Ceramic Tile Failure
DEAR TIM: While cleaning the tile above my bathtub yesterday, several tiles in the bottom row appeared loose. After investigating further, they actually fell off the wall. Behind them was what appeared to be drywall or plasterboard with a green paper. What puzzles me is that none of the grout was cracked or missing. After talking to some of my neighbors, I come to find out that this problem is widespread in my subdivision. What can I do to get my ceramic tile to adhere as in my parent's 65 year old house? F. R.
DEAR F. R.: You are not the first person to suffer this fate. Nor will you be the last. The root of your problem, as you have probably already determined is water. Water simply got behind the tile and was absorbed by the paper covering the drywall. After a period of time, the gypsum core softened. This caused the bond between the paper and the gypsum to fail. You can clearly see in your photographs that pieces of the green paper are still attached to the back of the tile.
Your parent's house has what is often referred to as a mud and metal lath system. This was a very popular method used to attach ceramic tile to vertical wall surfaces. This system involved nailing metal lath identical to that used by plasterers to the wall studs. The tile setter would first apply a thin coat of cement mortar to stiffen the wire. On the second day, he would return to apply a second coat of cement mortar. This coat was approximately three-quarters of an inch thick. After this layer of mud had set up, a thin cement paste was then used to set the individual pieces of ceramic tile. Water that gets behind this tile will not dissolve the cement paste or mortar. The bond between the tile and mortar bed is very permanent.
The problem with this system, however, is that it is extremely labor intensive. While there are tile setters who can still install tile in this manner, very few people can afford the cost. This is where your green paper drywall and some other products enter the picture.
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The green paper drywall is a product manufactured by gypsum manufacturers. It is marketed as a moisture resistant drywall. The gypsum core is treated with special chemicals that make it moisture resistant. The green paper simply tells installers that it is different from ordinary drywall. Note that there is a big difference between moisture resistant and waterproof.
There are two other types of products that are available as well. One of them has a gypsum core that has been treated with silicone. This gypsum core is then surrounded by an inorganic glass mat. The mat, on one side is then surfaced with a co-polymer coating which is both water and vapor retardant.
The other product is referred to as cementitious board. These products use the same cement mortar as described above. However, it is sandwiched between thin layers of fiberglass mat.
All three of these products can be installed in a fraction of the time of the old fashioned method. The key to successful tile adhesion lies in installing them properly. These products must not be allowed to come into contact with the top ledge of the bathtub. Water sitting on the top ledge of the tub may come in contact with these products. This water can be easily absorbed by some of the products such as your green drywall. This water can cause numerous problems such as the one you experienced.
The Tile Council of North America recommends that you should leave a gap between the bottom edge of these products and the tub. This gap should then be caulked with an elastomeric caulk such as mildew resistant silicone. If you should use green drywall, be sure not to cut off or trim the factory paper-bound edge. After the tile installation is complete, the gap between the tile and the tub should also be caulked. Do not install grout in this horizontal gap!
Grout can also be very misleading. Many people think that grout is water retardant. In fact, it is the opposite. Untreated grout can and does absorb water. Always treat your finished grout job with grout sealers.