently unaware of a screed that doubles as a tile adhesive. I know that screed needs to cure for several days before you tile over it, otherwise the trapped moisture will simply pop the tile off. If I recall correctly the waterproofing membrane is the last layer to go on top of both the tile underlay and the screed. Perhaps you can tell us more about the conditions of your bathroom project and we can provide recommendations in regards to screed, tile and adhesive installation. 

Let me call on our experienced members @Nailbag, @Dave-1 and @AlanM52 for their recommendations.

If you have any other questions we can help with, please let us know.   

Eric

[Nailbag]

Hi @TryingMyBest 

There are no shortcuts when it comes to bathrooms. Here are the layers:

1. Floor base timber or concrete slab

2. Tile underlay if on timber floor

3. Screed for walk-in shower base

4. Waterproofing membrane

5. Tile adhesive

6. Tiles

Nailbag

[Dave-1]

Good Morning @TryingMyBest 

I havnt had a lot of experience in screed or tiling I must admit. The steps that @EricL and the list that @Nailbag has specified are the ones that keep things in line, "There are not shortcuts when it comes to bathrooms" That @Nailbag has said is perfect Anytime I hear of someone thinking of a workaround I imediantly think of the issues it raises. And I am one of the people who has  and keeps on having ideas for workarounds for my bathroom It may seem easier but nope, stick to that list and it works best.

Dave

[TryingMyBest]

Thanks @EricL, @Nailbag and @Dave-1 ! 

As far as I understand bonded and unbonded screeds have different design requirements. Bonded screed can be as thin as 15mm, while unbonded screed must have a minimum thickness of 40mm and embedded reinforcing mesh. Usually bonded screed is done on a concrete slab substrate using a cement slurry as a bonding agent.

I have a timber substrate, not a concrete slab, so I'm trying to figure out if there is a set of products I can use to put down a bonded screed onto fiber cement sheeting - i.e. Hardy Ceramic Tile Underlay. If that's not possible, I will have to go for an unbonded screed but the required thickness creates other issues with my design. 

Waterproofing is obviously a big concern in timber structures so I would like to do a double up - one membrane under the screed and one over the top. I'm aware under-screed waterproofing is not common practice in Vic, but I believe they do it in other states.

Any further thoughts on achieving a bonded screed on a fiber-cement sheeting substrate would be greatly appreciated!

[Nailbag]

I can’t offer anything further sorry as I don’t have any first hand experience with bonded screeding. 

As a fellow Vicco and with several bathroom Reno’s ticked off, as long as the top layer waterproofing is done correctly, then anything else is overkill. Plus the barrier is lost with every brad you put through it from the tile underlay. But if you really wanted extra insurance add another top coat. 

Nailbag

[MitchellMc]

I'd agree with @Nailbag here, @TryingMyBest. A membrane is used to collect and direct water to a drain outlet, like that in the shower area or a floor drain in the bathroom. Adding a second membrane under the standard could collect any water a failed membrane lets through, but it will then just hold it, which could cause issues. If your membrane fails, that really needs to be addressed in itself; a secondary membrane, just in case, is not really something that I've seen done.

If you are concerned about the timber flooring, you should look at installing compressed cement flooring sheets over it or replacing the timber flooring with them. You can then use polyurethane like Sikaflex 11FC to seal the cement sheets together.

Also, you might need to enlist the services of a professional waterproofer to install the membrane and sign off on it. If that's the case it would be best to run these questions by them for their professional opinion.

Mitchell