Waterproofing should extend around the edge of openings.

You would think a material that is unaffected by exposure to water would be the perfect finish for wet areas. While ceramic tile is not harmed by water, some types of tile will absorb moisture. Many tile installation materials absorb or transmit water, and many installation practices can actually promote water penetration and the subsequent damages this causes.

One of the keys to eliminating water-caused problems is to remember that throughout its long history, ceramic tile has never been used as a waterproofing material, rather, it has been used effectively as a durable covering for the underlying materials that provide the actual waterproofing - a fact well documented in Bath, England, where some tiled public baths have been holding water since around 50 A.D.

In 1968, I began my tile education as a helper to a tile repairman who specialized in repairing leaking mortar bed shower stalls. At the time, one thing not immediately apparent to me was that all tile installations were made up of several materials - not just tile - and it is this combination of materials’ properties that provide either good or ineffective waterproofing. Everyone in the trade told me that mortar bed installations do not leak, but the only showers I was fixing were built around a mortar setting bed. This contradiction started me on a journey to discover the best waterproofing methods and materials. We will start with tile itself.


Much history has been handed down to us because events were depicted on glazed tiles that adorned public and ceremonial buildings. There is no doubt that tile is durable, but not all tiles are the same and not all offer the same properties.

There are four absorption categories for tile:
  • Non-vitreous (7 percent or more)
  • Semi-vitreous (3 to 7 percent)
  • Vitreous (0.5 to 3 percent)
  • Impervious (>0.5 percent)
The absorption is caused by microscopic voids in the tile body. When these voids interconnect (as they do to varying degrees in all but impervious tile bodies), not only will the tile absorb water, but water can also be transmitted through the body of the tile. For this reason, impervious tiles make the best choice for wet areas, and the only reasonable choice in food service or preparation areas. Unlike non-, semi-, and vitreous tiles, impervious tiles are not affected by freezing conditions, nor will they harbor mildew or mold.

The author paints waterproofing paste around the edge of a mixing valve opening.


The level of exposure to water is the determining factor governing the selection of tile and installation materials. Obviously, a tiled deck built over a living space requires far greater waterproofing than the floor tiles in a guest powder room, or a steam shower at a spa compared to a tub surround used by one occupant of a small home or apartment. It’s important for durability and ease of maintenance to factor usage and exposure into your calculations for the best installation.

For my work, I develop specifications largely on the following: Does the tile installation have to retain water (pond, pool, etc.)? Is the installation exposed to the elements? Is the installation heavily used? Is the installation so lightly used that it easily dries out quickly between uses (or after cleaning)?

Just as there are differences in the level of exposure to water, there are differences in the way tile installations can be designed and built to handle varying degrees of exposure – not all wet-area installations need maximum waterproofing protection, but all need careful attention to details, regardless of the level of protection. An honest assessment of the anticipated usage is essential: One user or many? Work boots or slippers and bare feet? Decorative or functional?

The author embeds a reinforcing flashing strip with waterproofing paste.

Installation Materials

Some tile installation waterproofing materials only offer limited protection, and are completely ineffective when tasked with higher levels of exposure. For example, there are several trowel-applied membrane systems designed for light-duty waterproofing that are incapable of passing a submerged 24-hour water test. Nevertheless, these materials are routinely specified as shower pan membranes. A common misunderstanding among designers, specifiers, sellers and buyers of tile installation materials is that the addition of natural or manufactured (acrylic) latex renders grout and thinset powders into waterproofing materials. There are various forms and concentrations of latex, and each brand offers properties unique to that brand only - not all latex admixtures are the same!

Another misunderstanding confuses water-resistant to waterproof. For example, most cement backer boards are water-resistant in that they can be exposed to water or moisture without being damaged, but like other types of tile backer, and many mortar beds, they are capable of absorbing water and transmitting it from the surface of the installation to the surrounding structure where damage can occur. Water-resistant materials are important, but only when used in concert with an overall waterproofing strategy that includes a membrane. Having said that, I should point to a small number of specialty materials that offer adhesive/waterproofing/crack isolation properties. An example is Custom’s Mega Late crack-prevention mortar.

Waterproofing Strategies

Two thousand years ago, tile installers covered the masses of masonry supporting their tile works with layers of reinforced molten tar that, after cooling, was covered with very thick mortar setting beds. With today’s construction, there are two waterproofing approaches for tiling: install the waterproofing materials behind the setting bed, or install them on the setting bed surface. On light-duty applications, installing the waterproofing behind a mortar bed or backer board setting bed can provide adequate protection. For medium- or heavy-duty applications, it makes sense to use a surface-applied membrane. Fasteners required for lath or backerboard puncture behind-the-bed waterproofing, and in my opinion, render them ineffective - especially in areas where moisture within an exterior wall cavity is an issue. When properly specified and installed, surface-applied membranes (sheet or liquid) cover all the fasteners and seams, and offer reliable waterproofing.

