Design Issues and Value Analysis of Modern Stone Slab Coverings

Abstract

Nowadays, public buildings are clad on the outside, many with stone-clad facades. Energy requirements have changed a lot in the last 20–25 years, and the latest required value of the thermal conductivity of masonry is 0.24 W/m²K. The relevant requirements, available materials, and fastening technology options have changed significantly. Our research covers a comprehensive analysis of these systems, the selection of stone cladding materials, and the suitability and use of individual stone types for facade cladding, as well as an energy examination of layered wall systems and the development of fastening elements, including the material structure of the elements and possible design and fastening methods. In the original university research, we also developed an applied technology for several product manufacturing companies in order to obtain approval for industrial application. In this article, we summarize the results of our research, the building structure and building physics issues, the necessary fastening technology design, and the main aspects of selecting stone tiles regardless of the manufacturing companies. The goal of our university research was the introduction and structural development of assembled stone facade cladding in Hungary, a development that continues to this day. The assembled stone cladding system we developed has been used to cover the facades of thousands of buildings in Hungary.

1. Introduction

1.1. Research Background

The first installed stone cladding on our public buildings appeared in the 1960s and 1970s, e.g., on office buildings, dormitories, and hospitals. Typically, the stone cladding was laid directly on the reinforced concrete or brick wall, embedded in mortar with carbon steel fasteners. These are now at the end of their service life and need to be inspected, and often replaced.

In the past 20 years, there has been significant progress in the field of modern prefabricated facade cladding, including stone slab cladding. Architects and designers have an unprecedented selection of domestic and foreign stone materials, modern fasteners, and complete facade cladding systems that are well thought out from a building physics and structural perspective, which already include computer-aided design and a complete computer-controlled production process. Yet, we still encounter construction errors, poorly chosen cladding, and material problems. In the following chapter, we have tried to provide the main aspects and procedures by which these errors can be avoided or eliminated.

1.2. Research Propose

Our buildings now have to meet new thermal insulation and moisture technology requirements. The first energy effort was in 2006. At that time, the expected thermal conductivity factor was reduced from 0.7 W/m²K to 0.45 W/m²K. Therefore, in the early 2000s, serious development began to ensure thick thermal insulation and air gaps in new buildings, renovations, and replacements. Even if there was a need for thermal insulation before, or the supporting structure was very thin, it was only used in thicknesses of 3–5 cm, with an expected thermal conductivity factor value of 0.7 W/m²K. This concept had to be changed. In the original research, we developed an applied technology for several product manufacturing companies so that they could be licensed for use in Hungary. This work was performed in cooperation with colleagues, because there are also static, building physics, building structure, and material science issues. From this, we develop building physics and building structure issues, considering static results and material structure characteristics. The development is constantly updated and always needs modification, because in 2016 the expected value of the thermal conductivity factor decreased to 0.26 W/m²K, then in 2024 to 0.24 W/m²K, and this is already mandatory for new buildings. In Hungarian climatic conditions, heating is required for about 7 months: in winter, temperatures between 0 to −10 °C are typical, while in some weeks they are lower, but temperatures under −20 °C occur very rarely. In summer, cooling requirements also arise for 1–2 months. Although the required building physics values differ from country to country, the concept of our research should be applicable to other countries with similar continental climates in the region.

1.3. Research Aim

The aim of our university research is to determine the correct construction of assembled stone facade cladding as part of layered wall structures and to develop corresponding fixing brackets. Our research has included examination of the building physical properties of the wall structure, consideration of external damage, and selection criteria for stone materials. As a result of all this, assembled stone facade cladding systems have been introduced in Hungary. At the same time, this structural development continues to this day in cooperation with industry. Due to the ever-changing and stricter building regulations and energy requirements, we had to optimize the structure of the structural system several times. The last energy tightening was in November 2024, in response to which we had to adapt this building structural system. These are the results we are presenting now. Although these are based on Hungarian regulations, countries with similar climates can use the technical content presented here, even with slightly different national regulations.

2. Literature Review

2.1. General Information and Requirements

Today’s modern stone cladding is made of 2.5–6 cm thick sawn stone slabs and is not assembled with a back wall but is attached to it with an intermediate element.

This results in an aesthetically pleasing and durable, but significantly more economical, lighter, and thinner facade cladding. This significantly reduces material consumption and the weight of the masonry, thereby shortening construction time.

It is possible to design the wall cross-section with the best possible building physics, since both the necessary thermal insulation and the ventilated air gap are built into this system. This is now a clear requirement for all new buildings. However, when renovating existing buildings, installing new facade cladding is also an option.

Another goal is the construction of low-maintenance buildings [1], and building information model BIM methods can now help a lot with this [2]. In general, the structural system of a building accounts for 10–20% of the construction costs, and the cost of the facade is about the same. In the case of taller buildings, the structural costs are about 10%, but the cladding cost can be 20%.

The assembled facade system is not part of the structural system, but if it is not properly designed, it can have an impact on it. Since neither the facade nor the structural system is designed for this, a generally accepted system of rules currently ensures proper operation [3]. Traditional facades are more organically connected to the structural system; therefore, regular condition assessment of the structural system also affects the facade cladding [4]. In the case of assembled cladding, however, inspections are usually only started when a defect is detected, so there is no general regulation or guideline for the inspection of intact facades, but case studies are available, grouping the problems based on the defects that appear [5]. We have a somewhat detailed picture in the case of ceramic cladding, which has remained popular even when though load-bearing walls are typically no longer ceramic [6]. Buildings use a lot of energy, not only during their construction, but also during their operation. Thermal insulation and good engineering are very important, but correct operation is also necessary. In small buildings, e.g., family houses, this can be significantly improved by educating people. In small and medium-sized buildings, organization and operation take over this function, and in very tall buildings, automation is the only solution. In large buildings, different parts of the building must be scheduled differently, e.g., according to the cardinal directions. This can be solved with adaptive systems. A significant part of this can be the adaptive facade. In these cases, the material of the cladding and the material of the shading are of particular importance, although glass and solar panels dominate here.

