Water Coming From Under the Stone in Wet Areas: Leak Detection and Repair Sequence

One day the marble on a bathroom or shower floor starts to darken at the edges, the grout never dries, and a slab sounds hollow when you step on it. Most people's first reaction is to remove the stain or have the stone wiped again. Yet most of these symptoms are not on the surface but under the stone: water coming from somewhere has soaked the screed and the adhesive and is rising into the stone. In this article we explain how to recognize water coming from under the stone, how to find its source, and why the repair must be carried out in a specific order.

In short: With water coming from under the stone, the source is first found and shut off, then the floor is dried sufficiently, then the waterproofing and details are repaired, and only at the very end is the stone itself touched. If this order is broken, new grout, new slabs and new polish will soon face the same problem again.

Signs of Water Coming From Under the Stone

Water under the stone usually reveals itself gradually. The first sign is often the color of the grout: while the grout in the rest of the bathroom has dried, in one particular area it stays dark and damp. Then darkening begins at the edges of the slabs, especially in the bands close to the grout. On light-colored marble and limestone this darkening is very noticeable; on dark stones it is more often seen as white salt marks or dullness on the surface.

Common signs of water under the stone are as follows:

• Grout that does not dry: If the grout in a particular area stays dark even on days when the bathroom is not used, it suggests the water is coming from below rather than from above.
• Darkening spreading from the edges: A wet appearance that starts at the grout-facing edges of the slabs and moves toward the center indicates that the adhesive and screed beneath are constantly damp.
• Hollow sound and movement: A hollow sound when the slab is tapped lightly, or slight movement under pressure, is a sign that the adhesive has lost its bond because of moisture.
• White salt and crust: A white, chalky layer appearing on the grout or on the stone surface shows that the water is carrying salts dissolved from the screed up to the surface.
• Marks on neighboring surfaces: Swelling at the foot of a door frame, bubbling on the skirting board in the hallway, or a stain on the ceiling of the floor below shows that the water has gone beyond the limits of the bathroom.

Some of these symptoms can also have other causes. Darkening sometimes comes from an oily stain, and sometimes from the stone's own natural moisture exchange. We explained in detail how to tell whether a wet-looking stain is moisture or oil in our article on darkening in marble

Source of the Water: Possible Routes

Water can reach the underside of the stone in several different ways, and the solution for each one concerns a different tradesperson. The most common source is service water that seeps slowly through the grout and silicone joints in the shower area. When the silicone at the edge of the shower tray or bathtub wears out, when a gap forms in the shower glass profile, or when the grout around the drain cracks, a few drops of water pass under the stone with every shower. Over the years, this small amount can completely soak the screed beneath.

The second source is the plumbing itself. A small leak in a pressurized fresh water line supplies water continuously, whereas a leak at a waste pipe connection only acts when that drain is used. The third source is the drain connection: if there is no watertight connection between the drain body and the waterproofing, water spilled on the surface goes straight down to the screed along the edge of the drain. Less common sources include water from a neighboring apartment or the floor above, rain entering through an exterior wall, and condensation collecting on cold surfaces.

Finding the Source: Step-by-Step Diagnosis

Leak diagnosis starts with the cheapest and least destructive methods. The aim is to narrow down where the water comes from as much as possible before removing any stone. For this, you need to observe for a while, change usage habits, and take measurements.

The diagnostic sequence followed in practice is as follows:

1. Meter check: If the water meter keeps turning when all taps and appliances in the home are closed, it strongly indicates a leak in the pressurized fresh water line.
2. Separating usage: The shower is left unused for a few days to see whether the grout dries; if it does, the source is most likely service water or the drain connection.
3. Zonal water test: First, water is applied in a controlled way only to the drain, then only the shower glass profile or tray edge is wetted; moisture change is measured after each test.
4. Moisture map: The floor and the base of the walls are measured in a grid with a moisture meter that does not damage the stone; the point where the highest values cluster shows the area closest to the source.
5. Specialist equipment: If necessary, the plumber performs a pressure test on the lines, and the location of the leak is narrowed down with a thermal camera or acoustic listening devices.

Being patient at this stage reduces the demolition needed later. It is possible to strip an entire bathroom to find the source, but it is unnecessary; in most cases, careful observation points to an area of just a few slabs. It is also important to note down diagnostic results, measurement values and photographs; these records are useful later both for evaluating the success of the repair and for talking to neighbors or the building management.

Why Is the Repair Done in a Specific Order?

The most common mistake with water coming from under the stone is reversing the order. The stained stone is wiped, the grout is renewed, perhaps an impregnator is applied on top; a few months later the same darkening starts again. That is because the water is still coming, and new grout and new polish cannot stop it. Worse, a treatment that seals the surface slows the evaporation of the moisture underneath and deepens the problem.

