Water saturation under the tile base is water saturating through the grout joints around the tiles and finding its way to the mortar bed.
Beneath the tile base, there are three components—the mortar bed, the liner, and the preformed slope. The preformed slope causes the tile floor, mortar bed, and liner to slope properly toward the drain. Even with this slope, some water will saturate the grout joints between the tiles and move below the tile base. Here, the water wets the mortar bed. If the mortar bed becomes saturated with water, the water then wets the last line of defense—the liner. The liner directs the water to the drain. When these three components are installed correctly, they prevent water from reaching the framing.
Using a preformed slope
The preformed slope, which is installed first, is the component immediately beneath the liner. Its ¼" per foot slope directs water toward the drain. The greater slope the floor has, the less water will saturate the mortar bed. Installing a preformed slope helps save time, since it's light and easy to install, and eliminates possible scheduling issues.
Mixing and installing the mortar bed
The mortar bed is installed over the liner. The mortar bed has to be mixed and installed correctly to prevent water damage. The mortar mix should contain the correct ratio of ingredients, as well as a fine stone that won't protrude from the mortar bed. Once the mortar is mixed, it has to be applied evenly on top of the liner. Applying the mix evenly means creating a smooth, consistent thickness across the surface of the liner. Being consistent is important in maintaining the ¼" per foot slope. If the slope is inconsistent, excess water will rest on the tile and saturate the grout; grout absorbs water very easily, because it's porous. Once water saturates the grout, it can also saturate the mortar bed. If the mortar bed is unable to dry, it can cause unsightly mold to grow on the grout.
Water that saturates through the mortar bed and reaches the liner has to be able to drain away. At the drain area, the top housing is installed to create a watertight seal between the liner and the drain. It has weep holes (and weep slots, in some cases) that allow water to enter the drain. The weep holes are the small holes visible in the top housing of the drain assembly. If these holes are filled with mortar, water will accumulate on the liner. To keep mortar from filling in the weep holes, pea gravel is placed around the top housing at no less than ½ the width of the mortar bed. Pea gravel promotes drainage to the weep holes, because water can drain between the pieces of gravel. Without pea gravel, water is more likely to rest on the liner and saturate the mortar bed without draining.
Cause 1:
Poor slope toward the drain
If the installer doesn't adequately slope the mortar bed toward the drain, any water that penetrates the tile floor may saturate the mortar bed. When the mortar bed under the tile base is unable to dry because of the amount of moisture present, mold may grow on the grout.
Solution
Install a preformed slope composite with a ¼" per foot slope. Follow the installation instructions provided by the manufacturer; this will dramatically increase how well you achieve the correct slope and decrease the time and labor necessary.
ProTip!Once the tile has been installed and grouted, add two to three coats of grout sealer to the grout joints. Adding sealer prevents the grout from being saturated with water.
Cause 2:
Improper mortar mix ratio
Improperly mixed mortar can cause the mortar bed to dry too quickly and crack; cracks will cause water to pond or leak to the liner. Cracks create a direct path for water to reach the liner, which will then drain the water away. But if the mortar bed cracks too much, it can crack the tile.
Solution
Mortar should be mixed using a ratio of 4-to-1 (sand-to-portland cement). The mortar bed should be approximately 1-¼" to 2" thick and should maintain the proper slope of ¼" per foot. Fiber reinforcement is suggested to strengthen the mix to prevent cracking.
