Designing with natural stone cladding

by Katie Daniel | February 29, 2016 4:25 pm

By Michael Picco, P.Eng.
The use of natural stone on buildings and paving can be traced back to the beginning of civilization. There are countless buildings, monuments, and structures dating back thousands of years that have stood the test of time and left generations in awe. Some examples of projects built primarily of local limestone include Egypt’s Great Pyramid of Giza, the Mayan temples in Mexico, southern Italy’s round Trulli homes, the Parthenon in Greece, and the Roman Coliseum (which also included local travertine). Many of these great structures were built with no mortar. The stones were simply cut and tightly fitted together and dry stacked—a method of stone installation mimicked in modern-day construction and referred to as ‘dry-stacked stone.’

In many ways, our ancestors in the building industry applied sustainable practices by using the larger quarried blocks for building construction and smaller waste pieces for erosion control and walkways. They also used the gravel to pave small roads from town to town.

When natural stone is properly specified and installed, few cladding materials last as long and perform as well. Not only is it esthetically pleasing, but it is also a low-maintenance, sustainable material.

The greenness of stone
Natural stone’s inherent characteristics make it a green building product—it can be used without any additional finishes or wall coverings, has low maintenance needs, and is highly durable. (This author acknowledges the assistance of Stephanie Vierra, Assoc. AIA, LEED AP [Vierra Design & Education Services LLC], in developing the article’s section on stone and sustainability matters.) Unlike the many cladding materials available on the market requiring extensive manufacturing energy, natural stone is extracted from the earth and processed, slabbed, finished, and cut.

As more emphasis is placed on the whole building design and life cycle assessment, the concept of a product’s embodied energy is an important and relevant measure. Embodied energy is a measure of the carbon dioxide (CO₂) emitted from the time raw material is extracted to the point it is installed on the building. This process has been referred to as ‘cradle to gate.’

Since building designs may be using more materials (and/or more carbon-intensive products) to achieve lower energy use, an increasing proportion of the total energy use and carbon emissions for high-performance buildings comes from its materials and products. By taking embodied energy into account, a project team can ensure it is designing for net carbon emission reductions. In the case of natural stone, this could include the CO₂ required during quarrying, transport to the plant, energy required for slabbing and fabrication, delivery to the site, and installation. There is still some work being done by various lifecycle analysis (LCA) authorities to clearly define the definitions of what is, and what is not, included in the calculations for various products to ensure a consistent and fair measure is established. However, based on most studies and comparisons, natural stone is consistently rated as one of the building materials having the lowest embodied energy on many scales.

Different materials, different stone
With the advent of many new thin cladding manufactured products introduced to the market, there is a recent trend for designers to specify and detail very large cladding panels. When using products such as porcelain, recycled glass, glass-fibre-reinforced concrete (GFRC), fibre-reinforced polymer panels (FRP), or engineered/manufactured stone, care must be taken by architects and specifiers to make the clear distinction between natural stone and manufactured products. The design approach, attachment detailing, and structure can vary dramatically.

This author has recently seen a trend of substituting an engineered thin cladding product with large, thin natural stone panels, with the erroneous assumption they were basically interchangeable. The variability of natural stone requires a design approach quite different than manufactured products and, thus, has different limitations associated with panel size and connection design.

New natural stones are being introduced to the market on a continual basis. Many are a result of new quarry areas and stones being discovered, and many are a result of new technologies allowing extraction of stones previously impossible to quarry (described later in this article).

Some of these technologies include the use of epoxy injection at the quarry with stones that are heavily flawed and cracked. In other cases, vacuum epoxy injection is employed to stabilize heavily cracked and fissured blocks of stone. For the general consumer, these injections would be undetectable as they would just appear as another vein in the material.

The epoxy in the stone and the amount of veining can have a dramatic effect on the technical properties of the stone. In most cases, these heavily cracked and filled materials are restricted to interior use. Many of the very desirable and heavily veined onyx slabs on the market today are a result of these technologies. In the past, blocks large enough to produce slabs were impossible to extract in one piece.

Factors in selecting and detailing stone
Having been involved with stone detailing and design for more than 30 years, this author has come across numerous projects requiring complicated and costly engineering solutions due to insufficient planning and research during the initial stages of a project. The need to source a new stone material late in a project can lead to unexpected costs, schedule delays, and frustration.

In order to greatly reduce the possibility of these and other problems, it is imperative the design professional dedicates time and resources to research materials and methods of installation amongst many other details. In many cases, engaging an expert with experience in stone selection, design, and detailing to become part of the design team early can turn out to be a cost-effective solution.

In choosing a stone, several factors must be considered. Although the colour, texture, veining, and/or grain size are always critical aspects to architects and designers, there are other aspects that must override these.

Strength
Is the stone strong enough to withstand all the design loads at the thickness and size specified? Particularly when using stone on the exterior, climate conditions can make a stone suitable in one environment and not in another. Seismic loading governs many designs on the West Coast, whereas East Coast Canada is primarily designed for wind loading. It is important the design team consult both local codes and the authority having jurisdiction (AHJ).

Block size
Is the stone specified available at the size detailed on the drawings? Depending on the stone type, quarrying equipment, and technology available, blocks are extracted in several different sizes. Standard block sizes for most granites and marbles will be approximately 1.5 x 3 m (5 x 10 ft). Many limestones, as well as more exotic stones like onyx block, can be much smaller.

