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Unread 03-02-2021, 04:07 PM   #2
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Join Date: Jul 2001
Location: Boerne, Texas
Posts: 92,804
Welcome, Zoe.

You wanna do #3. You don't wanna fill or tape any seams that will be covered with Kerdi or similar sheet membrane. The membrane, properly applied, will result in the CBU seams being better treated than with the alkali resistant mesh tape usually recommended.

The CBU manufacturer, any CBU manufacturer, requires the use of a modified thinset mortar (ANSI A118.4) over his product. All other manufacturers of sheet-type direct bonded waterproofing membranes (ANSI A118.10) require the use of a modified thinset mortar with their product.

Only Schluter wants you to use un-modified thinset mortar (ANSI A118.1) with their product.

You gotta violate somebody's recommendations when installing Kerdi over CBU. I recommend you use a modified mortar for your entire project. See my warranty information below.

My opinion; worth price charged.

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