Ian Robottom of Lithodecor discusses lightweight natural stone ventilated rainscreen facades and the requirements of modern architecture.
The advent of modern architecture has fundamentally changed the way buildings are constructed. This is particularly relevant when looking at how the use of stone in construction has changed. As architecture has required more modern methods of construction, so too the use of natural stone in modern architecture has had to adapt and adopt modern methods, such as ventilated rainscreen cladding, to compete alongside other innovative facade finishes.
To fully appreciate how the use of stone has adapted we need to go back to the early 20th century. At this time skilled masons and a large workforce served the building industry at quarries throughout Britain. Today’s UK stone industry is substantially smaller, with fewer dimensioned stone quarries; however, quarrying principles are similar and only differ according to the extracted stone’s geological type. Block sizes depend on the quarry, which is a consideration when designing buildings, and are processed in primary saw sheds, where large blocks of stone are transformed into slices which are then cut to their final size using secondary saws.
In traditional ashlar construction, huge chunks of stone were often used to form a single leaf, which might be 3-400mm thick. Current handset ashlar cladding, covered by BS 8298:2010, is typically 75-100mm thick, often with 5mm joints. With this form of construction it is necessary to consider a number of factors such as weight and time constraints, wet trades and scaffolding. Additionally support is required, usually provided at each floor level on angle irons, with each stone requiring restraint fixings doweled into the beds of the stone.
Since many city centre sites had and still have limited storage/working areas, and in some instances, scaffolding restrictions, alternative construction methods were developed, such as stone-faced precast units. This construction method requires designers to consider how to divide facades into large, if possible, repetitive panels which can then be hung on the building frame. Two further major factors to consider however are panel curing times and weight.
With the growing popularity of rainscreen cladding products, there was a demand for natural stone to be fixed in a similar manner which leads us to the introduction of rainscreen cladding. Ventilated rainscreen cladding is a layered system, typically comprising an outer facing layer that forms the primary rain barrier, a ventilated air gap and an impermeable backing wall.
The principle involves allowing the ingress of air at the base of the system and its egress at the top. The ventilated cavity allows any water which penetrates the open panel joints to be removed by the stack effect and by running down the rear face of the panels and out at the base.
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.
The relevant standards for rainscreen cladding are BS6399 – wind loading design; BS8118 Part 1 – structural use of aluminium; CWCT (Centre for Curtain Walling and Cladding Technology) standards for systemised building envelopes and BS EN 8200 – Impact Load Testing for hard and soft body impacts.
Most of the primary stone types, ie. Limestone, Sandstone, Slate and Granite are available on a rainscreen system but limited in panel size by each specific stone’s geology. To create a typical panel, natural stone is quarried and cut into slabs, then bonded to an aerated concrete backing, circa 16.5mm thick. The large slabs are then cut into finished panel sizes, calibrated, honed and polished, before fixing points are drilled and ceramic plugs inserted.
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 3.75m². Some manufacturer’s systems incorporate an invisible fixing system and can provide bespoke specialist features such as fully mitred corners, window reveals and soffits.
The typical method of 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.
Some rainscreen cladding systems can incorporate a glass finish, with panel sizes of up to 4.7m². There are a wide range of glass colours available with some systems able to incorporate printed designs. It is possible to create a facade using both natural stone and glass using the same fixing system.
Ventilated rainscreen systems provide designers and contractors with a solution to combine the use of natural stone and glass, using modern methods of construction, in accordance with the architecture’s requirements, while also ensuring creativity in design.