Stanhope was sick of waiting around for screed to dry, so it asked some suppliers to work out a way of doing without it. Thomas Lane kicks off a flooring special by explaining how they did just that.

When it comes to levelling concrete sub-floors ready for a stone or tiled finish, a fast-track office project that uses all the latest high-speed construction techniques can come to a grinding halt. The office areas themselves aren’t a problem as raised-access floors quickly take care of uneven sub-floors, but public spaces usually need to be levelled with a sand and cement screed before being topped by a traditional hard floor.

According to Bruno Miglio, Arup’s stone and materials expert, this presents several problems. “It’s potentially a dirty process when you already have finishes in the building,” he says. “It can also take a very long time to dry and good site workmanship is important. Typical problems include cracking, edge curling and poor compaction.” Screeds are typically 50mm to 80mm thick and need one day per millimetre to dry, which doesn’t fit well with modern day fast-track construction.

Developer Stanhope didn’t want to put up with these problems at its flagship Willis Building project in the City of London’s Lime Street. “For some time we have been concerned about the 19th-century way of laying floors using sand and cement,” says Steve Moschini, Stanhope’s construction director. “With the lift lobbies and the number of floors in this building, we decided to solve this problem once and for all.”

Stanhope turned to one of its stonework specialists, Grants of Shoreditch, for help. “We challenged Grants to come up with a more modern system for laying floors,” says Moschini. Grants worked with Arup and Lindner, a manufacturer and product development specialist, to find a solution.

The answer is a prefabricated system called Technik, which completely cuts out the need for a screed. It consists of the same type of adjustable pedestals that are used for raised access flooring, onto which a specially developed tongue-and-groove board topped with the desired stone finish is placed. The tongue-and-groove boards lock together, and after 24 hours, the gaps between the boards are grouted so it looks like a traditional stone floor laid over the screed.

Grants says it is twice as fast to install than a traditional stone floor, 15% cheaper, and you don’t have to wait for a screed to dry. Another advantage is that the gap underneath the finished floor can be used for services.

For some time we have been concerned about the 19th-century way of laying floors using sand and cement

Steve Moschini, construction director, Stanhope

According to Miglio the key to the whole system is the specially developed board to which the stone is fixed. This is made by Lindner from 95% waste paper mixed with gypsum and water to form a paste, which is poured into moulds, squeezed under pressure then dried and cut into 40mm thick boards.

The size of the boards is determined by the desired appearance of the finished stone floor, as they have to be the same size as, or a multiple of, the stone tiles.

Not only is the board very sustainable because it is made from recycled paper and gypsum recovered from flue desulphurisation, but less stone is needed. Because the adhesive used to bond the stone to the boards is factory applied it is evenly distributed, so a thinner stone can be used – between 12mm and 15mm thick rather than 20mm to 30mm thick.

The board was tested by BRE to see how it performed in damp conditions and what happened if there was a flood. Damp conditions were created by suspending the boards over tanks of water. “Distortion was barely measurable and the board didn’t loose any strength,” says Miglio. The boards did lose strength when saturated with water but regained this on drying. Miglio admits that both types of flooring would probably need relaying after a serious flood, as compounds in a screed may leach through the stone finish to cause staining.

Installing the floor is straightforward. The pedestals are set out and glued to the sub-floor. These are adjusted using a laser to ensure these are all at the same height, an adhesive is used to lock the thread adjuster in place and the boards are placed on top. Adhesive is applied to the edges of the boards as they are positioned. Spacers are used between the boards to create a small gap for the grout. This is applied conventionally, 24 hours later.

Walk across the finished floor and it looks and feels just like a solid floor. Other clients are convinced of its merits, too – the system is being considered for several UK shopping centres including one at Leicester, and will be used for the flooring for the executive lounges at Heathrow’s Terminal 5.