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Blistering tile and priming drywall with liquid waterproofing membrane
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Ask the Experts Q&As are culled from member inquiries to NTCA’s Technical Team. To become a member and make use of personal, targeted answers from NTCA's Technical Team to your installation questions, contact Jim Olson.
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QUESTION: Can you prime drywall with liquid waterproofing membrane before tile?
I have been using TCNA B431-20 for shower receptors for years, although I am beginning to run into shower designs that require a topical system based on material availability. I am planning a larger shower than I’m used to; is it acceptable to combine TCNA F141-20 and F145-20 in the same application? I've attached a detail of the specific project for clarification.
ANSWER It is possible to prime drywall with a liquid waterproofing membrane. Doing this takes the place of dampening the substrate with water to create a Saturated Surface Dry (SSD) condition and helps ensure the thirsty drywall does not pull the hydration out of properly-mixed thinset/bond coat, making it cure too quickly. Please contact the manufacturer of the membrane you are planning to use to make sure you are choosing the correct product for this application and to make sure you understand how to properly apply it.
You didn’t ask about this, but this is very important: Please take a look at your up-to-date edition of the TCNA Handbook Methods W243-20 for ceramic tile and W243-20 for natural stone tile. These are the methods for installing over gypsum board on an interior wood or metal stud framed wall. Besides tiling over gypsum, a number of other conditions need to be met, such as a maximum of 16” on-center stud spacing and specific requirements for metal studs in commercial applications. You will find these in the “Limitations” and “Requirements” sections of the method.
In the “Preparation by Other Trades” section, Method W243-20 for Stone refers to the Gypsum Association standard GA-216, which describes the requirements for installing gypsum board when it is to be used as a substrate for tile. This section outlines the how the gypsum board joints and fasteners are to be treated with tape and only one coat (bedding coat) of joint compound. It does not allow for texture or paint on the drywall.
GA-216 and GA-214 also describe the fastener schedule for drywall that is to be tiled. Fasteners are to be placed more frequently than for other finishes, and the fastener heads must not penetrate the paper layer. This is necessary to properly support the tile installation. I suggest you check with the gypsum board installation contractor to make certain they installed the drywall per GA-216 and GA-214 to properly support your tile installation.
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QUESTION: What causes blistering tile over existing terrazzo flooring?
Attached are two pictures from a project. Both are on-grade over existing cement terrazzo flooring. Obviously, one has spot bonding. The other one, well, I have never seen before – literally the tile has blistered upwards. A video of the project shows that it still sounds solid in most of the areas, but you can hear hollowness in a 1/2″ area.
Have you ever seen a glazed product do this? Can spot bonding shrink down so hard at the perimeter of each blob to literally bend the tile?
We will remove some pieces and a few more to verify the substrate when we are there onsite. I believe it was a two-part thinset. I will place a straight edge across it to see if I can see light. I suspect the mortar pulled the tile down. I do not know if it’s porcelain or not. The distributor, I think, that supplied it is reputable though.
No one is being blamed. The owner is redoing the 12,500 sq.ft., but hoping the terrazzo is salvageable.
There is some concern as maintenance department replaced several tiles, and mentioned it smelled damp.
ANSWER The Technical Team has been having a few discussions and thinks this looks like agglomerate tile. We think these are the sorts of issues that could occur with the agglomerate tile when spot bonded or with voids in the bond coat. And the damp smell could be an indicator of it being wet from who knows what, causing the tile to bubble. One of our team has seen sort of similar issues on large agglomerate installations.
Thanks for sharing this with us and for keeping us updated! It helps build our knowledge base too!