In general, with the spread of increasingly affordable digitalization in all areas of life and the ever-increasing energy prices/carbon dioxide emission reduction targets, adaptive facades will gain ground even in smaller buildings. Although for now, not only is the price an obstacle, but also the fact that, due to their complexity, they can only be effectively developed in new buildings and are designed directly to the needs of the specific customer, and therefore they are not marketable in already completed buildings [7]. In adaptive facades, in addition to glass, everyone primarily thinks about movement, but the mechanics can also be adaptive and programmable to work independently of the cladding material in air-gap facades [8].

2.2. Stone Types Used

In Hungary, with few exceptions, sandstone and limestone coverings were made until the early 1970s. There are three types of limestone: porous limestone, dense limestone, and freshwater limestone. Dense limestone can be used in the same way as marble. Freshwater limestone can be used as a paving stone with the correct cutting direction, where the cutting is perpendicular to the layers: this is called travertine.

The porous limestone material came mostly from the Süttő and Budakalász quarries. Today’s architects can order granite, diabase, porphyry, marble, and other stone materials from catalogues, but more affordable limestone coverings also remain popular.

Traditional stone cladding and ventilated cladding may differ in materials and geometry, and attention must always be paid to the construction and environmental—geological, meteorological—conditions [9,10]. The facade structural system of one building cannot be automatically transferred to another without taking these into account [11], and during renovations, it is necessary to be aware of the interaction of old and new materials, especially since facade stone slabs are exposed to significant temperature fluctuations and, in some cases, severe frost effects [12].

2.3. Air Pollution in Urban Environments

Clean air contains 77.9% nitrogen, 21.1% oxygen, and 1% other gases or compounds [13]. Compounds and values that differ from this may indicate air pollution. The air quality of our large cities differs significantly from this.

Nowadays, we can find additional pollutants in the air that can be blamed for the harmful effects on building materials, such as

• sulfur dioxide,
• nitrogen oxide,
• ozone,
• hydrogen fluoride, hydrogen chloride, hydrogen sulfide
• dust and soot particles,
• carbon dioxide, if its concentration significantly exceeds the natural proportion in the air (0.029%).

As a mode of action, we distinguish between dry disposition of gaseous pollutants and wet disposition, which involves harmful substances dissolved in moisture, as well as particle-shaped deposits on the surface of building materials.

The damage can be

• efflorescence,
• material deterioration, thickness reduction, rust, weathering,
• cracking and peeling, frosting,
• corrosion of fastening elements,

All of these factors can add to the construction damage caused by natural aging weather and the already mentioned faulty design and construction of external building parts.

The resistance of building materials used in external facade elements to air pollution varies greatly. The limit values described so far in research and the literature, which determine the extent of the harmful effects of air pollution, make it difficult to draw a line between the natural aging process and damage caused by air pollution, especially when these forms of damage are also accompanied by construction damage. This is confirmed by recognition that the weathering process, the natural aging process, and damage caused by air pollution, especially the effects of harmful substances, accelerate natural aging in most building materials used for external structural elements, as has been demonstrated in different countries and stone types, including Hungarian sandstone [14], Hungarian limestone [15], Czech sandstone [16], and Italian marble [17].

2.4. Typical Damage to Stone Elements on Facades

The stone materials used for facades are primarily exposed to weather conditions, which are closely related to geographical location. Differences in precipitation and temperature fluctuations mean that the possibilities and their evaluation cannot be standardized. There are types of stone used in large quantities for which methods have been developed for typical failure and testing. Laboratory tests were conducted in parallel with real cladding elements, including destructive and non-destructive tests [18].

Brito et al. [9] developed an evaluation system in which natural stone cladding defects were classified into seven classes: surface discoloration, breakage or cracking, looseness of stone fixings, biological damage, thermal expansion defects, joint defects, and defects in fixing elements.

The two most significant stone deteriorations were caused by climatic factors and by inadequate cleaning or inadequate protective treatment procedures. That is, the defects were mainly caused by the selection of materials that were not suitable for the environmental effects or by the use of poor protective materials.

Three intervention categories are proposed: intervention within six months, intervention within one year, or further observation without intervention. Discoloration was the most common, followed by damage to various fixings. A method for selecting covering materials was also developed [19].

In this analysis, it was found that natural stone paving, with proper maintenance and cleaning, has the longest lifetime, at 70–177 years, which is clearly more favorable than plastered or ceramic-clad surfaces. Several groups have studied discoloration of paving stones and developed regular cleaning methods [20] and life assessments [21]. In many cases this is essential for horizontal coverings, but this knowledge can also be used for facades. Other studies also emphasize that maintenance and good maintainability are key issues for facade coverings [22,23].

Limestone represents a significant proportion of the stones used in the construction industry in Hungary. It is used as a raw material and binder at construction sites, for example for making mortar and concrete, and for exterior and interior wall coverings on facades, giving buildings an excellent, aesthetic appearance. In the case of coverings, floor coverings, stairs, counters, windowsills, and elbows can also be made from it.

