There can be a fine line between ostentation and good taste. NSS talks to some of the people involved in drawing that line about how their use of stone keeps them on the right side of it.

Stone is more popular than ever among the better off parts of society, both for their homes and their offices, which has helped many in the stone industry fare better than those in some other industries in the economic downturn.

It is not surprising that in times of uncertainty people look for a rock to cling to, and nothing smacks more of ageless solidity and timeless style than… well, rock.

One of the ways it is increasingly being used in the homes of the well-heeled is as self-supporting staircases – often referred to as cantilevered or spiral staircases, although in fact they are neither spirals nor cantilevered.

They are not spirals because a spiral winds its way outwards from a centre. They should be described as helical. And they are not cantilevered because that means something fixed at one end only, whereas stone staircases essentially rely on the steps below them to transfer loads to the ground. Typically the steps go into a wall only a few centimetres and one step on its own fixed like that might collapse under its own weight, or under the weight of someone standing on it.

The pictures here show an Arabescato marble staircase in a private house that Sean Daly of consulting structural engineers Solid Structures in Hook Norton, Oxfordshire, engineered and stone specialist Chesney’s built. Some of the steps do not go into the wall at all as they pass in front of windows at either side – although all is not quite as it appears.

Normally, says Sean, a wall is an essential part of the construction because it acts as a rotational restraint. Without it, what we shall call a spiral staircase for the sake of convention would fall over sideways, because the forces in its construction make it want to twist. The steps will typically go into the wall about 10cm but the forces are pushing the steps in towards the wall, so there is no danger of it coming adrift.

The treads are rebated to fit together so the weight passes from one tread to the one below all the way down to the ground.

Setting out and building are critical and should be carried out by a skilled stonemason used to working to fine tolerances. The stairs must start and finish where they are supposed to. Some masons use piano wires to ensure they are following the required lines.

Sean Daly has made something of a speciality of engineering spiral staircases and in the past eight years has worked on about 50 of them, giving designers the confidence to build stairs that look disconcertingly – but deceptively – insecure.

Even so, he says designers do push the limits all the time and he does sometimes have to tell them that there’s a degree of risk in what they propose.

Even after he has made all the calculations and is as confident as he can be he has achieved a successful solution he admits he does sometimes have sleepless nights. Perhaps it is because he does that none of his stairs has failed – not even when they had to pass in front of windows.

Sean’s solution to being able to sleep at night as well as engineering steps that spanned the gap in the wall on the project pictured here was to create a steel box section hidden by the stone and dowelled into it along one side to act as a restraint.

That was not the end of the problems, though, because at the top, the landing could not be fixed where Sean wanted to fix it and the stairs rest on a 10m long, curved steel support. What worried Sean was that the stairs might start to sway or bounce, like the Millennium Bridge in London did before the problem was rectified, with the rhythm of someone ascending or descending them.

He was careful to ensure the problem was avoided but it highlights a point he makes that it is not just the stairs he has to consider but also the structure around the stairs. He says he has had to recommended architects to make changes to their designs to accommodate so-called cantilevered stairs.

Not all stonemasons seek the advice of an engineer before installing stairs, relying instead on years of experience, which is usually successful, although Sean says he was consulted by a stonemason once when stone stairs in a project started cracking.

The stairs were unusual in being like the steps in a church tower, coming out from a central column. However, unlike a church tower, where the outside of the step goes into a wall, these were unsupported on the outside edge. Sean’s solution was to reinforce the treads with steel dowels.

“Very often I work with the stonemason doing these projects,” says Sean. “The architect draws it in and expects the stonemason to make it work.”

He says the hardest part of engineering spiral and cantilevered staircases is working out the loads. He believes he has the problem solved these days but when he started he referred back to old books on stonemasonry for guidance. He has since devised his own methods of checking loading, which has not let him down.

While cantilevered stone stairs are proving popular in domestic properties, in the commercial world, feature walls of stone are adding instant impact to reception areas and atria.

The pictures here show two such walls – one in Portland limestone at Finsbury Circus (on the right) and the other in white onyx at 199 Bishopsgate (below), both projects in London on which Putney & Wood was the specialist stone contractor.

The architect at Finsbury Circus was Damian Howkins of Fletcher Priest Architects, who used Belgium blue limestone for the reception floor, black granite flooring to lift lobbies and Portland Basebed for the lift lobby walls, as well as the Portland Whitbed for the feature wall. The architects had originally intended to use Basebed for the wall but, in consultation with Putney & Wood and Albion Stone, which supplied the Portland stone, decided the slightly livelier Whitbed would be preferable. The leaf motif was milled by APS Masonry on its Brembana

G-Mex CNC workcentre.

