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Expansion joints/ humidity
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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.
QUESTION I understand I need to have expansion joints in my exterior tile installation. My question is how frequently do I need them, and particularly how wide do they need to be? The customer wants an 1/8" grout joint. Can I make my expansion joints the same size? ANSWER Correctly designing and specifying expansion and movement joints for an exterior tile installation is a critical part that will determine the long-term success of the installation. The Tile Council of North America Handbook for Installation of Ceramic, Glass and Stone Tile includes Method EJ171, which gives guidance for movement accommodation joints. Method EJ171 requires a knowledgeable design professional to specify this part of each tile installation. The minimum allowable width of an exterior movement accommodation joint is 3/8" for joints 8' on center and 1/2" for joints 12' on center. Minimum widths must be increased by 1/16" for every 15 degree Fahrenheit temperature change over 100 degrees from summer high to winter low. An example of this is if an installation can experience a 115 degree Fahrenheit change in a one-year period then an additional 1/16" of an inch should be added to the minimum width. Another way to explain this is where the temperature varies up to 100 degrees Fahrenheit, the joints should be no more than 12' on center based on a 1/2" width joint. The joint width needs to increase the minimum width by 1/16" for each 15 degree Fahrenheit temperature change greater than 100 degrees. For exterior installations, the frequency of movement joints is given as every 8'-12', dependent on the width of the installed movement joint. Other factors must also be considered to correctly design the expansion/movement joint for an exterior installation. Please refer to all of the information in TCNA Handbook Method EJ171 for more guidance. QUESTION Can tile be set over a substrate with 90% relative humidity? ANSWER This RH level likely should not affect the bond of ceramic or porcelain tile to a concrete substrate when using a cementitious bond coat, provided that the substrate and back of the tile are clean and free of any bond breakers, the concrete is properly prepared for tile, and correct bond coat coverage is achieved. For a high RH application 95%-100% bond coat coverage is beneficial. Other considerations will factor in the long-term appearance and performance of the installation. For example, water or vapor percolating through the concrete substrate could result in visible efflorescence in the grout or the body of the tile. A vapor barrier, waterproof membrane or uncoupling membrane will help to reduce the potential of efflorescence. Selection and application of the correct membrane will be critical to ensure it can handle the water or vapor pressure coming from the concrete to which it is adhered. It is beneficial and sometimes necessary to reduce the amount of RH in the substrate. To do this, the source of the humidity has to be discovered and mitigated. Sometimes landscaping, roofline water runoff management systems, etc. need to be designed and installed to redirect the water from flowing to or residing beneath the concrete slab. Water barriers can be installed beneath the slab before it is poured.