The most common compound found in limestone is CaCO₃. The appearance rate of calcium carbonate in the stone can vary significantly depending on the formation conditions. The compound dissolves in acidic water, e.g., acid rain, a process that can be described by the following equation: CaCO₃ + H₂CO₃ <-> Ca(HCO₃)₂. As a result, limestone slabs can be significantly damaged by acid rain, which can even be combined with biological pollution [24].

The dissolution of limestone by rainwater is a natural process and already occurred with pre-industrial rainfall. However, acid rain in cities has further worsened this process.

In parallel with the deterioration of urban air quality, the amount of sulfur dioxide (SO₂) emitted has also increased, which has accelerated another form of limestone degradation. Sulfur dioxide formed from heating equipment, car exhaust, and other industrial activities combines with precipitation water (H₂O) and the oxygen (O₂) content of the air to form sulfuric acid in a manner described by the following reaction: SO₂ + H₂O + 0.5 O₂ → H₂SO₄. Sulfuric acid converts the calcium carbonate material of limestone into gypsum by the following process: CaCO₃+ H₂SO₄ → CaSO₄(gypsum) + H₂O + CO₂.

Gypsum, similar to dissolved lime material, precipitates on the outside of the stone as the stone dries due to the solutions moving in the pores of the stones, so this process also contributes to softening of the inside of the stone, thus also reducing the strength of the stone. This process is also most characteristic of coarse limestone, which is more porous and therefore more permeable to solutions. The hard, often dark grey, layer deposited on the surface of the stone material can lead to even more intensive stone material destruction if the necessary conditions continue to exist.

This process can cause significant aesthetic problems, especially in areas characterized by high concentrations of particulate matter, since, during precipitation, a significant amount of dust, soot, etc. can be incorporated into the structure of the outer surface of the stone, which can turn even snow-white lime surfaces into soot-black within a few decades. This can be observed in the case of many buildings in Budapest, so everyone has probably encountered this phenomenon.

A good example of this is the many historical buildings in Budapest, such as the Matthias Church or the Parliament building [25], where the porous limestone slabs had to be replaced with travertine in order to avoid practically continuous cleaning.

Blackening of limestone was also observed in the case of the Hungarian Parliament Building (Figure 1). Originally, only freshwater limestone was used for the external wall cladding on wall surfaces that were considered more exposed (e.g., plinth), while in other places much cheaper but lower quality porous limestone was installed, which was delivered to the Parliament Building from the Sóskút and Biatorbágy mines. Deterioration in the quality of the Sóskút limestone due to its high porosity (it can reach up to 37%) was visible even after 25 years, so, since the completion of the construction of the ascending walls of the Parliament Building in 1894, the white, coarse limestone cladding of the building had to be cleaned on several occasions to prevent the aesthetic quality of the building from deteriorating. As a result, one of its wings was almost continuously scaffolded from 1925 to 2014, where sometimes only cleaning took place, while sometimes stone replacement took place. Since the 1970s, only high-quality, low-porosity, spring-water limestone travertine has been used to replace cladding elements. The renovation lasted with minor interruptions until 2014, after which it has not been necessary to repair the stones of the Parliament Building [26,27].

However, at the same time, it can be seen in the example of the Parliament Building in Budapest that, at the time of construction (1885), the stone chosen was porous limestone, which was still suitable, because at that time horse-drawn transport was used, which did not cause damage. Motor vehicles were not mass-distributed in Hungary until 1960. The explosive growth of urban transport caused significant SO₂ and SO₃ emissions, thus causing mechanical and aesthetic damage to limestone cladding. See the black surface on the right side of the Parliament Building in Figure 1.

Marble is a catametamorphic version of limestone, so its mineral composition and chemical characteristics are the same, but its porosity is lower, making it more durable. However, it reacts to chemical influences in the same way, and this means exposure in all major cities around the world [28].

As a result, great care must be taken when selecting stone material. In conclusion, we can conclude that stones with high porosity, such as looser-structured, more porous. soft limestones, are not advisable for use in urban polluted environments for cladding facades, as they turn black and start to weather prematurely. Urban acid rain dissolves and damages limestone slabs. Various marble coverings also easily lose their shine and color under harsh conditions.

Granite is beautiful and durable, and despite the fact that its cold, precise effect is somewhat alien to Hungarian architectural traditions, granite coverings, which are durable and withstand environmental influences very well, have become one of the most important cladding materials in bank, office, and hotel architecture (see: Hotel Kempinsky).

Some exotic stone materials, e.g., serpentinite, cannot be used due to the traces of iron contamination they contain. This can cause reddish-brown runoff on the facade, which spoils the overall effect and aesthetics of the cladding.

2.5. Traditional Paving Joints

The nature of the covering, in addition to the stone material and surface treatment, is determined by the joint pattern, which can be restrained or very pronounced. The design of the gap is determined by the shape and size of the facade and the maximum dimensions of the stone slabs. The gap design can be very diverse, although it is primarily a matter of thinking about regular patterned stone slab divisions, but it can also follow the pattern of the carved stone wall or create a random effect, like the facade stone cladding on the MŰPA building (Figure 2). MŰPA is one of the most modern cultural institutions in Budapest.

The layout of the joint pattern gives a theoretical stone layout, from which the clear dimensions of the stone slabs can be obtained by subtracting the size of the joints. In many buildings, the joint pattern is used exclusively as a decoration, although joints of only a few millimeters are unsuitable for “moving” large surfaces. Traditional joint design schemes are shown in Figure 3. Traditional stone cladding is made flush with the back wall, while modern, state-of-the-art stone cladding is built as a thinner layer, hung on the load-bearing masonry with fasteners, with an air gap and thermal insulation.