The architects had worked with Putney & Wood before and had confidence in them, although Skanska, as main contractor, still went to tender for the specialist. “You would look to someone like the team from Putney & Wood to supply answers, certainly at the site stages,” says Damian Howkins.

The alterations to the building gave the architect a 36m long wall connecting the two sides of the building and the spaces beyond by bringing aspects of the exterior inside.

Damian told NSS that because of the dimensions, it was important to avoid simply producing a long, plain wall. He worked with Putney & Wood and APS on deciding how the motif could be transferred to the stone. “There was a great learning curve for us: How far you can push the material. Milling was new to us.”

The leaf artwork was commissioned and the architects experimented with the design, creating an interesting shape across the wall and making a big play of the contrasts.

How the design would be transferred to the stone went through a number of iterations, each being produced on 3 x 4m panels to show the architects the result.

They started with the idea of building up the picture from dots, like a printed picture, but in this case created by drilling holes of different depths into the stone to create the tone. Unhappy with the result, they moved on to the idea of intaglio carving of the designs using CNC milling, experimenting with different variations of the depths of cut with, again, the variations creating tone. Some are particularly lightly etched to give the impression of fossilisation. The wall is made of 150, 1000 x 600mm panels.

“The big thing is how you light it,” says Damian, and lighting specialist Watermans Lighting Design worked closely with the architects to provide the solution. Damian: “It’s quite surreal. When you’re there you wonder if it’s just screened on to the wall.”

At 199 Bishopsgate, another retrofit, this time by John Robertson Architects (JRA) of a 1992 SOM building, the feature wall is onyx with the emphasising lighting coming from behind and other lighting carefully designed into the walls of the new entrance in the centre of the main facade that replaces SOM’s low-key, single-storey corner entrance even at the cost of sacrificing a little of the lettable area above it by making it two-storey (although some lettable space has been recovered in other ways).

The clients, British Land, wanted to reposition the building and one of the ways JRA has done that is by leaving you in no doubt about its gravitas the moment you enter. The height and the onyx feature wall, as well as the two-tone travertine of the floor, play a major part in setting that scene.

John Robertson told NSS that he has always admired the work of Skidmore, Owings & Merrill and their use of stone, and it was because they had so effectively used travertine in buildings nearby that he wanted to use it at 199 Bishopsgate. “It seemed to me we should bring some sort of sympathetic material into the project,” he says.

He had never used travertine before but had also wanted for some time to work with the Italian stone company Mariotti, which has a travertine quarry and major processing works just outside Rome. The quarry was visited several times by JRA Director Festus Moffat, who worked on the project. “Festus spent quite a lot of time with them selecting the block,” says John. “What attracted me to Mariotti is the quality of their stone – it really is that beautiful ochre yellow travertine.”

Both a dark and a light stone were selected and were separated so that bands of the darker shade could be used to decorative effect.

The white onyx of the feature wall was sourced by Putney & Wood from Iran and John Robertson says: “Putney & Wood did a stunning job of that.”

The stone was carefully selected for the purity of its whiteness but also to maintain the figuring. It is backed by glass for strength and lit from behind with LEDs so there are no pools of light showing through on the viewed side, but rather a uniform illumination. John: “It’s a very rich glow. It makes a massive impact. British Land are delighted with it.”

There is also LED lighting in the walls, which helps create an overall brand in the foyer.

John Robertson says natural stone is synonymous with headquarters buildings in London. “I love the richness and beauty of the natural material. It has a corporate feel to it. It smacks of wealth and success. That’s the reason we select it.

“Putney & Wood help us with procurement, but I’m keen to be involved in the stone selection.”

He admits that budgets in the City of London are higher than in many parts of the country but clients still expect value for money. At 199 Bishopsgate “there wasn’t actually a huge budget”, says John (the construction cost was £21.2million of which the stone package accounted for £1.2million), but the client agreed that travertine and onyx would be a good investment.

“They got the bang for their buck,” says John, although adding: “Our work is about understatement and simplicity. We tend to shy away from decoration and we also don’t like these overly ostentatious entrances. We want to create a sense of drama and want the building to be noticed but we don’t want to resort to bling. It’s a question of taste.”

The housing market

Black granite is still the most popular choice for worktops in kitchens, even though there are considerably fewer kitchens being built or upgraded currently than there were at the height of the market in the first half of 2008.

Anecdotally, the kitchen market has shown some recovery in the second quarter of this year and the wholesalers supplying stone for interiors say there is a bit of a swing back from engineered quartz to natural stone (Report: Wholesalers, NSS July issue).

The most notable change in the domestic interiors market, at least in the South of the UK, has been a steady growth in demand for marble and polished limestone – not because it is being used in kitchens (except for floors) but because more bathrooms are being upgraded using natural stone and the stones being used are most often marbles and polished limestones.