Using multiple layers of HardieBacker under Kerdi shower tray.

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Russell Zinck

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Long story short, this is my first home build and I need to raise the floor under my shower tray by 1.75 inches to allow the drain to connect properly. Subfloor is a suspended concrete slab - there is no option to cut into it and change the height now. I have a large supply of HardieBacker 1/4" sheets. Any problem with me layering them until I acheive my desired thickness?
My plan is to use unmodified thinset between the concrete floor and the first layer of HardieBacker. Should I use thinset between each layer? I'm aware that this might not be the best way to raise the floor, but I already have the materials to go this route.
 

Jadnashua

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The easier way to achieve your goal is to use some foam panels. Schluter shows that in one of their videos on shower building. Much easier to work with, cut, and you might be able to do it in a single sheet rather than 5-7 depending on how much thinset you use with 1/4" ones. You do want a lot of coverage, and trying to get a sheet that large to smush down the notches from the trowel into the thinset will be problematic. Hardiebacker is sort of like a big sponge...you MUST wipe it down well with a wet sponge, or it will suck too much moisture out of your thinset and prevent it from flowing. Keep in mind, thinset doesn't need to dry (unless it is a latex-modified one...there are other modifiers that don't need to dry to become stable), cement literally incorporates the water into its chemical structure, and that is how it cures and become stable. It literally grows crystalline spikes that interlock, and that's what causes the bond, sort of like Velcro, except the spikes are rigid, not flexible like the hooks and loops.

If you have some mechanical aptitude, you could forgo the foam pan altogether, and just use deck mud the desired depth to build your pan, then put Kerdi membrane over it. Deck mud is like wet beach sand...you mix it, dump it, pack it, and shape it. It's not really all that hard to do.

If you did it over concrete, you'd do what's called a bonded mudbed. Check out www.johnbridge.com for building your shower.
 

Russell Zinck

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Thanks Jim. I like the simple idea of just using some foam panels, but I have radiant heat in the floor, so that's a no go. It sounds like you would use thinset between each layer of HardieBacker instead of just stacking it, correct? I also want to confirm that you would use a mud bed in this case - even if you had to buy the materials - instead of using the HardieBacker already onhand?
 

Jadnashua

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If you did use the Hardiebacker, yes, you'd need to bond them together, but as I said, the issue is, without screws, trying to get it gap free is much harder. A slant-notch trowel would work better as it ends up making a nearly flat layer of thinset after the notches fall over.

But, it would be easier to just put in a temporary dam across the front, fill it with deck mud, tamp it down, then screed it level. The stuff is really cheap. If you wanted, you could make your pan and forget the foam one. That takes a bit more skill, but it's not rocket science. Deck mud is like wet beach sand that gets hard once it cures...not all that strong, but great in compression when it is contained on the edges.
 
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