Another reason for the order is that the job involves more than one tradesperson. The plumber, the waterproofing applicator and the stone crew come to the site on different days; if each starts without accepting the previous one's work, responsibility becomes unclear. That is why doing a short check at the end of each stage, taking photographs and noting moisture values also prevents disputes later on. Before starting stone repair, we like to see these records and verify the floor with our own measurements.

The correct repair sequence and the work done at each stage are summarized in the table below:

Drying: The Most Frequently Skipped Stage

Once the source is cut off, the water that has accumulated in the screed and adhesive does not disappear on its own right away. Stone, especially with a polished surface, slows the upward evaporation of water, and the grout joints act like narrow doorways. For this reason, a soaked bathroom floor can take weeks to dry. Thick screed, poor ventilation, and cold seasons extend this period even further.

To speed up drying, the bathroom door and window are kept open, a suitable dehumidifier is run, and the room temperature is kept at a reasonable level. In areas where moisture is heavy and the tiles are already hollow, lifting a few tiles increases the screed's contact with air and noticeably shortens the process. The decision that drying is complete is made by measurement, not by eye: the moisture meter readings are expected to approach those in an area of the bathroom that never got wet. We have also described the effect of screed moisture on stone flooring in our article on screed moisture separately.

There are also a few things that should not be done during drying. Pointing a heater directly at the floor to heat the stone quickly dries the surface but draws the moisture underneath deeper into the stone, which can increase staining and salt efflorescence. Filling the joints or applying a protective coating before the floor has dried closes off the moisture's exit path. Even using the bathroom for short showers during the drying process disrupts the measurements; if possible, an alternative bathroom should be used throughout. This waiting period is often the stage homeowners find hardest, but it is also what makes the rest of the work last.

The Same Problem in Shower Walls and Under the Countertop

Water coming from behind the stone is not seen only on the floor. A similar picture develops on the marble cladding of shower walls: tiles darken around the fixture, at the pipe outlet of the shower head, or in the corner where the wall meets the floor, and white salt appears on the lower edges. Because water runs down the wall by gravity, the darkening is often concentrated in the lower half of the tile and along the skirting line. The most common sources are the body seals of concealed mixers, concealed cistern connections, and the voids left behind fixture rosettes.

The situation is similar under the washbasin countertop. A small drip at the siphon connection wets the base of the cabinet supporting the countertop and the wall cladding behind it; the stone at the front looks dry while a halo forms on the back edge. Such leaks are often found with a check as simple as opening the cabinet door and looking with a flashlight. For walls and countertops, the order is the same: first the connection is repaired, then it is dried, and finally the stain and surface of the stone are dealt with.

Waterproofing and Details

Stone is not a waterproofing layer on its own. Marble and limestone absorb water, and grout joints develop hairline cracks over time. What actually stops water in a wet area is the liquid-applied waterproofing layer beneath the stone. These liquid-applied products used under ceramic and stone cladding can be cement-based flexible mortars, dispersions, or resin-based coatings; in Europe, the properties of these products are defined by the EN 14891 standard. Whether a product is compatible with natural stone should be checked in the manufacturer's documents.

It should be remembered that waterproofing often fails not on the surface but at the junctions. The corners where the wall meets the floor, pipe penetrations, the drain body, and the door threshold are the weakest points. Flexible waterproofing tapes are used at corners, collars at pipe penetrations, and a flange connected to the waterproofing at the drain. The floor slope of the shower area should direct water to the drain, and the door threshold should be designed to prevent water from spilling into the hallway. These details are the job of the waterproofing installer; the stone team starts work only when this layer is sound and dry.

Then the Stone: Tile, Grout, and Surface Repair

Once the source has been cut off, the floor has dried, and the waterproofing is complete, it is the stone's turn. Tiles that have lost their bond due to moisture are either removed and relaid or, if their condition allows, fixed in place. We have described the method of fixing without removal in our article on hollow tile injection repair . Tiles that are broken or permanently stained are replaced, if possible, with the same stone and with a matching vein direction.

Stains remaining on the stone surface largely fade with drying; for halos that do not fade, a poultice is tried. When grout joints are renewed, low-absorption grout suited to wet areas is preferred. The final step is to renew the surface with in-situ honing and polishing, then apply a sealer once the stone has dried thoroughly. Sealer protects the stone against stains but does not stop a leak underneath; that is why it is left until the very end.

If you notice darkening, joints that won't dry, or tiles that sound hollow in the stone in your bathroom, we recommend first making observations and measurements to narrow down the source. Once plumbing and waterproofing repairs are complete, you can reach us for stone repair and restoration through the service request form .