Finish
Is the finish suitable for the proposed application? When using stone as paving, slip resistance is an essential property to consider. Finishes such as sand-blasting and flaming can make the stone more absorptive and potentially more susceptible to freeze-thaw deterioration.

Exposure conditions
Is the stone exposed to high levels of pollution? Will the stone undergo several freeze-thaw cycles? Is the stone exposed to high levels of sodium (coastal exposures)? In severe exposure conditions, stone selection becomes more critical, and choices
may be more limited based on the material’s technical properties.

Testing
Most stones quarried and sold have readily available ASTM test data including density, compressive strength, flexural strength, absorption, and abrasive resistance. Many materials are supplied with testing performed by different jurisdictions such as EN (i.e. European Union) and CE marking of stone also introduced in the European market.

Current test data is valuable for providing the information required for the initial evaluation and selection of a stone for a particular application in a given location. Current test data must be performed in order to determine the actual strength of the stone being quarried and used for the project. In most cladding applications, the most common test giving the most useful information is ASTM C880, Flexural Strength Test for Dimension Stone. Individual anchor tests should be performed with the proposed anchors being suggested for the project. These tests are also outlined in the ASTM standards.

Backup selection
The types of backup used by the building designer can, in many cases, determine the success or failure of a cladding project.

Concrete
Concrete tends to be the preferred backup for exterior and interior cladding. Anchors can be placed anywhere, allowing the stone designer the flexibility to design an efficient, installer-friendly anchoring system. Anchor capacities are typically highest when installed into concrete.

Masonry
When used for interior cladding where panels are stacked, concrete masonry units (CMUs) can comprise a very suitable backup assembly. If used for exterior cladding (i.e. where anchors are designed to support the weight of each panel), the CMUs must be solid-filled with masonry grout to ensure anchors can achieve the required capacity.

If masonry is not properly filled, positive anchorage is very difficult to achieve and may require the use of adhesive or epoxy anchors. These anchors are more costly and require time to set up, particularly when ambient temperatures are low.

Steel sub-frames
When designed by the stone installer’s engineer, steel sub-frames can be a very effective and economical backup. However, if the frames are not designed to accommodate the stone designer’s anchors, they can be ineffective and very often require either additional steel, or for the stone designer to engineer an expensive anchoring system to suit the steel provided.

Pre-panelized sub-frames
Pre-panelized sub-frames can also be an effective installation method, allowing much of the installation to take place in a shop protected from the elements. However, this method of installation requires close co-ordination with the architect and prime engineer on the project.

Metal stud
When used for interior cladding where panels are stacked, metal studs serve as an acceptable backup. When used for exterior cladding, however, it is the least preferred. This is because the capacity of anchors into metal studs is limited. The placement of the studs is critical and usually does not align with the stone connections. Consequently, use of a continuous horizontal formed channel along each horizontal joint to facilitate the stone anchors is required.

There is also some concern with the service life of the anchorage into metal studs, given the drilled anchor must cut through the metal stud—possibly compromising its corrosion resistance at the anchor. This results in the stone having a much longer service life than the anchors.

Specifying anchors
In several cases, anchors are misinterpreted by architects and designers as ‘masonry ties.’ In the transition to thinner veneers, the traditional masonry tie and stacked stone has continued to be incorrectly used in many thin cladding projects.

Anchors must be capable of independently supporting all the loads imposed by each stone. Generally, stones should not be stacked. However, in some conditions, with proper engineering, a stacked stone system can also be used with mechanically engaged anchors. The failure or breakage of a stone should neither affect any adjacent stone nor cause the failure or collapse of the stones above.

It is highly recommended all anchoring components be manufactured from Grade 304 stainless steel; for particularly harsh environments and coastal projects, Grade 316 stainless steel is often specified. While hot-dipped galvanizing secondary anchoring components are sometimes a little more economical, use of dissimilar metals can result in corrosion problems if not properly detailed and installed. The cost to upgrade to an all-stainless anchoring system is usually negligible when compared to the total cost of the cladding project. All stainless offers peace of mind the anchors will perform as detailed for life of the building.

There are also many new proprietary anchoring systems that are available, many of which are back-anchored into the stone, allowing for an open-joint rainscreen design.

Conclusion
Provided it is correctly designed, stone can be an everlasting, maintenance-free material. Nevertheless, this author has seen scenarios where stone has the potential to be disastrous when used and detailed improperly, as evidenced by the many re-cladding and re-installations required because of either poor design and installation or the wrong stone selection.

There are readily available resources from the Terrazzo, Tile, and Marble Association of Canada (TTMAC) including the second volume of the Dimensional Stone Guide for stone 19 mm (¾ in.) and thicker, along with Specification Guide 09 30 00–Tile Installation Manual for thinner stone. Further, version 7.2 of the Marble Institute of America (MIA) Dimension Stone Manual is another very useful resource.

Michael Picco, P.Eng., is president and CEO of Picco Engineering. With 30 years of experience in natural stone cladding and structural engineering, his expertise includes stone sourcing and selection, consulting, manufacturing, installation, testing, as well as the design and detailing of stone anchors and shop drawings. Picco has spoken to various associations including Terrazzo Tile and Marble Association (TTMAC) Trade School, Saskatchewan Masonry Institute, Marble Institute of America (at Stone Expo), and appeared at various international conferences. He was elected to the board of directors of the Marble Institute of America (MIA) in 2015. Picco can be reached via e-mail at mpicco@picco-engineering.com[1].

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