ASK THE EXPERTS QUESTION I have an outdoor elevated deck approximately 300 sq. ft. We removed the existing ceram- ic tile to reveal a very rough and uneven aggregate concrete slab. This slab also seems to have sunk in the middle a bit. The client wants us to waterproof with a waterproofing membrane system including a liquid-applied membrane, and then install porce- lain tiles about 3/8” thick. Since the slab is pitched away from the house and is not level, should we try using self-leveling material? Skim coat? Apply the membrane and tile all in one step? Please advise. Thank you! ANSWER Exterior decks, especially those over occupied spaces, are very prone to failure and must be con- structed correctly. As you noted, the first concern on this project is the suitability of the concrete substrate. It sounds like the existing slab may be a can- didate for repair with an appropri- ate topping coat or possibly with deck mud. But your description of its overall condition and that it has sunken in the middle might indicate that the original slab pour might not have been done cor- rectly and the supporting structure may not have been able to prop- erly support it. These existing con- Sponsored by Ask the Experts Q&As are culled from member inqui- ries to NTCA’s Technical Support staff. To become a member and make use of personal, targeted answers from Technical Support staff to your installation ques- tions, contact Jim Olson at jim@tile-assn.com. e-Learning Program Industry and product information in short, easy segments Train from the comfort of your home or office anytime Fast and easy to access, listen to, and complete A-8744-0519 ©2019 LATICRETE International, Inc. All trademarks shown are the intellectual properties of their respective owners. Register today at laticrete.com/university 24 TileLetter | June 2019