Even if the fasteners do not corrode or are replaced in time, thermal expansion of the stone slabs can have an uneven effect when using elements of different sizes in stone cladding hung on a facade.

The stone slab cladding solution of the Finnish architect Alvar Aalto gained great recognition in the 1960s, as it achieved a striking architectural effect even with the use of 2 cm wide joints. However, the connection of the fixing elements with the stone slab was fixed, so the stone slabs were trapped between the fixing elements. Thermal expansion could not take place, and uneven thermal expansion occurred, which curved the stone slabs. A deformation problem arose [11,29]. The cause of the error was not clear at the time, so it was not clear during the reconstruction. The stone slabs of Alvar Alto’s Helsinki Hall deformed again over time (Figure 4). So, this was not a quality problem of the stone material. Here it was a fortunate case, because the stones did not come off, and their curved surface has a special architectural aesthetic effect today [30]. Today, the wavy stone surface of the building has also become iconic among architects.

2.6. Building Physics and Fastening of Traditional Stone Cladding

Traditional stone cladding was built without thermal insulation, either assembled with the masonry or directly attached to it using clips. Traditional stone slab claddings, 30–40 years ago, were usually made with an un-ventilated air gap. From a moisture technical point of view, a more correct solution is to have free ventilation of the air gaps. The fastenings in each case represent a critical connection, which are hidden on the finished structure, and special attention must be paid to their execution, taking into account the weather conditions as an outdoor structure [31].

On the other hand, in our older buildings, there was no separate thermal insulation under the stone slab cladding; this was provided by the thick, 51–100 cm brick wall. External stone cladding, especially in the case of plinths, was built together with the back wall (Figure 5).

Thermal insulation placed on the inside for structural reasons did not work due to building physics reasons. A 2–3 cm thick air gap was created between the wall structure and the cladding. The gap behind the stone slab was useful for draining the moisture that had diffused through the wall, but only if the air was allowed to enter and exit from above and below, and the moisture to escape. In places exposed to greater stress (plinths, window frames, etc.), thicker carved stone elements were installed, and the gap behind the stone slab cladding was filled with mortar.

In the past, stone slabs were built with fixed wire clamps fixed to the wall at points (Figure 6), but based on current knowledge, it can be said that a flexible connection is more appropriate. The fastening was performed with hot-dip galvanized or tin-coated steel fasteners and quick-setting gypsum or cement mortar. Pitting and contact corrosion on the coating of the connecting elements significantly shortened the service life of the otherwise reliable connection. The connecting elements were made of wire or sheet metal.

2.7. Fire Resistance of Ventilated Stone Cladding

The air gaps of modern layered ventilated stone cladding can be a significant source of danger in the event of a fire, as the facade can burn out much more easily in the air gap [32]. Therefore, Hungarian National Fire Protection Regulations stipulate that, when constructing ventilated facades for buildings, the entire system must be qualified for the facade fire spread limit, and each of its structural elements must be fireproof, i.e., non-combustible. In the case of stone cladding, the cladding material itself is non-combustible, and the fixing clips are made of non-combustible material. The bottleneck of the system is the insulation included in the layer order. We use a variety of insulation materials for our buildings, including EPS, XPS, PIR, fibrous stone wool, and glass wool thermal insulation.

Sandstone and limestone are damaged at temperatures above 300 °C [33,34]. This is also evident from their discoloration, which is diagnostic [35]. The reason for this coloration is that not only physical, but also mineral, changes occur [36]. Load-bearing walls can also be damaged, especially if the material of the structure is concrete that has not been designed or has been inadequately designed for fire resistance [37]. However, steel is always the most critical factor [38], in this case, as the fastening element.

3. Results

3.1. Modern Assembled Stone Cladding

In Hungary, mainly limestone and sandstone (Figure 7), which are metamorphosed stones, were used in the past, but due to changes occurring, such as urban traffic, acid rain, and air pollution, we prefer harder, denser stones. Furthermore, after the change of regime, transportation from abroad is no longer a problem; anything can be bought and imported.

The most commonly found stones worldwide are granite and basalt (Figure 8), which, in addition to many other constructions uses, are also very good paving slabs. These igneous stones have appropriate hardness, density, and wear resistance, which makes these tiles popular for both horizontal and vertical surfaces.

We consider these to be the most important characteristics of modern stone coverings:

• the facade stone cladding is made with a free ventilation air gap,
• the thermal cross-section of the boundary facade wall—without cladding—satisfies basic thermal insulation and moisture protection requirements,
• the thickness of the stone slab cladding is at least 2.5 cm, and the largest panel size is not larger than 30 times the thickness,
• the suspension connection of the lower and expansion panels is solved,
• the intermediate anchorages—inside the expansion panel—are flexible,
• the connection and fastening elements are partially or completely resistant to oxidation—rainwater hitting the facade can be perfectly drained from the surface,
• the movements and forces resulting from solar radiation and variable heat load are equalized 50% within the element suspension and 50% within the expansion panel,
• the facade doors and windows must be closed with the inverted casing in such a way that the results of their mechanical movements remain within the casing field without reducing the quality of the window installation,
• defective elements can be replaced without opening the facade.

