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Sheet membranes and shower curbs

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Ask the Experts Q&As are culled from member inquiries to NTCA’s Technical Support staff. To become a member and make use of personal, targeted answers from Technical Support staff to your installation questions, contact Jim Olson.
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QUESTION Some manufacturers allow a sheet membrane to be continuously wrapped around inside corners. If that movement is such a problem for grout to crack, then what happens to the bonded tile assembly on the membrane when the same movement occurs?

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ANSWER: The sheet membrane is flexible. Tile and grout (even single component grouts) are not.
The change in plane joint – the space between the tiles – when filled with a flexible sealant appropriate for the environment, will allow for structural movement, expansion, etc., when properly installed.
If the corner is hard-tiled (no open joint between the tiles) or filled with grout, then the whole tile system is rigid and becomes subject to the forces and dynamics of structural movement, expansion, contraction, etc. There can be many types of failures such as de-bonding of the tile from the substrate, or perhaps fissures or cracks in the bond coat when there is not an open change in plane joint, or when it is grouted.
If there is a rigid substrate, the change in plane joint must be open there also and for the same reasons, with or without a sheet membrane over the change in plane.
QUESTION I’m a homeowner. On my recently-completed bath renovation, we’ve found that the shower curb collects water and it takes a long time to dry even with ventilation. I’m trying to determine if this condition meets industry standards or if it would legitimately be considered a defect. Someone online recommended NTCA may be able to provide help.
ANSWER The Wet Area Guidelines in the Tile Council of North America (TCNA) Handbook state that horizontal surfaces must be sloped to direct water to the drain. The slope recommended is no less than 1/4" per foot. In the picture you sent, it appears there is a metal end stop trimming out your curb. If that end stop edge is above the edge of the tile it can sometimes act as a dam on curbs that are sloped.
Many shower systems incorporate surface-applied waterproofing that lays right under the tile and can be easily damaged by removing tile. I would suggest tiling over the existing curb tile using a chair rail or some type of specialty trim to maintain the integrity of the waterproofing below the surface. Guidelines for tiling over the existing tile are found in the renovation section of the TCNA Handbook for Ceramic, Glass and Stone Tile Installation in section TR711. Scarification or primers are sometimes needed. Always make sure to use the correct adhesive for tiling over tile in a wet area.
RESPONSE Thank you very much for the prompt and helpful response! Much appreciate the advice to tile over rather than break tile and risk waterproofing damage.
You are right. There is a stainless-steel edge on the curb. It seems to be proud and acting as a dam.