Shower base on Plywood or over Hardibacker?

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Mike Rock

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Hi all, getting ready to renovate my small 6X6 master bathroom, and it will be a total gut.
I am putting down new 3/4 Plywood on top of subfloor, but was wondering if I should install the shower base first with mortar directly over plywood, and then install Hardi-backer on rest of floor?
Or should I Hardi-back the entire floor, and put shower base on top?
Its only a 1/4 inch difference in height, and I am thinking it might add more stability to the base if I go on top of the Hardi, but any disadvantages in doing it like this?
 

Jadnashua

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No need to put the backer underneath the shower.

When installing the stuff, make sure to follow the instructions which call for thinset underneath. That thinset is there to fill any minor imperfections so the board is fully supported with no voids, not really to hold it in place...that's what the fasteners are for. ALso, you need to use the alkaline resistant mesh tape on the seams, that can be added as you're tiling unless you're planning to add waterproofing over it, when then means those seams must be taped and cured prior to adding the waterproofing.

Most shower receptors require that you (or highly suggest) embed them in mortar. This does two things, makes the bottom well supported so it can't flex when in use, and provides a means to exactly level the pan so that the walls will fit in properly, and the pan will drain properly. You make piles, not try to make a platform, and you can put some plastic both on the floor and over the piles. That will help keep the moisture from being pulled out of the mix prior to the stuff curing. Note, curing literally incorporates the water into the mix...excess allows you to push it around, and will evaporate eventually. But, drying it out too much means there may not be enough for the chemical reaction to proceed for best strength, thus, the plastic helps keep it in while that happens.
 

Mike Rock

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Thanks, but this is a Dreamline Shower base, and their instructions state not to fasten to the studs, just set in thinset. I already did one bathroom in my Den with a Dreamline, but that was over a concrete slab.
This one is a little different since over plywood, so the thinset is the only thing that's going to keep in place, other than the tiles locking it in, but I did not want to depend on just that.
Also if I put plastic down on floor, would that not prevent the thinset to bond the pan to the plywood? It could still move around.
 

Jadnashua

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If the foam on the bottom is full coverage and flat, then, no, you don't need any mortar underneath, thinset would work. Many shower pans are not flat, and don't sit on the floor fully, so when you step into them, without support underneath, they'll flex. Those require, or at least should have, some mortar underneath. That's not to secure it to the floor, but to hold it up and level so it doesn't flex when you step in. Once you put up the walls, that can often lock things in place. Many have a lip that can be secured to the walls. Placing the finished floor out in the room, would also prevent it from sliding out that way.
 
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