Floor without joints: is it possible with porcelain stoneware?

The grout line in ceramic floors: What is it and why is it necessary?

The grout line is the minimum distance between one tile and another that must be provided during the installation process and is filled with cementitious grout or epoxy grout in colours that match (or sometimes contrast) with the colour of the ceramic surface. As we mentioned, unlike the materials previously reviewed, a porcelain stoneware floor without joints is highly discouraged, as it would not comply with the safety standards set by the UNI regulations, would not withstand the slight movements that the tiles can undergo over time, and would contribute to creating an environment conducive to moisture, which tends to damage the floor itself.

This applies to both rectified and non-rectified tiles. But what is edge rectification? 

At the end of the production process, the four edges of each square or rectangluar tile will be perfect right angles, as will be the 90° angle where the tile surface meets the edge.

• In the case of formats that are not rectified, whose section is an elongated isosceles trapezoid, the surface of the tile is narrower compared to the side that will be glued to the cement screed. Therefore, even when the tiles are placed next to each other (which should never actually be done), an unavoidable gap will remain between them.
• As for rectified tiles, it is always advisable to provide a joint of at least 2 mm because, according to international standards, porcelain tiles have a minimal tolerance in linear dimensions. Although insignificant, even a tolerance of a few tenths of a millimetre will make a floor without joints unstable and unappealing (especially in the presence of underfloor heating).
• A smaller joint, according to the most experienced installers, will not ensure the durability of the grout over time.

Regarding the possibility of not grouting the joints, here is another good reason to reconsider: joints that are not filled with grout will fill with dirt and water, providing a fertile ground for mould, fungi, and bacteria. And if the water pipe under the kitchen sink were to break? The floor will flood, and water will penetrate through the joints to the subfloor, risking compromising the adhesive and soaking the ceiling of the neighbours below.

In addition to the joints, for large surfaces, it is advisable to install expansion joints, whose purpose is to compensate for the stresses of mechanical loads, vibrations, and thermal phenomena affecting the building.

The function of grout lines in ceramic floors

As mentioned, jointless installation is not permitted. This is stated by the UNI 11493 standard, dedicated to ceramic tile flooring and wall coverings, and it applies to any type of ceramic flooring: 

• Single-fired and double-fired tiles;
• Porcelain tiles (full-body, polished, semi-polished, glazed...), including large formats;
• Ultra-thin laminated porcelain tiles.

From a technical standpoint, what are the purposes of grout lines? Grout lines serve several fundamental functions in flooring:

• Structural Stability: they help to stabilize and reinforce the ceramic floor, preventing detachment and/or cracks in the coating in case of movements of the underlying support;
• Stress Dissipation: unlike tiles, grout lines are elastic; therefore, they absorb minimal contractions and expansions caused by temperature fluctuations;
• Load Distribution: grout lines help to evenly distribute the load across the entire floor, preventing localized damage;
• Waterproofing: grout offers good resistance to water; some are even waterproof, preventing water from seeping under the floor;

• Aesthetic appearance: grout lines create visual order and cleanliness; additionally, they mask any dimensional disparities between tiles, especially if they are not rectified.

How do we achieve a floor without grout lines?

Since this is not possible for ceramic and porcelain tiles, we can instead ask how to minimize the presence of the grout lines. There are essentially three aspects to work on in this regard: the choice of the format, the colour of the grout, and the type of effect.

Let's look at them in detail, with some examples from the Cotto d’Este catalogue:

• Large, rectified formats allow for wider spacing of the grout lines and reduce their width (since they are rectified). Laminated stoneware, in particular, is characterized by the exceptional size of the slabs, which reach (in the case of Cotto d’Este) 100x300 cm and 120x278 cm. Thanks to these dimensions, the material becomes the protagonist of the space, with its textures, colours, and surface effects, making the grout lines less noticeable and significantly reduced in number. Excellent examples of the role that large formats play in effectively reducing the number of grout lines are two very light marbles: Paonazzetto (from Allure collection) and Bianco Statuario (from Vanity collection)
• The brightness of these marble effects brings us to the second aspect on which we can work on: the colour of the grout in the same shade as the tile tends to camouflage the grout lines and is particularly important for predominantly monochromatic looks, such as the Cotto d’Este X-Beton, collection, a rolled cement in three shades; or like Black-White, with a very high degree of purity; or again, Pietra d’Iseo, inspired by the highly sought-after Ceppo di Gré; and Cement Project, another cement effect, very refined in the finish called Cem.
• The aesthetics of the ceramic texture also make it difficult to spot the grout lines. This is particularly true for Cotto d’Este collections with richly veined or clouded patterns, such as Orobico or Alaska, two marble looks from the Allure collection. But it is equally true for many wood effects, especially with rustic styles such as Woodland.

Grouting large tiles: additional useful tips

Creating the best grout lines for your floor, combining aesthetics and technical aspects, is not always simple.
The first obvious but always valid advice is to rely on expert installers, who can judge and evaluate the correct placement of the tiles, ensuring the most suitable grout lines.

The second suggestion is to ensure the use of high-quality materials, both for the adhesive and the grout. In this regard, it is important to distinguish between cementitious grout and epoxy grout

• Cementitious grout has cement as its main component: it is available in various colours and grain sizes; it has excellent abrasion resistance and good resistance to water and alkaline chemical agents; and finally, it has the same fungal and bacterial resistance as epoxy grout.
• Epoxy grout, until a few years ago, was used only in special situations (such as swimming pools or saunas), but today it is widely used in residential settings as well. It is a high-performance grout because it is more elastic than cementitious grout, a feature that is useful for accommodating the structural movements of buildings; it also has greater abrasion resistance, is water-repellent, and resists common chemical agents very well.