Although admired primarily for their beauty, ceramic tiles have always been perceived – falsely – as a waterproofing material. Ceramic tiles are not affected by moisture, which makes them the ideal finishing material in wet areas. But tiles are only the protective covering – not the waterproofing.

There are other common misconceptions regarding ceramic tile installations and moisture. One of them is that the use of latex grout eliminates the need for a membrane in wet areas. There is no question that when properly mixed, applied, and cured, latex grout is effective at slowing the penetration of moisture, but neither latex nor regular grout are effective waterproofers.

Another misconception is that latex thinset mortar can substitute for a waterproofing membrane. Even when properly mixed, applied, and cured, both dry-set and latex-modified thinset mortar are adhesives, not waterproofing membrane.

Waterproofing membrane systems can be highly effective moisture control materials, but the use of a membrane, alone, is not enough to manage moisture in several key areas:

• The porosity of the tiles being installed in a wet or exterior area
• The density of the grout framing the tiles
• The adhesive contact between a tile and the setting bed
• The integrity of the sealant filling the movement joints.

Waterproofing membrane systems are available as a sheet or mat, or they may be built up from a variety of gels, pastes, liquids, and reinforcing fabrics. Regardless of its composition or form or cost, a membrane can be wasted if the above aspects of the installation are faulty.

Tile Selection

The most obvious part of a wet-area installation is the tiles. In addition to a variety of colors, designs, and sizes, tiles are also categorized by their rate of absorption.

Impervious tiles, with an absorption rate of .5% or less, are ideal for wet area use because they do not hold water – a leading cause of mold and mildew. Non-vitreous tiles, because they can absorb 20% moisture, can become so water-logged that they are able to wick moisture upwards.

Non-vitreous tile bodies, once saturated, can harbor mold and mildew that is almost impossible to remove. Non-vitreous tiles installed in an exterior location can also harbor mold, and in a freezing climate, are subject to spalling and delamination. Some stone tiles can absorb more than 20% moisture, putting them in the non-vitreous class, and unsuitable for wet areas.

Membrane Selection

Once it is encapsulated, excess reinforcing fabric can be cut away.

Not all membrane systems are created equally. Some may be designed solely for providing light resistance to moisture, while others are rated for heavy-duty, full-immersion installations demanding the highest levels of protection. There are two types of moisture control membrane for use with tile: sheet and liquid-applied.

Sheet membrane systems consist of a reinforced sheet that is laminated to the setting bed surface with an adhesive.

With liquid-applied, a reinforcing fabric is attached to the setting bed with a paste, gel, or liquid and built up with one or more applications of the liquid component.

Make certain that the system you intend to use is rated for the job and installed according to each manufacturer’s instructions.

Adhesive Layer

This sheet membrane is factory-made with integral reinforcing.

Because porous, dry-set thinset mortars can absorb more moisture than latex mortars, they are not the best choice for wet-areas such as showers or tiled kitchen countertops. But composition is less important than how the thinset mortar was prepared, installed, and cured.

Thinset mortars (dry-set or latex) mixed with too much liquid lack their anticipated strength, and are more porous than when mixed properly. Mixing is important, but even the best thinset mortar can cause moisture-related problems if it is not applied correctly.

For wet areas, the industry standard is 95% uniformly distributed adhesive contact. When this coverage is not attained by the installer, the result is a honeycomb of voids beneath the tiles. This reduces the tiles’ compressive strength, which is another problem, but it also allows moisture to collect where it can attract and harbor mold.

Thinset mortar is the choice of professionals but some installers (architects and general contractors, too) prefer to use organic adhesive, sometimes called mastic. When most tile mastics were solvent-based, moisture was not a big issue, but with latex-based mastics, especially when used to install large-format porcelain tiles, exposure to water can have serious consequences. First, the amount of adhesive needed to properly adhere and support the tile is so large that mastic may not completely cure beneath the tile, leaving a soft spot that has a weak compressive strength. Second, because the mastic may not be completely cured, it is subject to re-emulsification: the mastic goes from hard to soft and useless.

Grout

Voids under a tile admit moisture and harbor mold.

Grout is the first line of defense against water intrusion into the adhesive and setting bed layers and yet few installers (architects and general contractors, too) understand the function of grout, which is to protect tile edges, and slow down the penetration of moisture. Grout that is stored, proportioned, mixed, packed, cleaned and cured properly offers high water resistance, but when one of these steps is done poorly, weak, porous grout is the result. The process of installing grout is not for sissies. It requires hard work, concentration, and consistency to produce good grout. Of course, the addition of latex (instead of mixing water) increases the strength and performance of grout, but latex grout, too, has to be done correctly in order to achieve that performance.

Prior to grouting, each joint should be open and clean to a depth equal to two-thirds of the tile’s thickness. Grout has to be packed into the joint, rather than just swished in lightly. The absolute bare minimum of water should be used when cleaning grout. Too much exposure to water can weaken grout when it is in its plastic state.

Finally, grout must be damp-cured for its full strength and anticipated properties to be achieved. Grout that has been improperly cured powders, cracks, crumbles, and is easily stained.

Like any chain, the strength of any tile installation, and its ability to shed moisture, is dependent on each link.