Setting a new track record

It is nearly five years since Lincolnshire’s Princess Royal Sports Arena was clad in “revolutionary” Thermowood. TiC braves the icy whip of the east coast to see if it stands up to its claim.

JUST OVER five miles away from the North Sea, Lincolnshire’s four-and-a-half-year-old Princess Royal Sports Arena is exposed more than most to the force of Mother Nature. There are no barriers in the landscape to protect the building from the buffeting icy chill that blasts its way across the reclaimed marshland. The £7.2m complex was built in two stages and funded through registered charity, Boston Sports Initiative.

The first phase of the project was completed in October 2003. It took just nine months to assemble the first half of the club pavilion and a separate fitness pavilion.

The grandstand and the remaining half of the club pavilion were completed in June 2004.

Originally destined to be steelframe, budgetary constraints dictated that the original design far outstripped the available funding – and timber could fit the bill more cost effectively.

Finnish product

BGP McConaghy Architects and project manager PGA Management elicited the services of Finnforest to discuss what options were available to construct the building in timber.

The structures of the pavilions were created from glulam beams and laminated veneer lumber columns. These were pre-fabricated and shipped over from Finnforest’s factory in Lahti, Finland, to the port of Boston. The composite timber panels for the roofs were made in a warehouse near the site.

The façade of the building is split into upper and lower layer tiers with exterior grade plywood cladding used for the upper layer and Finnforest’s Thermowood for the lower. The latter is made from Scandinavian redwood that has been heat-treated for extra durability and hardness.

Using both plywood and Thermowood for the skin of the building broke up the composition of the structure and was also a measure of the endurance of Thermowood against against a traditional substitute. “Using both added a bit of variety,” says project architect and director of BGP McConaghy, Nick Reynolds. “The pavilions are big buildings and there is a natural break in the structure anyway so using the two gave it a nice horizontal flavour.”

Thermowood has already been extensively used on the continent, but was relatively new to the UK at the time, says Reynolds. “In a way there is always a certain element of risk with a new product but it had already amassed an impressive track record in other parts of Europe.”

Having no particular axe to grind in using timber products, designing with them was as much a learning exercise in specification and performance as anything else, he says.

The design team considered using western red cedar and European larch before hearing about Thermowood. Their decision was based on finding a material that was both durable and would last untreated.

“It is not hard to see where Thermowood fits into the current timber cladding market,” says Finnforest’s UK construction projects manager Jonathan Stone. “It is cheaper than its competition and the heat treatment process enables the Scandinavian redwood to be used without a chemical preservative.

“A lot of architects want to use timber and a lot of them think their only other choice of a cheaper, maintenance free material is western red cedar, but Thermowood is half the price.”

However, the scale of the Pavilions caused a few butterflies in the stomachs of the timber suppliers and Finnforest was sceptical about using untreated Thermowood on the project.

Sustained

“We were reasonably keen not to stain it,” says Reynolds. “Once you start treating it you enter a maintenance cycle. But because of the scale of the building it was a showcase of Finnforest materials at the time. Finnforest was fairly insistent that it wanted to stain the wood, so we did. The results have been very exciting.”

The architects specified maximum lengths of 3.6m that were approximately 20mm thick. The cladding sat on 38mm treated square battens and a 9mm breather membrane. “It worked out reasonably well to handle,” Reynolds says. “It was light and more durable than a substitute softwood.”

Nearly five years on, the wood is due for restaining, but Reynolds is confident it won’t need it. “It’s got a good couple of years left in it still,” he says

The wood is being used unstained on Asda’s £16m superstore in Bootle, Merseyside, which is due to open in November. Previously, Tesco used unstained Thermowood to clad its Wick store in northern Scotland.

So is it better to use the wood treated or untreated?

Stone says the only notable difference by staining Thermowood is that it will silver quicker if you without treatment. “Clients feel a lot better about it going silver,” he says. “Both Tesco and Asda know that it will silver over time and they’re happy with that. It greys and silvers a lot quicker that western red cedar and a lot more evenly.”

Competing against Thermowood are softwoods that need to be preservative treated. Stone says that clients often neglect the staining process and then come back to the architect when the product is deteriorating. He says Thermowood limits that risk. “A lot of architects specifying Thermowood are saying to the client, if they want it stained, then that’s great. But ultimately they know that they’ve got a Building Research Establishment-certified product that will last minimum 30 years, irrespectively.”

Results

After reviewing the building in January, Reynolds says the cladding is standing up well. “There has been more deterioration in the plywood,” he says. “The Thermowood has remained pretty sound. Parts of it were covered by large overhangs, but even where it wasn’t covered it still looks good nearly five years on.”

Reynolds says that the plywood exposed at a higher level and not sheltered by overhangs looks as though it has suffered water ingress. “It looks as though there is a bit of dampness or deterioration in the finish,” he says. “It is a question of either giving it more treatment or possibly more drastic maintenance like replacing the panels.”

Reynolds says he has no qualms about specifying Thermowood for future cladding projects.

“We use anything form brick and timber, to aluminium for exterior cladding. A lot of it comes down to detailing and making sure you’ve got a decent specification on the actual timber product itself and thinking about whether the client would go for an untreated timber that would potentially cost a little more.

“In a maintenance scenario, you’ve still got to make sure the client is aware that timber is something you have to at some point look after more than brick.”