What bonded screed over Hardie fibre cement sheet?

Question

Hi there,

I've re done my bathroom subfloor and installed James Hardie Secura fibercement sheets (22mm) (following all manufactures indications) Now I need to continue installing screed and tiles.

I have limitation on the height, so I want to minimise the screed thickness, hence trying to go with a bonded screed.

However, I noted that Ardex only recommend un-bonded screed for FC. Do you know of other manufactures who recommend bonded screed? (and which prod are compatible with JH Secura?). If none, how risky would it be to go ahead witha bonded one?

The bath dimension is 1.8x1.8 mm, not a huge area and the subfloor and joist are quite solid so i expect minimal movement (if any) of the sheets)

Thanks,

Martin

Nailbag · Answer

Hi @MartinM

I would definitely agree with @JacobZ on not using the bonded screed. It's very risky going with anything thats not compatible especially when talking sub-tile substrates. This has the potential for a flow on effect to waterproofing, and then things get very serious. Should anything fail down track then both product and insurance claims would be void. I would go notching the joists (assuming you're not on a slab). Its not a hard job and will solve the issue of height and allow for a walk in (sill-less) shower floor.

Nailbag

JacobZ · Answer

Hi @MartinM,

I’d be pretty cautious going with a bonded screed in that setup, especially if the manufacturers are not supporting it.

If a bonded screed over FC sheeting does crack or debond, the main issue is what happens on top of it. Any movement or failure in the screed will usually transfer straight through to the tiles, so you can end up with cracked tiles, loose tiles, or grout lines opening up. In a bathroom, that also creates a risk for waterproofing failure, which is where things start to get expensive.

Once water gets past the tile layer, it can affect the substrate, cause swelling or movement, and potentially lead to ongoing issues that are not easy to fix without pulling everything up.

Even in a small area, those risks are still there. That’s generally why manufacturers push toward unbonded systems over FC, as they help isolate movement rather than transferring it.

Given the consequences, it’s one of those situations where it’s safer to stick with a system that’s actually specified, even if it is a bit more limiting on height.

It might be worth looking at other ways to reduce height, such as notching the joists if reasonable to do so or even accepting a slightly raised finish if it means sticking with a system that’s known to perform.

Allow me to tag @Nailbag to see what he thinks.

Jacob