Waterproofing is an extra step.
Waterproofing does not happen because tiles are used, but because a waterproofing system is planned as a part of the overall tile installation. The purpose of this article is to examine how people use wet-area installations, what is allowed in the industry, and how you can eliminate leaks and problems. It is also important to understand how and when many of our current standards were developed, and why we may have to take a second look at our waterproofing perspective.
In 1968, one of the first things pounded into me by my first tile boss-a tile repairman-was that only mortar bed installations were any good, and that all thinbed installations were doomed to fail. He conveniently failed to notice that 95 percent of our work involved repairing mortar bed installations that leaked. What I noticed was that some mortar bed installations fared better than others, and that some thinbed installations gave years of reliable service. After starting my own business, I realized that what mattered was how an installation was detailed rather than what materials were used in construction, and how craftsmanship plays a pivotal role in the durability of any tile installation. More on that later; for now, let's jump to 2005.
There are now more bathrooms per household and less sharing, with higher capacity water heaters and even multiple water heaters in some homes. Instead of a quick 7-minute shower, many people plunk themselves down into the corner shower seat and stay there until all the hot water is gone. And because we are all so busy multi-tasking, many don't take the time to mop up spills or get the wet towels off the floor. The result is that some installations do not dry out between uses. This situation does not sit well with traditional waterproofing schemes which only have to shed water. With limited exposure to water, the old ways worked OK, but with today's water usage patterns, waterproofing should be active. Normal use today might have been called extra-heavy duty 40 years ago.
For example, the traditional way of providing waterproofing for a cement backer board tub shower installation (yes, cement backer boards need waterproofing) is to install either a 4-mil plastic sheet or ship-lapped layers of tar paper over the studs before the boards are installed. In addition, industry standards call for beads of sealant in the gap where the sheet or paper meets the tub, and in the gap required between the first course of tiles and the tub. Sounds good on paper, but in reality, installing the boards results in about 150 holes in the waterproofing, and the sealant beads some times dam water that has penetrated the tiles and grout. Trapped moisture sets the stage for the growth of mold.
A much better approach is to replace the plastic and paper behind the boards with a trowel-applied or sheet membrane system mounted on the surface of the boards. This covers all the fastener holes, and isolates the boards from moisture. With no industry standard for using plastic or paper sheets mounted behind backer boards, this is the only way to waterproof a floor installation, and it is the only waterproofing recommendation I make for thinbed work. Another option is to use a gypsum tile backer board with a factory-applied waterproofing membrane, or a foam tile backer whose core itself is waterproof.
Surface-applied waterproofing is more effective even on mortar beds.
Fastener holes through plastic film or tar paper cleavage membranes, resulting from the mounting of reinforcing wire for mortar bed installations, can also allow moisture to pass through to the structure. Mortar beds, like cement backer boards, can absorb moisture in heavy use even if they are produced with latex. For maximum protection, instead of relying only on the cleavage membrane required behind the mortar bed, I cover the finished and cured mortar bed with a surface-applied membrane system (see Photo 1). With this strategy, I can begin any tile installation knowing the setting bed has been isolated from exposure to moisture, but I can still greatly improve on this by installing the right tile, grout, and adhesive mortar.
Tiles and the installation
The best tile to use in a wet area is an impervious tile, the body of which can absorb no more than .5 percent moisture. With less moisture to absorb, impervious tiles do not provide a good home for mold or odors. The low absorbency theme should continue with the use of high-solids latex (or acrylic) thinset mortar and grout rather than regular mixes. High-solids means there is more latex and less water. This is important because quality liquid admixes or factory-blended dry acrylic thinset mortars and grouts are less permeable than non-latex mixtures. I avoid pre-mixed thinset/grout materials. Finally, to get the most life from your waterproofed installation, protect the membrane system, and to satisfy installation and warrantee requirements, make sure you build a network of movement joints into the installation (see 2005 TCNA Handbook, page 68-69).