Although tiles are generally considered to be virtually waterproof, they do not protect against water damage. And it is not only unpleasant, but can also be expensive and even dangerous. In this article you will learn what causes moisture under tiles, how to find a wet spot and how best to eliminate the problem.

Risk factor moisture

In principle, moisture in buildings is not a bad thing, in fact it is important. In the air, it contributes to a pleasant indoor climate and almost all building materials and materials only retain their stability permanently through the regular absorption of water or water vapour. However, only as long as the humidity remains within limits.

Too much moisture under tiles can sooner or later lead to water damage – and thus inevitably to a rat tail of negative consequences. These affect not so much the tile itself as the substrate on which they are laid. So musty smelling rooms and dark spots or salt efflorescence on the wall are the lesser evil. It is far more serious if the damage already affects the entire floor structure or the entire masonry, and if mould which is a health hazard has already formed on it. Because in these cases a complex and expensive complete renovation is usually no longer to be prevented.

Possible causes for water damage under tiles

Basically, water damage under tiles can occur in two ways:

  1. Because moisture is trapped under the tile covering, or
  2. Because water comes in from above.

For the former, improper tiling is usually responsible. For example, if tiles are laid on a floor structure that is not sufficiently dry or if there is so-called subsequent moisture from the substrate after laying. In these cases, moisture remains permanently under the tiles and can cause damage to the entire floor structure. For this reason, especially with freshly laid screed, it is important to check the readiness for laying by determining the residual moisture before starting to lay the tiles. Particularly in the case of large-format tiles with a low proportion of joints, it also sometimes happens that the moisture cannot escape sufficiently from the jointing mortar or tile adhesive and thus also remains under the covering.

The second cause of water damage is that wetness only gets over the surface and under the tiles afterwards. Whether it is due to acute exposure to water, such as flooding, burst pipes or the leaking washing machine, or due to regular exposure of the tiles to moisture, such as the steam in the shower or bathroom. However, the moisture does not penetrate through the tile itself, as the material hardly absorbs any water, but through the joints. Regardless of which filling material is used, joints are always water-permeable – even joints sealed with silicone can become brittle or perforated over time and thus lose their seal.

So the crux of the matter is: under tiles, water damage usually goes unnoticed for a long time. The coating probably covers the affected area so that the water can spread unhindered. And even if damage is then already apparent, the actual extent is usually still hidden under the tiles.

This makes it all the more important to start looking for the cause at the slightest suspicion or at the latest at the first signs and to obtain certainty by determining the moisture content under the tiles.

Measuring moisture: How it works

There are various possibilities for measuring moisture under tiles, which differ in effort and significance:

Direct procedures

In direct methods, such as the calcium carbide (CM) method known for determining the residual moisture of screeds, a sample is taken from the building material, crushed and mixed with calcium carbide in a pressure bottle. Based on the chemical reaction, the moisture content can then be determined using a manometer. This method is considered to be particularly reliable – and incidentally, it is also the only one recognised by the courts – but it is also the most complex and can only be carried out by a specialist.

Indirect methods

Indirect moisture analysis using moisture analyzers, on the other hand, is much simpler and therefore basically also practicable for private use. Numerous tile manufacturers offer a wide variety of models, most of which are already available at relatively low prices. Depending on the type and design, these devices can usually be used to measure floor or wall moisture even through tiles or other surfaces.

In principle, two variants are common for this:

Capacitive measurement

With the capacitive method, the moisture meter generates an electrical stray field in the sensor head, via whose permeability the moisture content at the respective location is determined at a depth of approx. 3 cm. The advantage of this method is that it is completely non-destructive and can be repeated as often as desired. The disadvantage is the relatively small measuring depth, which means that the results may not be reliable enough in the event of deeper water damage. In addition, a certain amount of expertise is advantageous for the correct interpretation of the measured values, as these can be influenced by salts or metals in the building materials.

Resistance measurement

In electronic measurement according to the resistance principle, current is conducted via electrodes into the presumably moist area. The conductivity of the material then provides information about the moisture it contains. The higher the resistance, the lower the measurement result and thus the moisture content. The advantages and disadvantages of such a moisture meter: the wall or floor must be drilled at the affected area in order to be able to insert the electrodes, but it is also possible to detect deeper water damage. Alternatively, however, it is usually possible to measure over joints.

Nevertheless, caution is generally advised when searching for water damage on your own. Since the results are determined differently for each measuring instrument and are displayed according to manufacturer-dependent scales, there are no general standard values. In order to obtain really reliable information about the moisture content, it is therefore always advisable to consult a professional.

Water damage – what now?

If water damage is detected, there is definitely a need for action. Whether it is sufficient to dry the affected areas sufficiently and eliminate the cause (e.g. renew leaking silicone joints in the bathroom) or whether major renovations are already necessary, however, again only an expert can judge. If the wrong measures are taken, the damage can sometimes be even worse.