When Putney & Wood completes its £1.2million contract at 12-15 Finsbury Circus later this year the project will provide a casebook of most of the stone specialist’s skills. And behind the finished stonework is a supporting structure of steel.

Winning the £1.2million stone contract at 12-15 Finsbury Circus is giving stone specialists Putney & Wood a chance to demonstrate a broad range of their skills.

The project in the heart of the City of London’s financial district involves Putney & Wood in the design, supply and installation of new Portland limestone cladding work for new build and the cleaning and restoration of the existing stone façade. In the reception they are creating an etched feature wall and laying limestone flooring in the reception and lift lobbies on all floors.

The current façade at 12-15 Finsbury Circus was constructed in 1991. Now the building is being re-developed from 11,520m² of office space into 20,000m² by creating an additional floor and re-designing the building layout.

Finsbury Circus has a history dating back to the 14th Century and contains listed buildings and architectural features from Edwin Lutyens and Francis Derwent Wood, so the site is sensitive. The site is also pivotal in the Cross Rail development, which is due to be completed in 2016.

Putney & Wood moved on site for the project at 12-15 in March and should have it finished by the end of the year or early into next year.

The Waterman Group behind the project has appointed Skanska as the main contractor and Fletcher Priest as architect.

On the north elevation of the project Putney & Wood is hand setting 50mm cladding in Portland limestone, which is being fixed back to a retained structure that has, nevertheless, undergone some fairly considerable alterations, not least to accommodate the extra floor that is being added.

The building is a reinforced concrete frame with infills. Concrete slabs 250mm thick have been retained, supported underneath by steel, which has made fixing difficult because the steel was not designed for the stone to be fixed to it. Mark Chapman in Putney & Wood’s design office says: “Designing the stonework was easy, designing the fixing was complex.”

The concrete infill that was there previously with a Bath limestone facade that had somehow acquired a red stain (probably indicating something behind was rusting), has been removed so that the windows can be enlarged. In its place now are Metsec steel frames with Tyvec waterproofing.

The Metsec spans the spaces between a floor plate and the one above it. If a stretcher bonded stone design were to be fixed back to the Metsec there would need to be almost a solid wall of Metsec, so the stone is being fixed using Halfen channels.

On the south side, the facade is retained and is being cleaned by Putney & Wood. There is also some minor restoration work required to remedy a few cracks that have appeared due to movement of the building.

Inside, Belgium Blue limestone flooring will be laid in the redesigned reception area that has been framed with stainless steel, giving it a modern look.

There are eight floors above ground and two below and Putney & Wood are also laying honed black granite in all of them.

In the reception area, a feature wall clad in 75mm Portland limestone will be etched with pictures of leaves, the pictures being built up from 1mm pixels with the depth of the pixel into the stone determining the shade of it in the finished picture. Some will be as much as 25mm deep.

The original pictures being used to make the patterns are photographs of leaves. They have been turned into 3D files that will be used on a CNC machine to create the picture in the stone.

Oxfordshire stone processors APS Masonry will be putting the pictures on to the stone for Putney & Wood using its Brembana G-Rex CNC workcentre. “Everyone’s watching us to see how that comes out,” says Mark Chapman.

The reception area flows from the main entrance in Finsbury Circus to its other access point in South Place. Apart from the feature wall, there will be smaller, 20mm thick Portland stone linings on the other walls in the area. There are also Portland stone wall linings and granite floors in the toilets.

A first for Forest Pennant

It is estimated that 70% of the people who went to the Olympic Park this month made their way there through the 175,000m² Stratford City development, a cornerstone of which is the Waitrose-John Lewis store. The prominent building is the first project for which Forest Pennant has supplied rainscreen cladding – 1,500m² of it in slabs 1,450mm x 500mm. The fact that Forest Pennant could supply slabs that size was one reason it was chosen. The specialist stone contractor was Grants of Shoreditch. Grants was handed over the primary steel frame and had to design the complete facade solution, including the aluminium extrusions of the backing structure and the insulation.

BACKING FROM THE SUPPLIERS OF STEEL BEHIND THE FACADES

Metsec supports main contractors and specialist sub-contractors by maximising the benefits of design, speed and efficiency delivered by its steel framing system (SFS).

Metsec views the complete system as being more than just high performance, lightweight steel framing components. Value engineered design and fully detailed construction drawings are just as important, which is why they are included in the Metsec package.

The company also manufactures the frames, ensuring the fast and accurate supply of materials to site ready for installation, helping Metsec approved installers to meet the tightest of building programmes.

Halfen also offers assistance to designers by supplying free software, which can be downloaded from its website (www.halfen-fixings.co.uk). It facilitates exact dimensioning and also helps with the selection of the correct choice of product combinations from its concrete anchoring, façade fixing and framing systems in its nine product categories.

The lighter way to a stone finish

The building pictured above is part of a gateway project to improve the east end of Glasgow ready for the Commonwealth Games that are being staged there in 2014. Called the Eastgate Project, it is clad in 1,200m² of red sandstone supplied by Stancliffe from its quarry near St Bees Head on the Cumbrian Coast, but the stone reached the building via America and the factory of Stone Panels Inc, where it was fixed to an aluminium honeycomb backing.

