Küche mit grau gefliestem Boden als Bild für Hohlklang bei Fliesen

Hollow spaces / Hollow sound under ceramic tiles - weber.sys 891

The special liquid weber.sys 891 removes hollow spaces under ceramic coverings easily and reliably due to its excellent creep properties. Costly removal and re-laying of the tiles is thus avoided. The ready-to-use liquid is suitable for walls and floors. It can also be used for elastic filling of hairline crack sealer under hollow ceramic tiles.

Elimination of hollow spaces under ceramic coverings

Large-format tiles in particular present tile layers with a number of challenges. Hollow spaces quickly develop, which can easily lead to consequential damage to the ceramic covering. But in this case, their removal is not always necessary. Careful renovation with low-viscosity injection liquids saves time and money.

The trend towards large-size tiles is unbroken - whether in new buildings or renovations. This often presents installation companies with many challenges, because the risk of structural damage increases with the size of the material being laid. In addition to handling, a number of things have to be taken into account, especially with regard to aspects of building physics, such as evenness and the drying behaviour of the tile adhesive. Since the proportion of joints is smaller with large tiles, moisture under the ceramic covering can only dry out very slowly due to the few joints present. If the wrong materials are used, this can lead to damage to the substrate at an early stage. An undersized layer of the adhesive can also cause problems. This is often caused by choosing the wrong notched trowel or not using the buttering-floating method.

In addition, the larger the tile, the more even the surface on which it is laid on, must be. The danger of hollow spaces under tiles increases. Damage and breakage can be the result if the tile is subjected to a corresponding load. In addition, the covering sounds partially hollow when walked on. In this case, building owners usually insist on removal and re-laying, often presenting the tile layer with problems, including financial ones.

Time- and cost-saving alternative

One solution is offered by low-viscosity special fluids such as the one-component injection liquid weber.sys 891. It has extraordinary penetration properties, fills hollow spots and can often be the alternative to costly and time-consuming removal. The acrylic co-polymer dispersion is poured into the opened joint without pressure and fills cavities with a force-fitting bond. It is also ideal for sealing hairline cracks, for ex. on mosaic and terrazzo floors as well as on screeds or, in some cases, renders/plasters.

Access via the joints

Grafische Darstellung wie die Fuge zwischen zwei Fliesen mit einem Werkzeug geöffnet wird

In order to be able to carry out a professional restoration of hollow layers, the damage pattern must first be determined. If there are no obvious cracks, the floor covering should be lightly tapped or scraped with an object to identify hollow spots. If spots are localized, they should be opened by cutting or drilling into them in the joint area. It should be noted that the dust formed in the process should be carefully removed using an industrial hoover, as an optimum result can only be achieved if the substrate is absolutely dry and free of adhesion-reducing components.

Injection in several steps

Grafische Darstellung der Injektion von weber.sys 891, um Hohlklang bei Fliesen zu beseitigen

Using appropriate auxiliary tools, such as funnel, squeeze bottle, storage container or pipette, weber.sys 891 is now poured into the opened covering. Depending on the size of the cavity, several working steps may be necessary. In this case, the liquid is poured in intervals of 20 - 30 minutes until the hollow sound is eliminated. Repeated re-pouring is possible within a maximum period of 24 hours and until saturation

The advantage of weber.sys 891 is that it remains elastic after setting and develops a high bonding strength. The extraordinary creep properties make it possible to pour the material with minimum effort even into fine hairline cracks, where it nevertheless penetrates deep into the structure. When it sets, it seals the crack. This prevents further penetration of moisture and ensures good bonding strength of the surfaces.

After drying through, a new knock test can be carried out. If a satisfactory result is achieved, the areas can be permanently closed again with appropriate materials, like the flexible grout weber.fug 877. A waiting time of at least 24 hours must be observed, as the injection fluid that has not completely set, can harden due to water release and thus impair the visual appearance of the joints. It is also important to determine the right colour of the grout in advance in order to exclude deviations from the existing joint appearance.

Partial removal and crack sealing

If major damage is found in the floor during the knock tests, partial demolition is often unavoidable. In this case, parts of the installed ceramic coverings are taken up in order to repair the screed underneath. The use of fast-setting, two-component epoxy resins, for ex. weber.prim 806, has proven successful. The resins have high compressive strength, flexural strength and bonding strength. The surface adhesion of the material, which is crucial for reliable waterproofing and force-fit bonding, is excellent, especially on concrete and screed. Epoxy resin is resistant to water, mineral oil, petrol and numerous alkalis and acids. The two components are mixed with the help of a slow-running drill with a stirring paddle attached. When no more streaks are visible after mixing, the material must be processed within ten minutes, otherwise it will lose the desired consistency.

Depending on width and depth of the crack and its penetration in the lower screed area, the resin is brushed, poured or injected. For an optimal result, it is also recommended to cut into the cracks and connect the slots with screed steel anchors (weber.sys Estrichklammer/weber.floor Wellenverbinder). In all cases, the cracks, and if necessary, the channels, are filled flush with the screed surface and then smoothed off with a trowel. Finally, oven-dried hard quartz material is sprinkled in excess onto the still tacky epoxy resin to provide the best adhesion with post-applied adhesive. After complete curing, the loose, excess sand is removed completely. If necessary, the repaired screed and concrete surfaces can be levelled with a self-levelling compound before laying the ceramic coverings.

Grafische Darstellung der Produktgruppe weber.fug 875 BlueComfort und weber.sys 891 als Produkte, um Hohlklang bei Fliesen zu beseitigen


The trend towards large-format tiles is accompanied by an increased risk of construction errors. Hollow spots or substrate cracks can be the result. The use of low-viscosity special fluids can prevent time-consuming complete removal, as these are injected directly over the joints and the cavities are thus filled. For larger damaged areas, it is advisable to remove part of the covering and seal the corresponding areas with a two-component epoxy resin.