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enter image description hereI recently purchased a solid surface pre-formed shower base. The shower base will be placed on 3/4" plywood. The manufacturer's instructions indicate that I should set the base in thinset. I e-mailed the manufacture and they indicated any thinset will work but unmodified is preferred. I've read that I shouldn't adhere the shower base to the plywood because the substrate will move and swell and contract. The shower base is very solid and doesn't seem to have any flex and the plywood is flat. The shower base will not be attached to the studs so that the thinset would be the only thing keeping it in place (other than gravity, it's heavy). I bought Mapei uncoupling unmodified thinset but wanted some feedback. My questions are as follows:

  1. What should I use to set the shower base? Thinset, bedding mix, something else?
  2. Should I put tar paper down on the plywood before putting the mortar down so that the plywood doesn't rob the mortar of its water and/or to provide a sort of cleavage membrane?
  3. How thick should the mortar/thinset be on the floor?
  4. Any other advice?

Thanks.

Dean
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    Follow the manufacturer's instructions to the letter. Otherwise you may invalidate the warranty or such. – gnicko Jan 20 '23 at 01:02

2 Answers2

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You need an unmodified thinset as the manufacture stated. What you purchased will be fine. Clean the plywood and be sure any nails or screws are set. The thinset is there to make the base immovable and to fill any dips or unevenness in the floor. You can apply the mortar directly to the plywood. However if it didn't cause other issues with the room I would screw down Durock or hardibacker over the plywood.

If the base is surrounded on 3 sides with walls it cant move very much. A base of 1/2 inch thinset is good. If it is a freestanding shower with something like glass on the sides you might want to add some extra thickness for the mortar to fill higher into the cell grid. In any case don't make the thinset so thick that it is deeper than the depth of the base cells

RMDman
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  • full half inch or use a half inch notched trowel? – Jasen Jan 19 '23 at 03:22
  • Again depends on the situation. with 2 or 3 walls , a 1/2 " notched trowel is good. If it is totally free standing , I would make a bed of thinset at least 1/2 inch thick. – RMDman Jan 19 '23 at 13:14
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Home depot along with this old house has a short but informative video here:https://www.youtube.com/watch?v=W8tGzK-OAeA

Note: Make sure the base is level when in place by dry fitting your pan before laying thinset.

They suggest: Prepping the plumbing as much as necessary before setting pan in place. Cutting out the floor exposing the subfloor and tacking down tar paper to the subfloor to prevent the thinset from traveling through cracks of the boards if it is not plywood (this will depend on your floors design whether or not you have 2 layers of flooring and if you want to cut out the top layer). Then lay 1" of thinset and set the base in it.

Always remember with plumbing to think 10 steps ahead. It is much easier than having to redo!

keshlam
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SOHR
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