Mountain bikers visiting the Coed y Brenin visitor centre in Dolgellau, north Wales will see a first – the first construction project using home-grown and home-produced Brettstapel in the UK.

Brettstapel is a solid timber construction system fabricated from softwood timber posts connected with hardwood timber dowels.

The £1.2m project for Forestry Commission Wales (FCW), which is part-funded through the European Regional Development Fund through the Welsh government, extends and adds to the original visitor and mountain bike centre, which was built in 2006.

This circular building with its conical roof has proved so popular – around 140,000 visitors per year and likely to rise to 180,000 – that more space was desperately needed. But it wasn’t just space that FCW specified in its brief.

"The brief called for high levels of sustainable design and the incorporation of affordable and transferable building techniques," said George Mikurcik, project architect at Architype. "It also specified that the building was to be a showcase of Welsh design and timber products."

Expanding the existing circular building didn’t offer a complete solution so the plan was to create a separate building that would complement the original.

The new building being delivered by main contractor Pochin Construction Ltd will add about 320m2 and feature a bike workshop and store on the ground floor and multi-use meeting spaces on the upper floor, along with two 24-hour access toilets.

The existing building will be extended by approximately 50m2, pushing out from the ground floor to give more space to its information point and retail area. A new bridge that will also act as a café overspill area will link the two buildings.

The construction method was originally based on traditional separated studs in combination with timber I-beams and wood fibre insulation but a visit to Austrian Brettstapel manufacturer Sohm Holzbautechnik GmbH’s factory proved inspirational.

Fitting the brief
"What really swung it for me was that although the other options would work with Welsh timber, the timber wouldn’t be visible and we didn’t feel that would fit the brief," said Mr Mikurcik. "Brettstapel is a load-bearing structure that is also the internal finish, so visitors will have a direct connection between it and what’s around them – the forest.

The next step was to tap into the expertise of Woodknowledge Wales (WKW). WKW is a FCW-funded initiative that works towards developing uses for Welsh timber in construction by working with, amongst others, BRE Wales, the private sector and Edinburgh Napier University. On this project independent consultant and wood scientist Dainis Dauksta, who has around 12 years of experience in the Brettstapel system, has led the technical development work.

Even he was cynical at the beginning, he said. "Not about the system but about making it in Wales. Technically it’s not that big a deal but the real challenge is kilning the timber. The moisture content of the lamellae can be no more than 15%," he said.

Three species, preferably Welsh-grown, were on the shortlist – Douglas fir, Sitka spruce and larch – and of these Douglas fir was Mr Dauksta’s first choice as there was kilning trial evidence to suggest that it could be dried to 12% without too many problems. No such evidence existed for Sitka or larch.

Kilning success
However, by altering its kilning schedules, Pontrilas Timber achieved some outstanding results. "We surprised ourselves by successfully drying home-grown Douglas fir and Sitka spruce down to 12%," said Sharon Poynton, sales director.

In fact, the resulting 120m3 of timber sent to timber frame and joinery specialist Williams Homes for cutting into lamellae and assembling into Brettstapel panels was a mix of Douglas fir and Sitka, with the latter accounting for around 85% of the mix. The two species are mixed randomly within the panels.

Williams Homes cut the lamellae on its planer moulder following the example of WKW prototypes. While these lamellae differ in size – the wall lamellae are 100mm thick and 50mm wide while the floor lamellae are 200mm deep and 50mm wide – the profiles are very similar.

"The development team wanted to get a good lock between the individual pieces before they are dowelled so a tongue and groove profile was used," said Mr Mikurcik. "A 2mm shadow gap on the exposed side of the Brettstapel was also developed by the team to hide any expansion or shrinkage in the future."

Once machined the lamellae were set out to the production drawings on Williams Homes’ purpose-made table, then clamped together in 600mm-wide panels, drilled and then beech dowels, dried to 6%mc were inserted. Apertures for windows, doors and main incoming services were pre-cut in the factory.

The dowels were supplied with a smooth surface by local timber merchant Roger W Jones and Williams Homes ran them through a spindle moulder to cut grooves in to improve the key. This key was improved still further by the fact that the holes themselves were drilled 0.5mm smaller than the diameter of the dowels so when they’re hammered home they’re hammered home for good.

Racking strength
OSB fixed to the exterior facing side of the Brettstapel panel fulfils a dual purpose, providing racking strength and an airtightness layer. This is followed by an insulation zone comprising 300mm Larsen trusses, fully filled with Warmcell, then an outer sheathing of 25mm Isoplaat board. The Larsen trusses comprise 50x50mm softwood timbers intermittently joined with 9mm OSB and are an elegant way of containing insulation and supporting rainscreen cladding, while minimising thermal bridging.

The panels are joined using simple half lap joints and screwed together diagonally from the OSB side.

The exterior rainscreen cladding will be either Douglas fir or larch – although Corten steel cladding will be used on the extension to the original building.

Exposed timber
The floors are Brettstapel, as are some of the internal walls. "Certain rooms will need other linings, in which case we’ve gone for simple studs," said Mr Mikurcik, "but those internal walls that are Brettstapel will be exposed on one side and the other will have two layers of birch plywood attached to them for racking."

Brettstapel for the roof was deemed overkill, so here timber I-beams "stuffed full" with insulation are the answer.

The first of 104 wall panels were delivered to site just before Christmas and have been installed by Williams Homes using conventional site handling machines. The company avoided using specialist lifting equipment by limiting widths to 1200mm (two 600mm panels joined together). The panels are storey-height – 2.7m on the ground floor and 3m on the first floor.

The windows will be a combination of traditional triple-glazed windows and curtain walling, the latter in timber with oak cappings on the outside, and Williams Homes is manufacturing and fitting these, too. "The fact that they are responsible for both the timber frame and the windows is great because they can ensure good airtightness," said Mr Mikurcik.

A U-value of 1W/m2K is targeted for the windows with 0.12W/m2K the goal for the ground floor and walls and 0.09W/m2K for the roof. "Coupled with an airtightness target of one air change per hour, the energy bills should be very low," said Mr Mikurcik. Subject to the Welsh winter weather, project completion is scheduled for the end of June.

Promoting Brettstapel
This project aside, WKW and Architype are hopeful that UK-produced Brettstapel will grow in popularity as a construction method. Already the technical expertise shared by WKW, BRE Wales and Edinburgh Napier University’s Centre for Offsite Construction and Innovative Structures (COCIS) has widened the pool of knowledge and, said Mr Dauksta, the resource and the capacity is there.

"We’ve pretty much cracked it with the kilning and the nice design aspect of Brettstapel is that almost every good joinery workshop has a planer moulder and would be able to manufacture it," he said. "Now our challenge is to promote awareness and get other architects interested.

"WKW is working with some county council architects on plans for a new school and is considering curved Brettstapel walls for that," he continued. "We’ve got over the initial hurdles and now we’re really keen to push Brettstapel out there."