With the sandstone reduced to 7mm thick (it can be even thinner when a denser stone such as granite is used on the panels), the StoneLite panels have a weight of just 16kg/m².

The light weight was a major factor in the decision by Simon Walsh of Cooper Cromar Architects in Glasgow to specify StoneLite, which is sold in the UK by Brick-Works UK in Newark, Nottinghamshire.

It was a decision made easier because Cooper Cromar had already used StoneLite on two previous projects in Glasgow. Simon Walsh thinks it is an elegant solution to cladding – it speeds up installation time and can be installed from a scissor lift (as it was on the Eastgate Project) without the need for scaffolding, takes the stone off the critical path, means the stone is not part of making the building air tight and, because it is so light, makes it easy for him to fulfil his health & safety obligations under CDM.

“The absolute beauty of the Stone Panels system is that it’s a one stop shop,” he says.

He says that although the panels are slightly more expensive per square metre than hand-set stone, the price is more than compensated for by the speed of erection and, if the lighter weight is considered at the design stage, the savings on materials for the frame of the building can be considerable compared with hand set stone, which would typically be 40-75mm thick. And the panels can be fixed by semi-skilled cladders who do not cost as much as masons.

Another attraction for Simon Walsh from a design point of view was that the panels could be as large as 1.5m x 900mm, whereas he doubts he would have got stones half that size if they were hand set.

Simon says the main contractor, Dawn Construction, was a little concerned about the length of time involved in sending the stone to America and getting it back again, but the architect says: “You can bring the panels on to the project at any time, unlike hand setting, which is on the critical path at an early stage.”

Even the BREEAM rating of the building was scarcely affected by sending the stone all the way to America and back because relatively little stone had to be shipped when it was only being used as 7mm thick veneer.

Simon says: “I would definitely use it again – speed, health & safety, air tight, modular, larger panels so you can do more with design… I don’t think there’s much choice.”

David Turner at Clad UK Ltd, the company that fixed the StoneLite, is also a fan. Clad UK was sub-contracted to the glazing sub-contractor, Solar Glass (now called Glass Solutions), because of the interface between the glass and the stone, with the stone being hung on the building at some points and on to the curtain walling at others.

Clad UK installed the Kingspan steel frame system that the StoneLite panels are secured back to, so they had made sure it was accurate. There is Kooltherm K15 rainscreen insulation behind the panels and the walls achieve a U-value of 0.23W/m²K.

The panels are fixed with 5-6mm joints that have been left open as a rainscreen. David say that the design incorporates some quite complex five sided corners that lent themselves to the StoneLite system, where all the solutions are worked out beforehand and the completed panels, fabricated into the sizes and shapes needed, are simply fixed to the backing. Clad UK worked with the architect to design the detail of the individual cladding elements.

On the Eastgate Project the savings on the frame were not great because the heavyweight floor plates still needed to be supported, but David says he worked on a project last year where the use of StoneLite meant a saving of 300tonnes of steel in the main frame.

He says: “We’re pricing quite of lot of this type of work at the moment and are talking about using StoneLite agin. I’m a fan of it. It’s light and fast to build with.”

The verdict is: Exemplary

Vetter UK was responsible for the design, supply, manufacture and installation of 2,400m² of 75mm thick external cladding in Cadeby limestone, incorporating a feature lattice wall and a hand carved stone Coat of Arms, that creates a stunning new entrance to the front of Westminster Magistrates Court.

The stone is used in various heights between 250mm and 900mm in random lengths for the cladding and manufactured off-site so that it could be installed more easily and more rapidly on-site.

It included the hand-carved stone Coat of Arms, made for Vetter by Cathedral Works Organisation (CWO) in Chichester.

Vetter UK worked alongside Main Contractor Laing O’Rourke South, Concept Architect Hurd Rolland, Construction Phase Architect Clifford Tee+Gale and Engineers Arup Facades at various stages throughout the project.

Both the main and feature lattice facades were engineered with off-site manufacturing principles in mind.

The internal backing wall of the main façade was built using the Kingspan Offsite system, where storey-high panels approximately 3m wide were manufactured by Vetter UK, transported to site, hoisted into position and hand fixed by Vetter’s masons.

The large stone mullions of the feature lattice wall were designed to be hung from a concrete sub frame, achieving an imposing monolithic look.

Individual stones were produced by Cadeby, which supplied the stone, in South Yorkshire and transported to Laing O’Rourke’s Explore Industrial Park for assembly and post tensioning into 27 individual 9m and 3m mullions.

The mullions were delivered to site on a flat bed lorry in protective steel cradles and hoisted from the lay-down area over the building using the tower crane and then installed into their final position by the Vetter UK masons.

Stone selection was paramount for the newly commissioned Coat of Arms as the stone needed to match the cladding and be of good carving quality.

CWO carved the piece while Vetter project managed the works from conception through to installation.

Following agreement of the final design of the crest, a half-scale maquette was produced for inspection and sign-off by the client and architect before carving commenced. A team of six mason carvers worked on the eye-catching piece, which now sits proudly above the main entrance to the building.