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Unread 02-13-2020, 08:00 PM   #2
Veteran DIYer- Schluterville Graduate

STAR Senior Contributor

Join Date: Jul 2004
Location: Nashua, NH
Posts: 13,485
Do some homework and read the manufacturer's instructions on how to build a shower with their products.

Trying to tie any waterproofing on a wall to a new pan is a tough situation that often doesn't work unless you remove at least enough tile so that it's above the top of the curb.

The type of drain you have now will likely dictate how you can build your shower pan. Not all drains can be used with a surface applied waterproofing membrane.

Spongy could be from the fact that the floor is deteriorating, or, the installer may have used a mastic versus a cement-based mortar. Pretty much nobody recommends their premixed adhesive product to be used on a wet shower pan, but do allow it on a shower wall, depending on the size and composition of the tile.

As to the stone on your floor outside of the shower...that can be problematic, as it requires a substantially stronger structure than normal ceramic AND a two layer plywood subflooring.
Jim DeBruycker
Not a pro, multiple Schluter Workshops (Schluterville and 2013 and 2014 at Schluter Headquarters), Mapei Training 2014, Laticrete Workshop 2014, Custom Building Products Workshop 2015, and Longtime Forum Participant.
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