It's wood all the way

The contractors working on the site faced these Arctic conditions last winter when they assembled the timber grandstand and sports pavilions next to the track. David Boughey, project co-ordinator for project manager PGA, says it was the coldest conditions he has ever endured — and he lists Warsaw and the Czech Republic among the places he has worked.

As well as the wind and rain, the contractors had to contend with the lightness of the hollow glulam timber elements that made up the superstructure of the buildings. "It meant we got blown about a bit before we could fix the beams and columns into place," says Tony Davies, senior project manager at PGA.

The decision to use timber had a profound affect on the specification and detailed design of the stadium buildings. Neither PGA nor architect BGP McConaghy had worked with the glulam and laminated veneered lumber elements before (see "Woodn't it be good?", page 8) and it required close liaison with supplier Finnforest and structural engineer Finnmap to ensure that the timber components were specified and assembled correctly.

When finished, the Princess Royal Sports Arena will be the home of Boston and District Athletic Club and Boston Rugby Club. The client, Boston Sports Initiative, comprises the two sports clubs along with Boston council. As the arena will also host large disabled sporting events, it has been designed to be accessible E E to as many people as possible (see "An arena for all", page 10).

Phase one of the project has just been completed and includes a fitness pavilion, which houses a gymnasium, swimming pool, sauna and steam room. The athletics track is overlooked by a half-finished grandstand containing meeting rooms for the athletes and rugby players as well as the local community. Known as the club pavilion, it also houses changing facilities, a first aid room, drug testing facilities and a prosthetics workshop. The grandstand is due to be completed, as part of phase two, in June 2004. This will also include a 60 m training pavilion, which will have indoor soft playing surfaces suitable for athletes and rugby players.

The pavilions have been built and clad almost entirely in wood. The structure is made from glulam beams and columns, the facades and roofs have composite timber panels and weatherboarding, and interior walls are made from timber studwork made from birch-faced plywood.

Surprisingly, given how well it works, timber wasn't the first choice of material. BGP Conaghy's initial concept designs for the project had a steel-framed structure, but the involvement of Finnforest, which has a factory nearby, lead to a switch in materials. The original design remained intact, though: "The concept design was very flexible and was readily adaptable to the use of timber frames and products," says Nick Reynolds, associate architect at BGP McConaghy.

For Reynolds, the challenge came in working out how the timber interacted with the rest of the building. "You can make assumptions about steel that we couldn't do with timber," says Reynolds. "It has been a learning curve. Using glulam and laminated veneered lumber was a very interesting exercise. It's very simple, but as a lot of the structure and fittings are hidden you have to learn how it works." Although structural engineer Finnmap did the details of the timber interfaces, the architects had to understand how they worked because they were responsible for signing off the drawings.

As Finnforest is a major local employer, it was asked by the Boston Sports Initiative whether any of its materials could be incorporated in the arena's design. The project team went on a fact-finding mission to Finland to see timber sports stadiums, and came back convinced that the whole building could be built in timber.

As well as the promise of a sponsorship deal, the location of the site helped to swing the decision Finnforest's way. There is a port in Boston with a wharf that Finnforest uses to import timber to the UK. By specifying the firm's products, the project team realised it could save on transport costs.

The existence of the port also changed the nature of the construction. "The original plan was to transport timber from Finland in a raw state and prefabricate the elements in the UK, says Peter Griffiths, principal of PGA. "But the transportation costs for this were looking high, as the few places in the UK that could fabricate the timber elements were far away."

The team therefore took the decision to import the large ready-to-fit timber elements directly from Finnforest's factory in Lahti, Finland. Prefabricating in the UK would have meant moving 30 m elements across the country, which would have meant prohibitively expensive transportation.

However, it was more practical to prefabricate the timber panel cassettes in the UK. The team set up a carpentry workshop in a rented warehouse next to the site, where 30 local carpenters were employed by contractor Driftwood Construction. "We tried to use local labour where we can," says Griffiths. The warehouse also provided a sheltered workspace and a dry storage area for all the construction materials.

Specifying wood had aesthetic benefits too, according to Davies, who says that the material was good for introducing "architectural niceties". The striking wooden rotunda that forms the reception area would have been too expensive to construct in steel, says Davies, because the curves would have been too difficult to reproduce. The wooden interior also gives the building a warm and intimate atmosphere that counteracts the harsh external environment, and one of the first things visitors will notice is the evocative smell of freshly cut wood.

The natural appearance of the interior is the result of the decision not to stain the ply panels. "We like the way the appearance of the panels vary naturally," says Davies. He was less pleased with the rough finish, which was caused by the need for a high level of fire protection. A thick intumescent coating was required to achieve a fire rating of 0, which meant the smooth natural texture of the ply was lost.

The timber panels lining the walls of the covered street that connects the three pavilions have a contrasting darker finish. The dark stain extends to the external facade, which features Finnforest's ThermoWood, a timber that has been more durable by heat treatment.

The one area where wood wasn't used was in the swimming pool, and shower and changing rooms. Instead traditional blockwork, plasterboard and tiles were specified. As well as being wary of the effects of moisture on timber, Davies says that any replacement of composite timber–laminate panels in the wet areas would have caused a lot more disruption than if tiles had to be replaced. He also appreciates that white tiles never go out of fashion. "They have a timeless quality," he says.

Building this sports arena in wood makes sense – which is ironic when you consider the virtually treeless surrounding landscape. Next winter local athletics will fully appreciate the benefits of this weatherproof material. Rather than shivering out on the gale-blown track, they will be running in the warm forest of timber that is the indoor training pavilion.