Most of our home inspections completed by Bay Area Mold Pros include an attempt to determine if moisture is present behind bathroom wall tile. Moisture often makes its way past wall tile, often through the space between the tile when grout is missing, or when the grout was improperly applied, or was never properly sealed. Homeowners are often surprised to learn that conventional grouts aren’t really waterproof, unless they have had a proper grout sealer applied, and re-applied when necessary.
Once water has made its way past the tile, it contacts whatever tile “substrate” was used. For decades, this substrate was made of mortar, and was “floated” onto the wall, generally onto chicken wire that had been nailed with spacers over some type of building paper. In the San Francisco Bay Area, this building paper was often the same “tar paper” used for roofing, and was also “asphalt-impregnated building paper” that was often used under exterior siding and stucco. These mortar substrates often lasted for decades, and indeed, many still survive throughout the area. Our family home in San Francisco, built in 1936, still has its original Art Deco bathroom wall tile in both the shower and above the bathtub. All of this tile was installed over a mortar substrate.
But mortar substrates require a certain skill level to apply them properly, and many modern tilers prefer to install tile over other simpler (and less costly) substrates, such as cement board and Hardi-board. When properly installed and waterproofed, these newer substrates will last for years, and although not as stable as mortar, they are very stable. But starting in the 1950’s, many builders started using standard drywall as a tile substrate, and over the ensuing years, these tile jobs have mostly failed. My wife and I purchased a home in the San Francisco Bay Area in 1980, and the shower tile was so “mushy” near the shower pan that you could move it by simply pushing on it with your finger. I gutted the bathroom and determined that the tile had been applied directly over 1/2″ drywall, which was completely saturated with water was falling apart.
I replaced the original 1/2″ drywall with a product that was fairly new in 1980, “green-rock.” This 1/2″ drywall was faced with green (rather than white) paper, and did not have the standard gypsum contents present in standard drywall. At the time, lumber yards and the newly introduced “big box” stores were selling this green rock as “waterproof drywall.” You can see where this story is headed, within a few years, this green rock had failed, and the shower needed to be gutted again. Several years went by and green rock was replaced by “purple rock,” which was again touted as waterproof drywall. But neither purple rock nor its predecessor, green rock, were ever meant as tile substrates, and should only be used as backing behind a properly installed tile substrate.
The wall tile depicted in the photo above was present in a home that I inspected last week in the San Francisco Bay Area. The lowest courses of the wall tile were loose, and when I pushed on one of the tiles, it simply fell off the wall, revealing what I suspected was behind it, standard drywall. Once water has made its way past the wall tile and comes into contact with the drywall, the drywall will suck up the water like a sponge, and will then fail. Mold loves paper, and because drywall has a paper face on both its front and back side, it makes a perfect host for mold growth. So when drywall tile substrates fail, it is very common for mold growth to occur.
So the lesson here is straightforward, never allow wall tile to be applied to any type of drywall in a water-setting, which includes shower enclosures and the walls above bathtubs. These installations require proper tile substrates and proper waterproofing. They also require appropriate sealing of the grout, and proper maintenance, which may include re-application of the grout sealer. And if the tile surface is properly maintained, it can last for many years. The tile in our family home is now 84 years old…and is still going strong!