3.2. Relationship Between Porosity and Density and the Excretory System

Stone tiles have a very diverse appearance, with a wide range of colors and grain structures, which facilitates architectural aesthetic design, and their physical and chemical properties also make them an optimal cladding material. The size and thickness of the tiles vary depending on the type of stone; the two measurements are naturally related, as, within a given type, a larger surface area corresponds to a larger thickness, and vice versa. The largest dimensions of stone tiles are—in addition to the above—also influenced by the possibility, method, and means of extraction. Consequently, the thickness and size of thin-sawn stone slabs that can be produced can be narrower and larger than harder stones, so in the case of marble and granite, it can be sawn to a minimum thickness of 2.5 cm, travertine to 4–5 cm, and coarse limestone to 5 cm, while the available slab sizes are 1.2 m² for granite, only 0.6 m² for softer limestone (travertine), or 0.75 m² for coarse limestone. In the case of special demand, they can also be produced in much larger slabs, but their use as facade cladding is not practical. The other aspect to consider is the weight of the stone types: denser, harder stones are heavier than more porous, softer stones.

Porosity is the amount of free space between mineral grains. The density range is inversely proportional to the porosity of the stone (

Table 1. Basic characteristics of the major stone types used in facades.

Stone Type	Density [kg/m3]	Min. Thickness [cm]	Porosity [%]
Sandstone	2000–2600	4–5	20–24
Limestone	2600–2700	3–4	0.25–1
Travertine	2100–2500	3–4	3–11
Marble	2600–2900	2–3	0.5–2
Granite	2600–2800	2	0.3–1
Basalt	2800–3000	2	0.5–25

). It is measured either as a decimal between 0 and 1 or as a percentage. In crystalline stones such as granite, which have closely packed mineral grains, porosity is usually quite low (less than 1%). At the other end of the spectrum is sandstone, with its large, individual grains of sand. Its porosity can range from 10 to 38% [39].

3.3. Surface Treatment of Facade Stone Coverings

The durability of stone coverings can be increased by grinding and polishing the surface, as the water absorption capacity of the surface is reduced, and the deposition of air pollution causing weathering is also reduced, or by applying protection with modern nano-technology compounds on the surfaces [40]. Laser technology methods have also appeared lately [41]. At the same time, these new methods are still very expensive. They are mostly used only for sculptures and occasionally for monuments, but in the future, they may also be useful on public buildings. However, a significant architectural effect can be achieved by various surface treatments. In the case of dense limestone and marble coverings, rubbed, roughly polished and finely polished surfaces are typical, but, for example, the travertine covering of the Hotel Marriott, which was roughly processed with discarded metalworking planers, also proved to be durable and aesthetic (Figure 8).

The surface finish of stones used for facade cladding can be honed, polished, or flamed.

• Honed—the surface of the stone is ground to a flat, even finish. In the case of stones that contain natural luster, such as granite or marble, the lens or shine is removed, creating a matte (unpolished) surface that is less reflective and has no protrusions or ridges.
• Honed and unfilled—imperfections and pores are left unfilled, giving the product a more rustic feel, especially in outdoor applications.
• Honed and filled—imperfections and pores are filled, and the surface is left honed.
• Polished—a mirror-like finish that creates a high-gloss finish.
• Polished—this finish provides an antique, rough appearance. Hard plastic or metal brushes are used to achieve the final shape.
• Flamed—a process in which granite or other stone is exposed to high temperatures and then cooled suddenly. This process results in cracks and texture on the surface of the stone, creating a rough, non-slip surface.

The most common surface treatments for granite coverings are polished, honed, and flamed. In Hungary, harder, low-porosity stones are increasingly gaining ground in the facade covering market, due to their low maintenance costs and long durability.

This also proves that, although there are very good material processing structures, the lifetime of stone paving can be significantly increased by reducing porosity. The most important thing is the correct choice of stone, taking into account climatic conditions and urban pollutants.

3.4. Impregnation of Stone Coverings

Another possible solution to improve the durability and sustainability of stone coverings is surface impregnation. Natural stone surfaces are extremely aesthetic and durable covering solutions, but without proper surface treatment they can lose their aesthetic value over time. Impregnation is a process that helps to preserve the original beauty and durability of stones, while increasing their resistance to dirt and moisture [42].

Stone is a porous material by nature, so it easily absorbs water, oil, and other contaminants. As a result, discoloration and staining can also occur. The impregnating agent penetrates deeply into the structure of the stone, closing the pores without changing the natural appearance of the stone.

Advantages of impregnation:

• The chance of moss and algae growth outdoors.
• Easier cleaning—Treated surfaces absorb less dirt, making cleaning easier and faster.
• Longer life—Impregnation protects the stone against environmental influences, freezing, temperature fluctuations, and chemicals.
• Aesthetic protection—Preserves the original color and texture of the stone, avoiding premature fading.

In summary, impregnation facilitates surface maintenance and cleaning and reduces water absorption, thus preventing the infiltration of various harmful substances into the stone surface, preventing staining and other damage, and maintaining the natural color and structure of the stone surface, which significantly increases the sustainability and life of the covering.

3.5. Modern Joint Design

The joints of modern assembled stone coverings, in addition to aesthetic considerations, play an important role in thermal expansion. Joint patterns often show geometric regularity (Figure 9). The joints formed on the external stone cladding can be closed with a clamped joint or a sealed joint, or left open, but in all cases, they are intended to equalize and dissipate the stresses resulting from thermal movement of the stone cladding.

3.6. Dehumidification of Installed Stone Slab Cladding

Traditional stone cladding was built without thermal insulation, either assembled with the masonry or directly attached to it using clips. Modern installed stone cladding is installed with a ventilated air gap and thermal insulation, which also has a positive effect on eliminating building physics problems. In the system, air continuously flows in the air gap under the stone cladding, cooling the building in summer, while in winter the thermal insulation protects the interior spaces from rapid cooling.

