A unique SKr7m housing project to build “attractive apartments for ordinary people” is causing quite a stir in Sundsvall, Sweden. Such is the interest, that the building contractor NCC has had to open an information office to deal with queues of up to 600 people who come to look at the high rise development.

Five six-storey buildings with space for 94 apartments are being constructed in Sundsvall’s Inner Harbour – and the whole development is being built in solid wood.

The project has been masterminded by property company Mitthem which launched a four-way contractor competition in 2003. The detailed development plan stipulated a solid wood construction.

Jenny Sundkvist, a building engineer with Mitthem, said the purpose of the project was to develop a technique to build high-rise buildings with solid wood.

The result is five apartment blocks with floor structures and load-bearing walls made of a laminated product, “solid wood”, from Martinsons, one of the largest sawmills in north Sweden and one of Europe’s largest producers of glulam.

Martinsons started manufacture of “solid wood” products in 2003 in a factory previously used by a caravan manufacturing company. Some SKr15m was invested in new machines and the factory now produces the floor structures and walls themselves.

Martinsons marketing manager Per Lundqvist said: “We had expected to sell 7,000m2 of ‘solid wood’ during the running-in year in 2004, but we have already passed 35,000m2.”

Framework elements of “solid wood” can be used in everything from small buildings to multi-storey buildings where there are stringent requirements for fire protection and sound insulation.

Mr Lundqvist said: “The structural weight for a framework element in ‘solid wood’ is approximately 75% less than for a similar concrete element.”

Several layers are used to achieve a high degree of sound insulation in the floor structure and walls. The “solid wood”, like glulam, consists of board layers which are put together in a ‘criss-cross’ effect and joined with glue to form solid slabs of wood.

One of the challenges was to meet acoustic requirements, and during the development work a two-storey wooden building was built specially in the factory to conduct acoustic tests.

More than 500 acoustic measurements were taken. “The most difficult thing has been to solve the sound bridges in the flanks, but we now have a patent for our solution consisting of an extremely strong rubber strip which absorbs the vibrations,” said Mr Lundqvist.

Ms Sundkvist explained that multi-storey buildings put high demands on stability, fire protection and noise abatement. “Wind causes high load, and requires a stabilisation system – and to achieve this we are using nail joints,” she said.

“The mid-wall plays an important role to compensate for the loss of the east facade in the stabilisation system, and this wall must take a lot of the load.”

Fire protection

She said the solid construction and cladding used give good protection against fire and added: “Because the buildings are fire-graded Br1, the surface layer for external and separating internal walls could not be made in wood – instead two layers of gypsum wallboard were used. The houses have a residential fire protection system. Every room has sprinklers – not because the houses are made of wood but because the facade has wooden boarding on all six floors.”

A weather protection system was used throughout the construction. Called a Gibson tower, it is a tent which follows the building up as it rises.

Ms Sundkvist said: “A weather protection system should be used in all projects, irrespective of the material, where the the building could be exposed to moisture.” She said the benefits are that no scaffold is needed; work can continue even when the weather is rough, so no time is lost; and the tent provides a good working environment.