When I started construction as a teenager I had no idea you had to waterproof the wall behind the tile. I didn't go to school to be a contractor - I mean who does? Is there even such a thing? Over the years, I ripped out countless bathrooms and most of the time I would find attached to the studs: cement board, thin set, and tile. If the contractor or builder was good, he used the special cement board screws that have a coating to resist corrosion, a special wide head with cutting flutes, and hi-low threads for a strong grip. A lot of times it would be ordinary drywall screws. As an alternative to cement board I've seen backer board used but this always seemed like the DIY remodels.
Fast forward many years, I became a licensed contractor and with the best intentions, I used cement board as my wall sheathing (backing panels) too. Then I started doing research. While tiles themselves seem to be waterproof they are not necessarily non-porous and moisture can get behind them. Additionally, grout is generally porous so it allows moisture in. Hence, manufacturers began mixing epoxy and other chemicals into the grout to make them non-porous.
Before I was a contractor I worked for several years as handyman in a luxury residential building with 500 units and began to think of all the leaks we had there. It was rarely the plumbing itself (water supply, waste line) as the leaks were almost always caused by poor workmanship in the installation phase or bad maintenance. Most of the leaks were caused by water splashing outside of the bathtub and finding its way under the floor tile or between the tile and bathtub.
What happens to the sheathing and THE REST OF THE HOUSE when water gets behind the tile and grout? Water can cause thousands of dollars in property damage.
Where the tiles meet the bathtub or shower pan, and in the corners where tiles butt to one another, those seems need to be caulked - not grouted - and the caulking needs to be maintained over time. There are specially formulated caulks that match the color of the grout for this purpose.
Additionally, the tile around the shower trims are never cut perfectly around the roughing and there are literally gaps in the tile and sheathing behind those finish trims so it is imperative to silicone around the finish trims; all of them need to be sealed. There are engineered prefabricated waterproofing seals with over-molded rubber gaskets designed for this purpose to protect moisture-sensitive backing panels but who uses them, right?
As a licensed contractor I was hired not only for remodels but at times we were also brought in for repairs where the tiles were literally falling off the wall. Even with remodels, but especially in every repair job, I found so much water damage and mold behind those tiles. In some cases, the studs and insulation were completely rotted out. It was evident that the leaks were going on for many years, probably from the first day the bathrooms were built. Sometimes the leak becomes visible in finished rooms below or adjacent but some of them aren't discovered until we rip out the bathroom to update it. In every case the sheathing was cement board, backer board, or just plain drywall. There was no waterproofing.
I was introduced to RedGard. It goes on like paint with a brush and roller right over the cement board to seal all of the seems and screw-holes. CONS: it is expensive and stinks. You need throwaway brushes and naps and wait for it to dry before installing tiles. PROS: it was better than nothing. I did a small remodel and used this method and three months later got a call from the client that there's a leak in her kitchen below the bathroom. I couldn't believe it because I used RedGard. The shower was spraying water directly at the caulk line where the tile met the shower pan and it broke the fresh caulk and water got under the tile and found its way through the cement board that was covered with RedGard. FAIL.
Never again was I going to use cement board and RedGard. I began using a product made by Wedi which is lightweight and waterproof. Amazing product. The seams and all the screw-holes are covered with a special sealant.
An alternative, which is probably even better than Wedi, is Kerdi made by Schluter because it is more versatile.
Whether you are going to do your own bathroom remodel or hire a professional make sure to use the best methods and materials available.