Geoff Wilkinson looks at the challenges of getting stone to comply with the regs
The recent appointment of Roger Scruton as chair of the Building Beautiful Commission has reopened the argument about the use of traditional styles and materials within architecture. With a call to prioritise the vernacular architecture of columns, cornices and pilasters over modern glass, concrete and steel boxes, along with the ban on combustible cladding materials, architects’ minds will more frequently turn to stone, so to speak.
Natural stone has been used in construction for thousands of years and is seen as a way to provide a safe and secure building. Traditionally stone was used to create structural solid masonry walls but in the past 70 years masonry design has moved away from solid wall construction and stone has fallen out of favour.
Stone is still used in some modern buildings but this is usually in the form of stone cladding panels fixed via bracketry to a steel or concrete frame. However, many of the UK’s indigenous stones, particularly softer limestones, are not suitable for use in this way as thin sections will not weather well under the action of frost (especially when saturated) and therefore do not lend themselves to use as rainscreen cladding.
An update to Part L meant it was no longer possible to use solid stone elements in the traditional manner
The use of stone reduced further when Part L of the Building Regulations was updated in 2010 and placed a focus on reducing cold bridging at wall/floor junctions and window and door reveals. This new emphasis meant it was no longer possible to use solid stone elements in the traditional manner and required instead that designers introduce a thermal break layer. The problem with this is that stone relies heavily on its thickness to provide its strength and, by splitting a stone element, you can easily render the stone structurally unstable.
Locations that require particular care when detailing include window reveals, sills, lintels, balconies, parapets, and the junctions and interfaces between walls, floors and roofs. The use of stone internally as masonry is much simpler, for instance in floor and wall tiling, fire surrounds and even in the internal leaf (provided it is isolated from the external leaf by an effective thermal break to ensure condensation on the stone is avoided).
Another concern about the detailing of stone is that it is heavier and less regular than other forms of man-made masonry, such as bricks and blocks. However, like bricks and blocks, stone used externally in cavity walls will still need to be adequately tied back to the internal skin in order to meet structural design codes. The correct design, location and installation of brackets and ties, therefore, are essential in such cases to ensure the strength and stability of the wall, but architects must also ensure that the outer skin is thermally isolated from the inner leaf.
In many cases, the very heavy loads of stone panels require substantial metal brackets to support their weight and to resist lateral loads and wind loads. These brackets, again, can create a potential cold bridge back to the main structure. Fortunately there are now a number of proprietary products on the market utilising the structural strength and low thermal conductivity of fibre-reinforced resins, which provide support but prevent the thermal transmission you would get using a metal product.
However, there remains the question of the irregular nature of stone. Even when building a cavity wall with 150mm coursed stonework, it will not meet Building Regulations guidance on the spacings of walls ties, especially where these need to be at 300mm vertical centres. The National House Building Council has recently published guidance to address this issue, which recommends that wall ties in such situations can be installed at 450mm centres, provided they are installed in pairs in the same bed joint and within 225mm of the edge of the opening or joint.
In conclusion, it is entirely possible to reintroduce stone into day-to-day use, provided architects take extra care in detailing and are aware of the unique properties of the material when deciding where and how to use it.
Further guidance on the use of stone can be found in Natural Stone Masonry in Modern Scottish Construction, published by the Scottish Stone Liaison Group, which can be downloaded free-of-charge from the Building Standards website at www.sslg.co.uk.
Geoff Wilkinson is an approved inspector and managing director of Wilkinson Construction Consultants www.thebuildinginspector.org