The short answer
Tile is unaffected by water but the mortar bed, backer board and subfloor beneath it are not. Water reaches them through grout lines, perimeter gaps and the substrate edge, and it dries very slowly through a tiled surface. Assessment requires readings at edges or access points, and drying frequently requires lifting tile.
How water gets underneath
Grout is not waterproof, and perimeter gaps at walls, thresholds and fixture penetrations give water a direct path to the substrate. Once beneath, it spreads across the bed.
In a bathroom, the perimeter of a tiled floor around a tub or shower base is the usual entry point.
Why it will not dry through
Tile and a mortar bed together form a dense, slow-drying assembly. Air movement across the surface achieves very little for what is underneath.
That is why a tiled area that has been wet frequently requires lifting tile to dry the substrate, even though the tile itself is undamaged.
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How the assessment is done
Readings are taken at edges, grout lines, thresholds or a small access point, and compared to an unaffected reference. Tapping to find hollow areas indicates where the bond has failed.
A meter on a glazed tile surface reads the tile. It is not evidence about the bed.
What indicates a problem beneath
The tile stays intact, so the signals are indirect and easy to dismiss.
- Hollow sound when tiles are tapped
- Individual tiles that have loosened or moved
- Grout cracking or discolouring in a pattern
- Efflorescence or staining at grout lines
- Odour in a room with an apparently perfect floor
- Adjacent materials wet where the tile is not
Lifting tile is a rebuild decision
Removing tile to dry the substrate means replacing it, and matching existing tile is frequently not possible. That turns a drying decision into a renovation decision.
It is worth raising early, because the cost of the tile replacement often exceeds the drying it enables.
Where the substrate is concrete
Tile over a concrete slab is a different problem. Concrete holds a great deal of moisture and releases it slowly, and drying a slab is measured in weeks instead of days.
Reinstalling flooring over a slab that has not dried is a common and expensive mistake.
Tiled entries and mudrooms in Vermont houses
Vermont houses commonly have tiled entries and mudrooms because of snow, salt and mud, and those are exactly the floors that take repeated wetting through a long winter without anyone treating it as a water event.
In older Greater Burlington homes those entries are frequently additions built on slab or over a crawl space, so a floor that stays cold and damp underneath is common. Where an entry tile floor smells or sounds hollow, the substrate rather than the tile is what to investigate.
Why tile can hide wet layers
The tile is fine but the layers beneath are not.
| Layer | Effect of water | How it is assessed |
|---|---|---|
| Tile and grout | Unaffected | Visual |
| Mortar bed | Holds moisture | Readings at edges or access points |
| Backer board | Can be damaged | Readings and access |
| Subfloor | Wet and slow to dry | Readings, sometimes lifting tile |
Clear answers
Common questions
What does a hollow sound mean?
The bond between tile and substrate has failed in that area, which commonly indicates moisture beneath.
How can tile sound solid and still be wet underneath?
Tile and thinset are effectively impermeable, so the substrate stays wet while the surface reads dry. The moisture leaves at the edges and slowly, or not at all.
Does the tile have to come up?
If the substrate is wet and there is no other way to reach it, usually yes. That is an inspection decision based on readings and on what the substrate is.
Authoritative references