Commercial developments using stone cladding – especially in city centres – are increasingly looking for lighter, faster alternatives to hand setting, especially if it takes the cladding off the critical path. NSS looks at some of the latest developments and projects.

Stone rainscreen and curtain walling systems are now accepted alternatives to hand setting stone. They are not always the answer, but they can make life easier for developers, contractors and specifiers, especially on modern, fast-track projects in crowded city centres.

One of the leading rainscreen systems in natural stone is Generix, sold by major quarry company, importers and processors of stone, Realstone.

It is a rainscreen system with the top and bottom edges of the stone cut at an angle to hide the fixings and minimise rain penetration. Not that too much rain should penetrate any rainscreen as wind pressure on the outer face is equalised in the cavity. Therefore, there is no significant pressure differential to drive the rain through the open joints.

Since our last report on stone façade systems (in the June issue of NSS last year) Realstone have been busy with further developments that continue to stretch the imagination and boundaries associated with natural stone applications to façades.

They are continuing to make it easier to decide to use their system and are now in the final stages of an eight-month programme of testing that has seen Generix gain two further CWCT accreditations for Peakmoor sandstone and Valente limestone, backed up by the success of a 60-year durability test in accordance with BRE Digest 346 November 1989.

Realstone decided to test their system to destruction for wind loadings and achieved a negative air pressure test to an impressive 5,626Pascals. That equates to a safety factor for UK wind pressures of 135%. Normally stone rainscreens are expected to achieve a 50% safety factor, which means surviving 3,600Pascals.

John Nolan, Director of Realstone’s Specialist Façades Division, told NSS that there has been no let-up of enquiries about Generix for projects such as the super Contemporary scheme at Peel Court, Manchester, (pictured on the right) where the system had to satisfy requirements that included the Pickard Finlason Partnership architect’s desire for the look of “a solid, square block of Peakmoor that has had openings clawed out by the hand of God”.

Realstone were contracted to design and supply the Generix system on the complicated façade with its free-standing columns, flying beams and angled returns. The Generix support system managed to overcome the difficulties associated with such features with careful design by Mark Heywood Associates (MHA) and the Realstone Façades Design Team. Intec façades worked alongside Realstone on the fixing. The building uses more than 1,200m² of 30mm thick Peakmoor sandstone from one of the Realstone group’s own quarries.

Another recently completed project where Generix was used, at Perth Prison in Scotland, also used Peakmoor, although Generix can be used with a wide range of limestone, sandstone and granite. At Glasgow Harbour, Generix was used with Black Granite, at Keele University in Staffordshire the chosen finish was 900mm x 450mm stack-bonded Velante White limestone and at Fort Vale in Bolton panels of Jura with a curved front were used to create a radiussed entrance.

The finish does not even have to be stone. Panels of terracotta, cedar, Corium clay brick, or metals can be used instead.

Given a good run, one man can achieve up to 12m² a day of finished stone cladding with Generix. And because it is still solid stone, albeit a lot thinner than could be used with hand-setting, it retains the look of traditional masonry.

Rainscreen cladding using stone at less than half the thickness it would traditionally be used at also helps answer sustainability questions and further reduces the encapsulated carbon of the cladding involved in transport because more than twice as much stone can be carried to site at a time.

“In a nutshell,” says John, “the Generix system is now offering more and more opportunities for the designer to create that special façade using a traditional material with a contemporary twist.”

There are various systems available as well as Realstone’s. Stancliffe, another of the UK’s largest quarry and stone processing companies, have a system they call PanAsh. TI Dynamic Façades offer 6-12mm thick resin-impregnated stone that goes under the name of Aerolite. Made in China, the stone panels have a glass fibre or aluminium backing (including an aluminium honeycombe version) that is fixed to their TI tracking rail system.

A German-made stone panel called Airtec was introduced by Alsecco, one of the world’s largest paint companies whose UK base is in Stone, Staffordshire, at the 2008 Natural Stone Show in London.

They received several leads from the show and the first project resulting from that, in Southampton, is now just reaching completion (it will be featured in a future edition of NSS).

The panels are made by bonding a stone face on to lightweight aerated clay concrete that is produced in blocks and sawn to the required panel sizes. Stone is bonded to the concrete using a patented system. In Southampton the stone used is Portland, laid on to large panels with hair joints to create a monolithic appearance. For fixing, holes are drilled through the concrete and stainless steel fixings attached to the back of the stone. The fixings can be positioned as required by the design of the frame the panels are being attached to. At 36kg/m², the Airtec panels are about 70% lighter than an equivalent area of 70mm stone cladding.

The classic lightweight stone panel is the aluminium honeycombe-backed panel with a veneer of stone that can be as little as 3mm thick on the face of it. There are several versions available now, but the original was pioneered in a quarry in Ireland 40 years ago and is now manufactured in the USA and represented on this side of the Atlantic by Stone Company UK in Newark, Notting-hamshire. It was originally called simply Stone Panels but five years ago was re-named StoneLite, although the new name has not been universally encompassed.

With such thin stone and such a light product (it weighs just 16kg/m²) sceptics have always been concerned that Stone Panels will bend, be crushed, be pulled off the building by wind pressure or in some other way fail. There are also concerns that the stone veneer will become detached from the backing.

To counteract such fears, the makers have put enormous efforts into subjecting the panels to every test imaginable – the latest (reported in the January issue of this magazine) was a blast test using 136kg of TNT, twice the amount usually used by the US government before accepting cladding products on to its tender lists. The panels came through practically unscathed.

