On a commercial job there is existing drywall on the walls. We are trying to avoid demolition of the drywall to install cement board. Do you think we could just apply a liquid applied waterproof membrane and tile over it? Would it be as solid and sound?
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.