Sloped Setting Beds

Although essential for shower stalls, sloped setting beds are also required for tile work connected to a drain system, or covering an exterior walkway or deck. Lack of slope for exterior work causes puddling, which causes wicking, leading to leaking, structural damage, and other problems. There are numerous sloped floor inserts that can be used to replace the mortar in a shower stall floor, and exterior slopes can be done with carpentry so mortar bed skills are not an essential anymore, but these components, unless designed otherwise, must be used in conjunction with a waterproofing membrane system. To avoid damaging the membrane, a network of movement joints must be built into the tile installation.

With this system, corners and seams are flashed with paste and reinforcing fabric.

The Role of Grout

Every morning in the 1950s, after the average family of four had used the bathroom, chances are the floor was dry, the towels were hung neatly, only a few drops of water were visible on the clean tile walls of the shower, and there was little evidence that anyone actually used the bathroom. The United States was in a post-war economy, energy use was relatively low, and most families imposed there own version of hot water rationing - water usage was very low.

One of the reasons those older tile installations lasted was that they never got really saturated, and most quickly dried out thoroughly between uses - conditions that make it nearly impossible for mildew or mold to take hold. Grout joints were small, and wet-setting techniques usually assured that all grout joints would be fully packed with no voids.

Today, we use very large amounts of hot water, are in the shower longer, and many of our wet-area tile installations remain permanently damp or saturated with water - most of it stagnant and ripe for mildew or mold. With membranes widely available, water penetration through to the structure should be less of a problem, yet absorption by the tile installation itself is a growing problem even as the usage of porcelain tile increases!

There are several factors that bring about absorption and subsequent wicking. One of them is that installers do not use enough adhesive (thinset mortar) to create a void-free bond between tile and setting bed that results in the 95% coverage required by the tile industry for wet or exterior areas. This can be solved by proper trowel selection and installer care. Another obvious factor is the use of materials that absorb minimal amounts of water, which, of course, can be solved by careful selection.

Another huge factor in absorption lies in the grout. When the grout joints are properly prepared and cleaned (leaving at least two-thirds of the joint space open for the grout), and when the grout itself is properly stored, conditioned, mixed, packed, cleaned, and cured, a wet-area installation’s grout joints present a formidable and effective barrier to moisture penetration. Unfortunately, most grout is improperly stored and conditioned, poorly mixed, loosely installed, and not given a reasonable cure: the result is grout that is variable in color, soft with a low resistance to scuffing and erosion, porous enough to readily admit water and stains, and prone to cracking, flaking, and powdering.

The cure begins with storage and conditioning: grout should be at ambient room temperatures before using. Extreme high and low temperatures allowed by the manufacturer should be avoided. Next comes mixing: wet and dry ingredients should be accurately measured (with no over-watering to make spreading easier). After mixing, grout (like all other materials made with Portland cement) should slake 10-15 minutes, and then re-mixed thoroughly before using. Grout should be packed with a grout trowel until all joints are filled with high-density grout - not simply fluffed in with no pressure on the trowel. Next, it needs to sit undisturbed for a short period (to allow it to begin to setup) before cleaning, which should be done with a tile sponge and the very least amount of cleaning water possible. Finally, the grout needs to cure for a period indicated by brand instructions. Sometimes this can be done dry, but often it requires periodic light misting or sponging with fresh water. After wet or dry curing, the tiles may require a light dusting to remove lingering haze.

The author rolls waterproofing gel over a cured reinforced mortar bed.

Waterproofing Sealers

The issue of sealing the tiles and/or the grout is a contentious one. At issue is whether or not a sealer can be applied to grout or tile to prevent water absorption. Even the most professionally installed grout can be stained, and for this, there are a number of highly effective sealers and impregnators (cheap sealers are usually not even worth applying). But to prevent water absorption, only a material such as varnish, or an absorbent synthetic resin is effective, and most of these will permanently affect the look of the tile and grout and should be avoided.

A tile installation’s ability to resist water should be inherent and not require the use of coatings or other treatments that need periodic stripping and replacement. Waterproofing adheres to the weakest link theory, and if there is an absorbent pathway in or around the tiles, water will find it. The trick is to asses the needs of the installation, use the right materials, and take enough time to do everything by the book.