The panels have been tested to all the required standards in their major markets. In the UK their credits include a BBA Agrément certificate and BS 8414 fire prevention standard. And with nearly 40 years of successful use as a cladding on buildings behind them, many in much harsher environments than they will ever face in the UK, worries about strength and durability must surely be nearing their end.

Some major projects in the UK have now incorporated StoneLite. The latest include Islington Wharf, Manchester, an award-winning apartment building developed by ISIS; the White City Shopping Centre in London; and the 156-bed Macdonald Hotel in Sheffield, where for the first time an English sandstone (Stanton Moor from Stancliffe) was used as a facing with StoneLite.

Current projects include regional police headquarters in Southampton, where the finish chosen is Portuguese St Hubert limestone, and Rotherham Civic Centre, where, once again, Stancliffe have supplied the stone for the finish, this time St Bees red sandstone.

Lee Slack of Stone Company UK, says they are receiving a lot of enquiries for StoneLite now.

In fact, a lot of the companies involved in rainscreen cladding and curtain walling are reporting an increase in enquiries this year. James Omerod, MD of Aliva in Reading, part of the Italian Ivas Group, believes it is a sign the economy is recovering. And he does not believe this month’s budget is likely to alter that because the spending cuts and tax increases expected have already been factored into the equation.

Aliva work all over the world designing bespoke cladding solutions for clients. One of their current projects is with stone specialists Putney & Wood in Grays, Essex, putting Jura limestone rainscreen cladding on Woolwich Civic Centre. This is Putney & Wood’s first rainscreen project.

The cladding goes back on to a Metsec frame. The original specification was for Moleanos limestone from Spain, but Aliva advised them that Moleanos would not be structurally sound enough. Aliva have also advised on the maximum size of Jura panel that can be used.

Architects tend to want bigger panels and thinner stone, and changes to BS 8298 to encourage tests and calculations to be used to determine the thickness of stone to make it appropriate for the kind of stone being used and the size of the panel is only encouraging that (see the technical article on the thickness of stone cladding in the December 2009 issue of NSS).

Using thinner stone does require greater care when drilling for fixings. With a 75mm thick stone it does not matter if the hole drilled for the fixing is a couple of millimetres out, but with a 30mm thick piece of stone, if you drill an 8mm hole there is only 11mm of stone either side of the hole. If the hole is a few millimetres closer to one side than the other, the break out strength can easily be halved.

Nevertheless, the desire of architects to use stone in novel ways has encouraged companies who make fixing systems to design new ways of holding stone on to a building and keeps specialist designers such as Harrison Goldman in Croydon and Stewart Design in Chipping Norton occupied.

A kerf top and bottom of the stone panel is the most cost effective way of restraining stone, but undercutting allows larger, thinner panels to be used.

New systems are being developed all the time to try to achieve what architects conceive. Domus Façades, based in London, have launched 16 new DFS series three-way adjustable support structures for their Technograniti range of rainscreens that include natural stone. They say the new structures will allow the end user to benefit from value engineering on the front end of the project. Many of the systems specifically address issues of fixing stone, rather than other materials, and Domus say the launch of the latest products reinforces their position in stone façades.

Halfen, meanwhile, are developing an external stone version of the WI Beam system they launched in April.

The system uses bespoke components to create a reinforcement that allows infill wall spans of up to 10m without the need to include windposts or movement joints. It can also be used to span openings of up to 2.5m, dispensing with lintels. They say it is a versatile system and believe the external stone version could be available in a year or so.

Fischer Fixings have their SUK stainless steel system that was used for the inclined and irregular thickness of the Turkish Sierra White limestone used on the Middlesborough Institute of Modern Art, among other other projects, and the subsequent ACT system (Advanced Curtain wall Technique). The system consists of a range of undercut anchors and a variety of support frameworks to facilitate the construction of rainscreen façades.

The ACT system was notably used on the £110million refurbishment of offices at 133 Houdsditch in London. Both the external entrance area and the inner walls of the atrium feature a mix of 30mm Burlington and Ardesia slate. Internal cladding of the main feature wall runs from ground level to the top of the building. But this is not a straightforward vertical wall. The 500m² slate wall is raked back and incorporates LEDs and protruding slate fins.

Not so random rubble walling

As well as larger panels that architects want for commercial buildings, many builders are also looking for easier ways of using stone for traditional-looking rubble walling. Several panel systems are now available.

Dorset Flint & Stone Block Ltd started making flint faced blocks and now offer the blocks in a variety of stone finishes under the name of TradLite. The stone is hand set into a lightweight backing of pumice and bloated clay. A composite backing is used to balance the blocks so they do not fall forwards with the weight of the stone while the pumice improves the insulation qualities of the blocks as well as keeping them light weight – each block weighs just 17kg.

Black Mountain have stone panels called Z-Clad, made in the Far East with a variety of stone finishes on a concrete backing. They are Z-shaped to avoid linear joints. Black Mountain say a bricklayer can lay 40m² of the panels in a day.

Beltrami have Stoneskin panels. The pieces of stone are sawn four sides with a split face in thicknesses of between 15mm and 30mm to produce a random texture finish to the wall. The panels are 600mm x 50mm bonded together with polyester resin. The result is an easy-to-install natural stone element that is ideal for external wall cladding.