What thin, rigid wet area backer for sheet vinyl flooring?

Nickoh

Working out the layering for an ensuite floor waterproofing system. After slab was poured and cured, I have been left with 4mm less height difference between the new slab level and adjoining floor level along the doorway edge than planned, so the floor system that I had planned would result in a wet area floor higher than the adjoining bedroom floor. For obvious reasons, that's a non-preferred solution.

Original plan was:

3~4mm tile adhesive on slab

4mm Wedi board waterproofing layer (integrates with waterproofing in the rest of the room)

3~4mm tile adhesive

6mm villaboard (rigid layer over the wedi board, required in order to use "soft" floor finishes such as sheet vinyl)

~1mm vinyl adhesive

2mm sheet vinyl flooring

Looking to shave that wherever possible to somehow save my 4mm. The obvious places are reducing adhesive thickness by changing trowel notch size and subbing out the villaboard. Alternative rigid layers I have come up with are:

 - 4.5mm HardieFlex. It is unclear whether the 4.5mm HardieFlex sheets have the same moisture tolerance as the slightly thicker Villaboard (this layer is above the waterproofing membrane, so needs to tolerate moisture).

 - 3mm PVC sheet. It's a bit ridiculous, but considering using 3mm PVC sheets (eg. Suntuf Handisheet ). Utterly water stable, thin and the wedi-PVC bond could be a thinner adhesive layer, but obviously not a "standard" component of a flooring system so unsure whether vinyl flooring adhesives would adhere to it. Maybe if roughened with sandpaper first? Also suffers from thermal expansion cycles and would be incredibly fiddly to install.

Otherwise, I have to redesign the entire waterproofing system for the room. Given the incredibly high failure rate for conventional liquid membrane waterproofing systems under even ideal circumstances, and a few features of this build that make it less than ideal conditions already, options there are limited. I have two backups I'm working on, but neither is preferable.

JacobZ

Hi @Nickoh,

Having a look at the system you are looking at using, unfortunately, due to the limitations on materials and the incredibly tight margins, it doesn't sound like altering any of the layers is really an option.

Unfortunately, James Hardie would never recommend using HardieFlex, which is typically used as an eave lining, as part of a wet area waterproofing system. Realistically, you shouldn't even be using Villaboard, which is a wall and ceiling liner. The James Hardie Ceramic Tile Underlay Flooring is what you should be using.

I would also not recommend the PVC sheets as they are extremely rigid and prone to cracking.

Have you considered having the concrete ground back by 4-5mm so that it sits at the correct height for your system?

This is the only solution that I can see working with your current system.

Allow me to tag @Nailbag, @Dave-1 and @TedBear to see what they think.

Jacob