In addition to the shielding effect, the air gap behind the stone slab cladding provides an opportunity for free flow of the wall’s vapor diffusion. The efficiency and vapor transport drying ability of the air gap depends on the boundary structures, the lower and upper inlets and outlets, and the height of the air column.

In built-in stone cladding, we primarily use open joints, which ensure continuous air supply and exhaust. The actual experimental model and computer simulations have proven that a minimum air gap thickness of 3 cm is required to eliminate any unevenness in the wall structure. This type of open joint arrangement ensures continuous air supply and removal. In the case of closed joint stone slabs, simulations have shown that at least 150 cm² of ventilation openings is required per 20 m² of floor area to achieve effective back wall ventilation.

The inner surface of the stone slab delimiting the air gap does not usually impede air movement, but the surface of the back wall or the thermal insulation boards can already influence the intensity of air movement, which can cause problems, especially in rooms with high humidity. The height of the ventilation air gap of larger, multi-story buildings can even multiply air flow of the air layer in summer, especially in sunny facades, facilitating cooling of the facade.

There are other methods to eliminate the vapor diffusion pressure: for example, not sealing the joints of the facade stone coverings, but leaving them as free openings, with a gap size of 4–6 mm. In this case, the free cross-section for inlet and outlet ventilation can be reduced, while the ventilation air gap can be 5–6 cm in size. Increasing the thickness of the air gap is not necessary for air exchange, but primarily for driving rain.

3.7. Thermal Expansion of Stone Slabs

Different building stones have a significant coefficient of thermal expansion, depending on their mineral composition. In particular, darker colored stones can absorb a greater amount of heat in the summer, which causes their volume to increase. Stone slabs, especially noble stones, are often made with tight joints, so the stones cannot move when the temperature changes. Thus, there is no room for dilatational movements, and therefore deformation, splitting, or even breakage may occur.

Therefore, when choosing the size of the joints, the coefficient of thermal expansion of the facade stone materials cannot be ignored:

$${\alpha }_{\mathrm{granite}}=7–9\times {10}^{-6}{\displaystyle \frac{1}{°\mathrm{C}}} {\alpha }_{\mathrm{limestone}}=5–20\times {10}^{-6}{\displaystyle \frac{1}{°\mathrm{C}}} \Delta l={\alpha }_{\mathrm{stone}}\times l\times \Delta t$$

In the 1970s, facade claddings with tight joints had the unfortunate consequence that, in the summer heat, the heated outer cladding could not expand properly, so the cladding elements “pushed” each other off the facade. Of course, this damage did not appear immediately, only decades later, when the corrosion of the carbon steel embedded in the mortar became so severe that it could no longer withstand the stress resulting from expansion of the facade stone slabs.

In our research, model calculations showed that, when using compressed or sealed joints, expansion fields should be created every 10–15 m² by incorporating a 6–10 mm expansion joint into the system, and in the horizontal direction every 3.0 m. In the vertical direction, 6–10 mm wide expansion joints filled with permanently elastic material should be created in the covering per floor height. Failure to do so may cause the covering to bend, crack, and break.

In the case of open joint covering, if the supports, console, and suspension elements form a continuous unit, expansion fields must be created at least every 30–50 m², so that one of their dimensions is 5–6 m and the other dimension is no more than one and a half times the previous one. In the case of a south facade, independent expansion fields should be created every 30–40 m²; in the case of a west facade, every 35 m²; in the case of an east facade, every 40 m²; and in the case of a north facade, every 45–50 m² [43]. Considering the cardinal directions is therefore an important aspect in the design, since the effect of solar radiation varies depending on the cardinal direction, and the heat received from the north and east directions always remains below the solar radiation coming from the south and west directions, which are the most dangerous (Figure 10 and

Table 2. Joint distances according to the cardinal directions, in m.

Type	Corner Design	North [m]	North [m]	South [m]	South [m]	West [m]	West [m]	East [m]	East [m]
		With Air Gap	Without Air Gap	With Air Gap	Without Air Gap	With Air Gap	Without Air Gap	With Air Gap	Without Air Gap
Granite	Open joint	14	10	9	6	8	7	12	9
	Closed joint	7	5	4	3	4	3	6	5
Limestone	Open joint	10	8	8	7	7	6	9	8
	Closed joint	6	5	5	5	5	4	6	5
Sandstone	Open joint	8	6	6	5	5	4	7	6
	Closed joint	4	3	3	3	3	2	4	3

).

3.8. Summer and Winter Thermal Conditions

The location where we plan is always an important aspect in architecture. As we move towards the equator, we can expect more and more intense solar radiation, which entails a significant heat load. Helsinki is located on the edge of the cold climate zone of the northern hemisphere at 60° north latitude. If Alvar Alto’s Helsinki Hall building were built in a city in the hot climate zone, it would be expected that the stone slabs would not only bend, but also break.

In modern layered facade stone cladding, solar radiation heats the external facade stone cladding in the built-in air gap, which also heats the air layer in the air gap behind it, which in turn starts a flow in the air layer, releasing warm air at the top and sucking in cold air at the bottom. This flow continuously flushes the air gap in the summer, cooling the structure and preventing the heating caused by solar radiation from reaching the inside of the structure. This significantly reduces the energy spent on cooling the interior spaces and cooling the stone cladding, too.

