The facade of a building is the first clue that tells us that structure may have something special to offer. Lightweight natural stone ventilated rainscreen facades are becoming more commonly used in modern buildings because of their benefits in efficiency and aesthetic appeal.
The way buildings are constructed has changed fundamentally over the past 35 years. While the original change in “modern” architecture — building stressed frames, on which less-stressed walls are attached — was developed at the end of the 19th Century, and brought us the skyscraper, it was really post-modernist architecture that developed the full use of the facade.
Where in the past facades were seen as decorative elements on the fundamental structure of a building, post-modernism buildings became more about their facades as a fundamental cultural expression of what the building was.
You could date post-modernism’s entrance into the mainstream back to the construction of the Portland Building, in Portland, Oregon, USA, in 1982. The architect Michael Graves (who passed away in 2015) conceived a building for the municipal workers of the city that was striking in that it attempted to bring human scale and dimensions to an outsize 15-storey block of a building. In particular, its pink exterior was unusual in buildings of the time.
Originally Graves had wanted to use a glazed terracotta facade, but, mindful of the tight budget for the building, he opted to use coloured reinforced concrete and fibreglass for the cladding and decorative elements. Functionally, the building has been criticised. The windows are unique and interesting from the outside, but from the inside, in a building of such depth, they lead to a strong reliance on artificial light.
This had led to recent calls for the building to be torn down. The architect himself has visited Portland to defend the building, and noted that many of the faults were the result of constraints imposed by tight budget restrictions. One reason the building is so dark, for example, is that the glass in the windows is tinted, as demanded by the government client, as a means to reduce expenditure on energy.
A more successful post-modern building, though not without controversy, is Graves’ Humana Building in Louisville, Kentucky, which was constructed between 1982 and 1985. The architecture critic for The New York Times, Paul Goldberger, described the effect of the building in June 1985:
“But for all the awkwardness of its overall shape, Humana is a warm and inviting building, perhaps the first skyscraper of our time to be both serious and visually alive. For too long architecture that has taken itself quite seriously has been a deadly bore, and architecture that has been lively has tended to be frivolous. But Humana is neither boring nor silly — it is at once a building of great dignity and a building of great energy and passion.”
What distinguishes the building more than anything else is its playful yet artful use of granite facade. Some 33,000 pieces of granite were used in the construction, in five different colours, each of which was sourced from a different country. The signature pink granite came from Finland, Brazil provided the green granite, Angola the black, and Sardinia the grey. Sourced as 20 tonne blocks, the granite was first shipped to the Carrara stone works in Italy, where it was cut into custom shapes and then shipped to the building site in Kentucky.
The rise of the façade
This has special relevance to the use of stone in modern architecture. While today the “excesses” of post-modernism have been increasingly moderated, the focus on the exterior facade of the building as a significant “character” in the design and aesthetic story has gone on to influence building. Colour and texture play an increasing role in making buildings that fit into a pre-existing urban landscape, and yet somehow also move the urban design story forward towards shapes and forms better suited to our modern times.
As architecture has required more modern methods of construction, so too the use of natural stone in modern architecture has had to adapt to modern methods. One such change is the widespread adoption of techniques such as ventilated rainscreen cladding, which now competes with other innovative facade finishes.
In traditional, “ashlar” construction, huge chunks of stone were often used to form a single leaf, which might be 300mm to 400mm thick. Today’s handset ashlar cladding is typically 75mm to100mm thick, often with 5mm joints. Using stone in this way carries with it a number of concerns and costs. The blocks of stone require additional support, usually on a floor-by-floor basis, through angle irons, along with restraint fixings doweled into the beds of the stone.
The modern alternative to this kind of construction is to use a form of rainscreen cladding, which both protects the building and adds insulating properties, faced with thin sheets of stone. Thin stone wall systems used for exterior building envelopes typically consist of stone panels ranging in thickness from 19mm to 52mm. Most panels are fabricated from granite. Marble, limestone, travertine and sandstone are used to a lesser extent.
The panels are lightweight and easy to position, often using scissor lifts, mast climbers or hydraulic platforms. A further advantage is that large format panels can be produced off-site to exacting standards. These are then quickly fixed to pre-positioned rails, with all application being dry fixed.
To create a typical panel, natural stone is quarried and cut into slabs, then bonded to an aerated concrete backing, around 16mm thick. The large slabs are then cut into finished panel sizes, calibrated, honed and polished, before fixing points are drilled and ceramic plugs inserted. Some manufacturers’ systems incorporate an invisible fixing system and can provide bespoke specialist features such as fully mitred corners, window reveals and soffits.
Typically, each panel is independently supported to the building structure or back-up system using an assemblage of metal components and anchors. In general, fixing utilises “helping-hand” brackets and vertical “T sections” which are used to support the rainscreen system. These can be installed with isolation pads to prevent cold bridging with horizontal rails attached to the T-sections and the support system fixed back to either light-gauge steel framing or blockwork.
Clips are attached to the backs of panels with ceramic fixing points, and all fixings are aligned and attached using the correct torque before panels are simply clipped on to the horizontal rails. Joints at the perimeter of each panel are usually 9mm in width, and are filled with sealant. A drainage cavity is usually located behind the stone panels to collect and divert to the exterior water that penetrates through the joints.
Panels are extremely impact resistant with the advantage of being one-third the weight of 40mm-thick natural stone and one-fifth the weight of 75mm-thick handset stone and can be produced in sizes up to 3.75 square metres.
Stone used in stone wall systems can have several finishes. For granites and marbles, a polished, highly reflective finish is common. Thermal finish is a rough textured finish that is often employed with granite. Also, smooth honed finishes are commonly used on all stone types used in stone wall systems.
Granites have had a long history of durable service. Certain marbles have a long history of successful use. Travertine, limestone, and sandstone have a good history of use as thick stone wall elements, but their service history as thin stone wall elements is fairly limited, particularly in terms of durability.
Most distress observed in stone wall systems can be attributed to anchors used to attach stone panels to the structure. Panel cracking, displacements, or other distress conditions can occur at locations where anchors are inadequately or improperly connected to the stone. Poor construction is often the result of poor quality control and out-of-tolerance fabrication or erection of the panels. Also damage from handling during construction can result in panel cracking, some of which may not become evident for several years.
When properly constructed, stone wall systems require relatively little maintenance as compared to other wall systems. In general, the only maintenance required is replacement of sealant in joints between panels. The time frame for this activity depends on the sealant used, but usually ranges between seven and 20 years. However, it should also be noted that periodic review and evaluation of thin stone veneers may be desirable in order to determine if any evidence of structural distress exists in the panels due to strength loss, and/or accumulated stresses at anchor points.