Hollow Flooring Tiles and Water
The story usually starts like this: “My tile floor got wet, and now it sounds hollow when I tap on it,” followed by, “It never sounded hollow before,” as if people regularly go around tapping their tile floors to ensure they don’t sound hollow. Sometimes, a homeowner notices the hollow sound, or it may be a savvy contractor who walks around bouncing a golf ball and marking tiles with tape. This often leads to a homeowner convinced their tile floor is damaged and a skeptical adjuster believing that tile is waterproof and cannot be damaged by water. The reality, however, is a bit more complex.
Ceramic Tile Flooring Over Concrete
Tile, concrete, and mortar are all waterproof, right? So why do tile floors sometimes fail when installed over concrete?
Slab Leaks
The first thing I always ask when a ceramic tile floor is claimed to be damaged by water is:
- “Did you have a slab leak?”
- If yes: “Was it a hot water slab leak?”
- Finally: “Did the plumber locate the leak?”
The answers to these questions won’t always definitively determine the cause of hollow tiles or prove damage, but they can provide helpful information for forming an informed conclusion.
Slab leaks, especially hot water slab leaks, can cause ceramic tiles to detach from their substrate. This makes sense if we consider how the bond between a tile and a concrete slab can fail.
A bond failure can occur in two ways: mechanical failure or deterioration—or a combination of both. In the case of a slab leak, both causes are possible depending upon how the floor was installed. A hot or cold water slab leak can cause the substrate or tile to expand. Clay-based tiles, as opposed to porcelain tiles, are more prone to expansion when exposed to water. If the substrate and tile expand at different rates, the resulting shear force can break the bond between them. This would be an example of mechanical failure.
This phenomenon is often seen in homes without water intrusion. Over time, concrete slabs can shrink, or clay tiles may expand. When the resulting shear force exceeds the mortar’s bond strength, the bond will fail. Porcelain tiles, while impervious, may still lose their bond if the concrete slab shrinks over time. Proper expansion joints (as specified in EJ 171 in the TCNA Handbook) can help prevent such failures.
If a slab leak causes hollow tiles, how can you determine if a specific water event caused the damage? The short answer is: you can’t always, but it may not matter. If hollow tiles are concentrated near the slab leak area, it is difficult to disprove a claim of water-related damage. Conversely, if hollow tiles are evenly distributed, it may suggest no water-related damage occurred. However, further examination is needed, including checking for deflection and inspecting grout for cracking or efflorescence.
Surface Water
Can surface water cause a tile floor to become hollow? It’s possible, but in most cases, no.
In Southern California, most ceramic tile floors are installed with thin-set mortar over an on-grade concrete slab (TCNA installation method F-113), which has a minimum Res2 (Residential Limited Water Exposure) environmental rating. This rating applies to areas like bathrooms, kitchens, and laundry rooms, where surface water exposure is limited and not continual. The Res2 classification suggests occasional surface water, such as a toilet overflow, should not damage the tile floor.
However, surface water can sometimes damage a tile floor. If water finds a pathway under the tile, it may soak into the concrete, causing expansion. In many cases, such damage reveals pre-existing issues with the tile installation—such as improper thin-set mixing, poor substrate preparation, or installation over unsuitable materials like paint overspray or abandoned vinyl flooring.
Ceramic Tile Flooring Over a Wood Subfloor
A wood substrate introduces additional moisture-related vulnerabilities to tile floors. While ceramic tile floors over wood subfloors should withstand normal moisture exposure (e.g., wet feet from a shower), prolonged saturation or improper installation can lead to damage.
For example, prolonged saturation may allow water to leach into the wood subfloor, causing expansion and bond failure. Additionally, moisture can enter through weak points like grout, openings at toilets, or poorly sealed perimeters. If water-soaked wood expands, the resulting force can break the bond between the tile and substrate.
Microbial Concerns
Microbial growth is often a concern when water is trapped under a tile floor. However, it depends on the materials underneath.
In Southern California, most tile floors are installed over concrete. Concrete, mortar, tile, and most crack membrane materials do not provide a hospitable environment for microbes due to their high pH levels and lack of organic material. In these cases, microbial risks are minimal unless improper materials, like scribing felt or asphalt felt paper, were used during installation.
For wood subfloors, the risk is higher. Non-destructive moisture testing often cannot rule out microbial growth with certainty. Destructive testing, such as cutting through lower-level ceilings or probing from crawl spaces, may provide more definitive answers, but these methods can be costly and invasive.
Contaminated Water
When contaminated water contacts tile flooring, removal is often recommended as a precaution, especially with wood subfloors. If microbes enter voids between tile and concrete, they typically die due to a lack of food. However, drying trapped water can be challenging, and a water mitigation expert may recommend removal.