In winter thermal conditions, the external cladding does not play a significant role. In this case, thermal insulation and its material and thickness come to the fore in the layered structure. According to our energy calculations, in Hungary, typically 15–20 cm thick thermal insulation is required under modern built-in stone coverings. This prevents cooling and heat loss of the interior spaces. This is true everywhere in the temperate zone with a continental climate. The thickness of the thermal insulation determined by calculations gives the necessary bracket dimensions.

In winter, the vapor flows from warmer interior spaces with higher internal vapor pressure to colder exterior spaces with lower vapor pressure. Here, the air gap in the layered structure comes to the fore again, since the vapor that passes through the masonry is removed by the air flowing in the air gap (ventilation effect), instead of condensing, preventing condensation in the structure. If condensation does occur, the air gap drains it out at the bottom, preventing water from causing damage to the structure.

3.9. Fire Resistance of Ventilated Stone Cladding

In our studies, we started from existing regulations to obtain the Hungarian system permit. We determined the materials that can be incorporated into the layered system of assembled stone cladding. During the qualification process, the entire layered system was built up and exposed to significant fire effects, as a result of which it was possible to prove that, even after construction of the specified materials, no fire spread occurred in the air gap. This result also means that the use of the materials we specified can be appropriate anywhere in the world.

According to the regulations and requirements for fire protection of assembled stone facades with air gaps, the requirements for the fire protection class of the external space-delimiting wall, the thermal insulation material and the wall cladding, the coating, the plastered thermal insulation system, and the interruption must be met.

In the case of assembled stone facades with air gaps, only non-combustible thermal insulation materials may be used, to prevent fire spread within the air gap. Only fibrous thermal insulation materials meet this requirement [44].

In summary, due to the continuous ventilated air gap, the facade poses an increased risk in the event of a building fire. Dozens of fires prove that fire spread here can be extremely rapid and devastating. The regulation accordingly makes it mandatory that the entire facade cladding system must be certified for the facade fire spread limit. Compliance with this is only possible with appropriate materials and certification of the entire system.

As a result, only materials that are not flammable can be installed. The most dangerous material in the planned layering system is thermal insulation. The installation of EPS, XPS, and PIR thermal insulations is prohibited. The only thermal insulation that can be installed is stone wool/mineral fiber thermal insulation.

3.10. Fixing the Assembled Stone Slab

The stone slab cladding of facades is fixed to the load-bearing masonry with fixing elements, which, passing through the thermal insulation, minimally interrupt it, thus forming point-to-point thermal bridges. We are currently conducting experiments to find out how to insulate these stainless steel fasteners without weakening them too much. At present, there is no perfect solution for this, but it can also be stated that the impact of this type of point-to-point thermal bridge on the entire building is not significant; according to our current knowledge, it is negligible [45].

3.11. Modern Fastening Methods

The increase in energy prices in recent decades and the new, stricter thermal standards have also helped the emergence of wall structures insulated under a layered external cladding, and thus the development of new fastening systems. The ideal general layer structure from a thermal point of view can be the following (Figure 11):

• 2–6 cm stone slab cladding,
• 3–5 cm ventilation gap,
• 12–20 cm thermal insulation (thickness varies depending on the climate),
• load-bearing wall structure.

The layer structure implies that the stone cladding and the load-bearing wall structure are further apart, so the traditional connecting elements are no longer adequate. We have developed new, modern clamps and fasteners together with the industry, in accordance with the new requirements. The development is not yet complete, and research is ongoing in several places, including the Budapest University of Technology and Economics, because the ever-tightening energy standards require thicker thermal insulation, so external stone cladding is increasingly further away from the load-bearing masonry. This results in greater reaction forces in the fastening elements, making them larger and larger, which in turn creates point-to-point thermal bridges, reducing the effectiveness of the thermal insulation [45,46].

3.12. Fasteners

Modern fasteners are high-strength, rust-proof “steel alloys” containing approx. 18% Cr (Chromium) and 10% Ni (Nickel). These steel alloys are also resistant to weak acids. In Hungary, it is still a problem—especially on smaller-volume construction sites—that, due to the high price of these steel alloys, hot-dip galvanized fittings are used instead. Fasteners made in this way are susceptible to pitting corrosion in the case of tin coating, as the coating metal is more noble than iron, so its electropotential is higher. In the case of zinc coating, iron is the nobler material; in this case, the zinc coating is destroyed first, and only then does the iron structure begin to rust. The further apart the electropotential of the two metals is, the faster the corrosion progresses. Corrosion can be further accelerated by atmospheric pollution, sulfur dioxides, nitrogen oxides, and hydrogen sulfide in the air. Corrosion is a slow process, and its danger lies primarily in the fact that these are concealed structures that cannot be checked even periodically, while the facade and supporting structure are exposed to moisture and chemical contaminants. Stone slabs are often located directly over the street, and a single element falling from a great height can cause significant damage, and even life-threatening injuries.

3.13. Fixing Methods

Wet fixing has lower material costs but can be built more slowly, because it requires more live labor, while dry technology is faster, but its material costs are also higher. In Hungary, the cost of live labor is also becoming more expensive, so investors have started to prefer faster dry technology solutions. Our research and development direction also favored dry fixing for these reasons.

Therefore, the most common type of new fixing methods is the so-called “dry” fixing method (Figure 11 and Figure 12a).

Figure 11 shows the basic element developed. Here, the load of the stone slabs is carried by dowels inserted into the wall. A perfect bond is ensured by a two-component synthetic resin adhesive. In this case, the design of the fixing element is more complicated, since it must ensure three-way adjustability even after fixing.

