How do they do it?

Tunnel-form construction is being used to build 235 new homes on the Nightingale Estate in Hackney, east London. The concrete ground- and first-floor "tunnels" have been constructed to form a short terrace of houses. Shuttering to form the gables is being craned into position ready for the concrete to be poured.

Gypsum block partitions

High-density, interlocking gypsum blocks will be used to construct the internal partitions. The blocks are glued together to produce a smooth surface ready for decoration without the need for wet finishing such as plaster. The softness of the gypsum allows blocks to be cut or sawn on site to form the openings for doors and hatches.

The tunnel-form construction means that internal partitions do not have to be loadbearing. This allows residents flexibility in room layout and, more importantly, allows the dwellings to be adapted to suit the changing needs of occupants.

Bolt-on wall panels

The front and back elevations of the homes will be clad with a prefabricated wall panel. The Nightingale Estate residents have selected a timber-framed panel, although any type of framed panel could have been used, including metal. The factory-produced wall panels consist of a timber frame to which is fixed plasterboard, insulation, an outer lining and fixing brackets. These panels are then craned into position and bolted to the concrete structure to form the inner leaf of the external wall. Additional insulation packs are added between cassettes to cover the exposed faces of the party walls before the outer cavity brick skin is added and tied back to the panels. Residents selected brick as a finish, but other forms of cladding or rain screen could equally have been used.

Factory-finished doorsets

Factory-finished doorsets will be used to speed up fit-out. These are installed by clamping a door frame to the partition using a series of Allen screws. The doors are then simply hung on the pre-fitted hinges. This allows the doors to be fitted after decoration, reducing the chance of damage and waste.

Roof panel system

Factory-made insulated standard roof panels will be craned into position to speed up construction. The panels are manufactured from timber beams backed with a plasterboard lining and filled with sprayed insulation.

On the roof, the panels are bolted to timber purlins spanning the tunnel-formed concrete gable walls. Openings are then created for rooflights before conventional tiling battens are attached and standard roof tiles are installed.

The method of construction is said to take about half the time of a conventional trussed rafter roof while creating a large, open, insulated roof space that adds about 25% to the volume of the house.

Tunnel-form construction

The structure is produced using a tunnel-form construction. The technique takes its name from the tunnel of concrete that runs from the front of the house to the back to form each storey. To construct the tunnel, factory-made steel shutters are craned into position and fixed. Openings for doors are blanked off. Steel reinforcement is slotted into the cavity that will form the wall and placed on the roof. Electrical services such as cable ducts and outlet boxes are attached to the reinforcement before the concrete is poured to form the roof and walls.

To cure the concrete, curtains are drawn across the ends of the tunnel and a thermal blanket is draped over the roof. Gas burners are used to accelerate the curing. Finally, the shutters are removed. The whole process should take a day.The advantages of tunnel-form are speed of construction, improved sound insulation and improved surface finish.

Twin-walled heating pipes

Twin-walled heating pipes installed in the reinforced concrete floor screed will distribute hot water to a conventional central heating system to keep the homes warm in winter. By burying the pipe in the floor, the walls and partitions are kept free of pipes to allow the interior layout to be easily modified. The pipe-within-a-pipe system allows the inner pipe to be withdrawn or replaced if it is damaged, without affecting the screed.