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CPD: Lightweight natural stone and glass ventilated rainscreens

The full title of this CPD is ‘Lightweight Natural Stone and Glass Ventilated Rainscreen Facades and the Requirements of Modern Architecture’

It is sponsored by rainscreen facade manufacturer and supplier alsecco UK, a member of the German DAW group. Its aims are to explain the distinction between handset and lightweight stone ventilated rainscreen panels, to demonstrate how to achieve robust lightweight rainscreen stone facades, to outline specification considerations and to summarise the principles and features of alsecco’s Airtec system.

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Part 1: Stone extraction

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Whereas in the early 20th century, skilled masons and a large workforce served building industry at quarries throughout Britain, today’s UK stone industry is much smaller, with few dimensioned stone quarries. Nevertheless, quarrying principles are similar and only differ according to the extracted stone’s geological type.

Sandstone is sedimentary and formed in layered deposits, which are pulled from their natural horizontal bed. In some granite slate quarries, wire saws cut blocks from the quarry face. Dorset’s Portland quarries use the latest methods, including cutting channels into the quarry floor and inserting huge airbags which, when inflated, push the large blocks from their original position to avoid damage caused by blasting. Block sizes depend on the quarry, which is a consideration when designing buildings, and are processed in primary saw sheds, and transformed into slices which are then cut to their final size by secondary saws.

Part 2: Traditional construction

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In traditional ashlar construction, huge chunks of stone were often used to form a single leaf, which might be 3-400mm thick, whereas current handset ashlar cladding is 75-100mm thick, usually with 5mm joints.

This form of construction is covered by BS 8298:2010 and it is necessary to consider weight and time constraints, wet trades and scaffolding. Support is usually provided at each floor level on angle irons, with each stone requiring restraint fixings doweled into the beds of the stone. For larger, and therefore heavier, elements such as window surrounds, extra support is required. Scaffolding should be of the correct type in order to allow larger stones to be lifted and sometimes intermediate platforms are needed. A limited number of courses can be fixed before being left to set and continuing with the next course.

Part 3: Modern construction

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Since many city centre sites had limited storage and it was not always possible for scaffolding to protrude into highways, alternative construction methods have been developed, such as stone-faced precast units.

Their designers have to consider how to divide facades into large repetitive panels, which are able to be hung on the building frame. Large wooden moulds are constructed at precast works, into which stones are laid face-down. Joints are masked and the backs of the stones are sealed to stop leaching of the concrete mix causing discoloration. The stone is then drilled and dowels are inserted at the correct angle before concrete is poured into the mould to create large panels.

Following curing at the works, completed panels are delivered to site and installed. The frame has to be designed to carry these large weights. Crainage and lead times need to be considered.

Part 4: Ventilated rainscreens

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With the growing popularity of terracotta rainscreen cladding, there was demand for natural stone to be fixed in a similar manner. Rainscreen cladding is a layered system, typically, comprising an outer skin with air gaps that forms the primary rain barrier, a ventilated air gap that prevents water ingress 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 panel joints to be removed by the stack effect and by running down the rear face of the panels and out at the base. 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.

Panels are lightweight and easy to position, often using scissor lifts, mast climbers or hydraulic platforms. One of the main advantages is large format panels produced off-site, which can be quickly fixed to pre-positioned rails. Additional benefits are low weight and dry fixing.

Part 5: Airtec Stone

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This form of ventilated rainscreen cladding is suitable for any type of stone. First, natural stone is quarried and cut into slabs, then the stone is bonded to a patented 16.5mm lightweight aerated concrete backing. Panels are cut into large individual slabs with natural stone facing which is calibrated, honed and polished, and finished panels are trimmed to size before fixing points are drilled and ceramic plugs are fixed.

Airtec Stone is one-third the weight of 40mm-thick natural stone and one-fifth the weight of 75mm-thick handset stone. It is available in panel sizes up to 4.7m2 and incorporates an invisible fixing system; fully mitred corners are available, if required. It offers very high impact resistance and is manufactured to exacting tolerances.

Part 6: Rainscreen installation

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Helping hand brackets used to support rainscreen systems can be installed with isolation pads to prevent cold bridging. Horizontal rails are attached to T-sections and the support system can be fixed back to light-gauge steel framing or blockwork. Clips are attached to the backs of panels with ceramic fixing points, which are bonded to the Airtec backing and the reverse face of the stone, and all fixings are aligned and attached using the correct torque before panels are simply clipped on to horizontal rails. Airtec Stone is fully tested and approved by the DIBt - the German Institute for Building Technology, the Taylor Woodrow UKAS Testing Laboratory and the CWCT.

Part 7: Airtec Stone projects

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These include the Portland stone-faced Southampton Regional Business Centre, Westfield Stratford City, Corby Enterprise Centre, London’s Farringdon Station Redevelopment in London and Portsmouth International Port, where an ammonite and shell-like finish has been achieved using Limestone Mirage stone. The Airtec Stone system was used internally as well as externally at both Farringdon and Portsmouth. Wigan Joint Services Centre has granite Luxor panels with integrated plinths at its base and the Steinerner Mantel in Dresden has wraparound corners. The Hotel California in Berlin’s Kurfürstendamm has charcoal-grey slate panels with bronze- coloured anodised aluminium components and prominent cubed projections.

Part 8: Airtec Glass

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Airtec Glass combines the ventilated rainscreen cladding system with a glass finish, with panel sizes of up to 4.7m2. It has a high safety factor as in the unlikely event that breakage of the glass should occur; it is retained on the backing panel. It achieves Class 0 for surface spread of frames and is available in all RAL colours. Individual designs can be incorporated in glass.

Airtec Glass projects in the UK include the Earth Sciences Department of Oxford University; the Marks & Spencer store at Gemini Retail Park in Warrington with black glass cladding; and the University of Potsdam in Germany, where Airtec Stone and Airtec Glass are combined with similar visual depth to create a patchwork finish.

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