In the “wet” process (Figure 12b), compensation for construction inaccuracies and precise adjustment are made possible by a larger-diameter hole drilled in the wall, into which the fixing element is reinforced with quick-setting cement mortar or synthetic resin [35]. However, this type of cladding made with subsequent cavity formation is increasingly being replaced by the already mentioned dry technology.

The stone slabs must be secured at a minimum of four points, 1/4–1/5 from the corners, depending on the size of the stone slabs, to ensure adequate load bearing (Figure 13). The stone slabs can be secured along the horizontal edges or along the vertical edges. Holes (6–8 mm) are drilled into the stone slabs from the side or bottom, and the steel pins of the fixing elements are slid into these holes. In the case of fixing elements placed in vertical joints, the lower element is the supporting, load-bearing element, while the upper element serves only as protection against overturning and as an expansion element. Stone slabs have the possibility of expanding through a polyethylene sleeve in the stone covering hole. In the case of fixing in horizontal joints, both the lower and upper fixing elements fulfill both a supporting and an anti-overturning function [35].

4. Conclusions

The results of our university research and development work carried out during the domestic introduction of modern assembled stone paving can be summarized in the following main points. This is important because, during the design and construction of stone paving, compliance with these rules and regulations is necessary in order to create durable, aesthetic stone paving. The recommended, not recommended, and forbidden cases are summarized in

Table 3. Recommendations for technology, material, and element selection for the masonry system of assembled stone facades.

	Recommended	Not recommended	Forbidden
Stone type	travertine, marble, granite, basalt	sandstone * limestone *	-
Air gap	all types of stone	-	-
Dry technology	fast construction execution	highly skilled workforce required	-
Wet technology	slower construction execution	lack of highly skilled workers	-
Thermal insulation material	rock wool, mineral wool	glass wool	EPS, XPS, PIR

* Not recommended without impregnation.

.

4.1. When Selecting the Stone Materials for Cladding, the Following Considerations Should Be Made

When selecting the stone material appearing on the surface, stones with high density and low porosity are preferred over porous stone materials, and they can be further strengthened by machining the surface.

At the same time, in the case of porous stones (e.g., limestone), it is advisable to seal and fill the pores during facade use, thus protecting the stone from subsequent environmental effects, increasing the service life and durability of the cladding and improving its aesthetics and appearance. The use of stones with low density and high porosity is not recommended for cladding external facades.

4.2. Technical Aspects to Check When Designing Modern Air-Gap Wall Structures

Modern layered facade stone slab claddings are made with a ventilated air gap. The thermal cross-section of this facade wall system must satisfy basic thermal insulation and moisture technical requirements, with the necessary thickness of thermal insulation built into the layer order, which results in an energy-saving and maintenance-free aesthetic facade.

Only mineral fiber non-combustible thermal insulation of A1-A2 fire resistance quality can be installed in the layer order of installed ventilated stone claddings.

In the case of an open-joint facade, driving rain does not pass through the air layer to the thermal insulation due to the internal air pressure and the minimum 30 mm air gap width, but dries or drips off on the inner surfaces of the stone slab due to the rising air flow.

4.3. Building Physics Considerations in the Case of Modern Assembled Stone Cladding

The basic requirement of modern construction technology is the technical installation of the elements in such a way that thermal expansion movements can occur without causing damage. The movements and forces resulting from the varying thermal load are not mutually transferred to the adjacent panels, but are balanced within the expansion joints: each stone panel moves independently by expanding

In the case of a closed-joint facade, expansion fields must be created every 10–15 m². In the horizontal direction by slab level, and in the vertical direction by cardinal directions, a joint of at least 20 mm wide must be filled with a permanently flexible façade-sealing material.

In the case of an open-joint facade, a minimum 5 mm expansion joint must be created. There must be at least 2 mm of free space between the flattened head between the joints and the side of the adjacent stone panel.

Layered facade stone panel cladding must be made with a ventilated air gap, which is the most effective in terms of vapor control. In addition to the shading effect, ventilated facade cladding promotes the vapor diffusion efficiency of the ventilation air gap. The air gap size is at least 30 mm, and the vapor transport capacity is determined by the height of the air column. An open joint should be used to equalize the vapor diffusion pressure.

In the case of closed-joint stone slabs, a ventilation opening of at least 150 cm² per 20 m² per level is required to achieve effective back wall ventilation, which can be achieved by leaving the joints open.

4.4. The Following Requirements Must Be Observed When Attaching the Supports of Modern Mounted Stone Facades

The advantage of the multi-point fastening system per panel is that it reliably and flexibly fastens larger and larger surface stone panels and enables quick, easy installation.

The facade panels must be supported at the two lower points with at least four fastening clips, through a fastening pin in the side hole or horizontal hole of the panel, while they must be flexibly anchored at the two upper points to prevent sagging.

At connection points—to masonry or windows—a joint of at least 15 mm must be used.

The fixing holes on the side of the stone slab must be filled with a sealant or expansion sleeve to avoid dry connection, as the hole may wear out or tear due to wind effects. On the other hand, if moisture gets into the hole, it can trigger erosion processes, in which case the stone slab may detach from the wall.

In the case of stone slabs narrower than 15 cm, the corner slabs must be fixed perpendicularly to the adjacent stone slabs with corner fittings.

When selecting the fixing clip, the total loads associated with driving rain and wind loads must also be taken into account.

The clips and fasteners must be made of stainless steel and resistant to oxidation in a moisture-diffusion environment.

The facade stone slabs cannot be loaded; only the external fitting built in front of the slabs can be fixed to the rear background wall.

All fasteners to be installed must have the certificates required in the given country.