Peter Wilson
Manifesto Centre for Architectural Research, Napier University

Nowadays the word cladding is applied ubiquitously to all manner of building façade systems, but as applied to natural stone its etymology stems from the development of steel and concrete frames and the consequent potential to clothe these structures with non-loadbearing exterior walls. The particular systems now used depend as much on the building type and structural solution employed as they do on the intended aesthetics of the architecture, but in each case the skin is designed to carry no weight other than its own.

Initially, the design of stone cladding systems was founded upon time-honoured principles of loadbearing masonry construction, but the many incarnations available today have evolved in parallel with the development of mechanical fixing systems in which metal corbels, cramps and dowels transfer the loads to the structural frame. With its loadbearing function thus removed, natural stone has become only one of many material choices available for use in cladding systems. Despite this, however, stone is often preferred for its appearance, durability, longevity, quality and symbolic associations.

As the technology of fixing systems for cladding has become increasingly sophisticated, so too has the thickness of stones used been progressively reduced. Whilst this has resulted in weight and cost savings, it has also generated new technical challenges for the designer since the ways in which stone performs when its thickness is reduced can be highly variable. Depending on the building type and the particular environment it is being constructed in, the rates of thermal expansion and contraction as well as differential movement between the cladding and the substrate or frame can, for example, vary substantially from those expected with greater material thicknesses. Similarly, moisture and drying cycles can fluctuate with the absorption and porosity of the thinner stone and the particular surface finish employed, whilst structural issues such as the effects of traffic vibration on fixings as well as wind load pressure on the panels can have considerable implications for the final design solution.

Natural stone from Scottish quarries is suited to three common cladding methods :

[Hand-set Cladding] [Rainscreen Cladding] [Stone-faced Precast]

The Design of Fixings and Fixing Systems

Because stone has a relatively low tensile strength, it needs to be supported and held in place when used as part of a cladding system in order to deal with the dead and applied loads involved. Essentially there are two kinds of fixings - load-bearing and restraint - with each type usually made up of three elements - the part attached to the stone; the section that spans the cavity; and the piece that is fixed to the backing structure. While the principal of fixing stone is simple to understand, the type of fixings to be used will be specific to the particular cladding system employed and the specific circumstances of each project. It should be recognised that the selection or design of fixings has now become a highly specialised area of engineering consultancy, with considerable expertise available not only on the structural issues but on the functional characteristics of different stone types.