Wales Forest Business PartnershipUnit 6, Dyfi Eco Park, Machynlleth,Powys SY20 8AX

Telephone: 0845 456 0342 Fax: 01654 700050

Email:timber.info@wfbp.co.ukwww.woodsourcewales.co.uk

woodknowledgeWALES

The Coed y Brenin visitor centre extension - a Case Study

woodknowledgeWALES

The site for the extension is located in difficult terrain, cut into a bank and with limited access for heavy machinery. Mass concrete was specified for the walls of the smaller room in the basement and Brettstapel for the two side walls of the larger basement room. On the upper ground floor above the basement, Brettstapel was specified for all walls except the south facing front wall which is mostly glazed curtain wall supported on a glulam frame.

Lightweight timber frames called ‘Larsen trusses’ (Holladay, 2011) for holding insulation layers within an outer sheathing panel were specified to be fixed over the OSB sheathing on exterior face of Brettstapel walls. ‘Warmcel’ cellulose fibre insulation could then be blown into the void created by the trusses between sheathing panels. A 215mm deep Brettstapel floor is supported by a whitewood glulam beam running north-south down the centre of the basement.

Brettstapel, literal translation ‘stacked planks’, is a type of solid wood panel pioneered in Germany which uses either nails or wooden dowels to fix parallel softwood

lamellae together to form structural panels. Hardwood dowels ‘super’ dried down to around 6-8% moisture content (MC) can be used with lamellae of considerably higher MC so that dowels absorb moisture from lamellae thus expanding and locking them together. Several lamellae are clamped together and drilled right through at right angles using an auger 0.5mm smaller than the dowel finished size. Dowels can be inserted simply with a heavy hammer. To speed up the process hydraulic or pneumatic equipped assembly lines are used.

This type of panel is anisotropic and behaves in a similar manner in expansion as would a large panel cut from solid timber, it is unlike cross laminated timber (CLT) panels which are restrained like plywood by the cross-tying action of orthogonal layers. However, because of the parallel alignment of lamellae (and therefore fibres) Brettstapel panels may perform better structurally than CLT both as diaphragms and shearwalls; under compression Brettstapel can take up to twice the loading of similar sized CLT panels (Smith, 2013).

page one

The Coed y Brenin visitor centre extension - a Case StudyBritain’s first Brettstapel building constructed using homegrown softwoods, the extension to the Coed y Brenin visitor centre, was completed in summer 2013. The design process to this achievement started in autumn 2010, when ‘Eco-minimalist’ architects Architype were appointed by Forestry Commission Wales (FCW) to design an exemplar building utilising Welsh grown softwoods.

Figure 1: Architype’s exploded sketch of the new extension (image: courtesy Architype)

At the moment Brettstapel is mostly manufactured in Germany, Austria and Switzerland; Merkle Holzbau <http://www.merkle-holzbau.de> and Kaufmann of Oberstadion http://www.kaufmannbau.com> are typical German Brettstapel manufacturers. Kaufmann started out as a small carpenter’s workshop and grew to a vertically integrated company which processes local softwoods using state of the art technology to create complete houses and municipal buildings. Two examples are shown; a newly completed Brettstapel house (Figure 2) and a large two storey Brettstapel kindergarten (Figure 3).

Merkle sum up the situation thus; ‘stacked planking’ is an ecological system of construction which although in its infancy, is nevertheless technically perfected (Merkle GmbH, 2013). Professor Kurt Schwaner of Biberach University agrees and adds that even in Germany there is still some inertia in utilising Brettstapel which he explains as being caused by the high regard in which masonry construction is still held. He also sees German carpenters and timber manufacturing SMEs as conservative and somewhat reluctant to embrace new techniques (Schwaner, 2012). Nevertheless, specifiers in Britain have expressed great interest in the product; three Scottish architects, Sam Foster, Matt Bridgestock and James Henderson manage the website www.brettstapel.org where useful technical information and links can be found. One of the best known manufacturers are Sohm who are based in Austria <http://en.sohm-holzbau.at/>; their distinctive wavy lamella profile is shown above (Figure 4) and sides of panels showing dowel ends below (Figure 5).

page two

Figure 2: A Brettstapel house by Kaufmann; with the render finish it looks like a masonry structure

Figure 3: A Brettstapel kindergarten by Kaufmann, outer timberwork being added

Figure 4: Sohm’s distinctive wavy lamella profile, top surfaces showing machined shadow gaps

Figure 5: The sides of Sohm panels showing dowel ends

page three

During winter 2010-2011 Dennis Jones of BRE Wales and George Mikurcik of Architype sought Welsh SMEs willing to create prototype Brettstapel panels in homegrown timber, however by summer 2011 only one firm had shown interest and no practical tests had been carried out. Therefore Woodknowledge Wales (WKW) commissioned Dainis Dauksta to undertake research and prototyping work in partnership with Edinburgh Napier University’s Centre for Offsite Construction and Innovative Structures.

One year after the design stage of the Coed y Brenin project commenced the first dowelled Brettstapel panel using homegrown spruce lamellae was produced in September 2011 and exhibited the same month on Napier University’s stand at Timber Expo (see Figure 6, above).

Tenders to potential main contractors should have gone out by autumn 2011 to allow a start on site in spring 2012 but site problems delayed the process. By this time it was known that around 100 square metres of Brettstapel would be required. Specifications were originally drawn up by Architype based on those of Austrian Brettstapel manufacturer Sohm Holzbautechnik which included the moisture content (MC) of 12% for spruce lamellae.

This low target MC for spruce was considered to be a significant technical challenge considering that spruce is generally kiln dried to around 20% MC in the larger sawmills and researcher Dr Barry Gardiner (then working

for Forest Research) was warning of high distortion rates encountered in drying UK grown Sitka spruce. In order to minimise risk to the project the decision was made between Architype, FCW and WKW to specify Douglas fir lamellae; TRADA rates this species as showing ‘small moisture movement’ (TRADA, 1999) making it one of the more stable homegrown softwoods for kiln drying. Larch was also considered amongst other species; however with little knowledge of distortion characteristics during drying the idea was dismissed.

Figure 6: The first dowelled Brettstapel panel in British homegrown timber at ‘Timber Expo’

Figure 5: The sides of Sohm panels showing dowel ends

Figure 7: Distorted larch down centre of picture sitting on much wider, stable Douglas fir board

In spring 2012 Pochin’s PLC were appointed as main contractor and Architype were in discussions with their preferred Brettstapel sub-contractor. Dainis Dauksta had spent five days with researchers from Strathclyde and Napier Universities visiting Brettstapel manufacturers in southern Germany, Austria and Switzerland. This study tour was organised under the auspices of ‘Limesnet’, an academic working group assembled by Dr Pete Walker of Bath University. The study group were advised by Peter Kaufmann (of Kaufmann GmbH, Oberstadion, Germany) that they used softwood at 15% ± 2% MC for their Brettstapel panels (Kaufmann, 2012).

It was becoming clear that German firms preferred to install Brettstapel at a higher MC than UK architects and researchers had assumed would be necessary. By machining shadow gaps into lamellae, subsequent lamellae shrinkage could be disguised. Scottish architect Sam Foster was invited to advise on detailing of Brettstapel installation because of his previous experience in project managing construction of Acharacle Primary School in Scotland where imported Brettstapel panels made by Sohm Holzbautechnik had been used.

page four

A workshop was held at Architype’s office on April 3rd 2012 to discuss the following topics:

● options for sourcing the timber, subcontractor own sourced or FCW donated to project: time and cost implications

● choice of sawmill, their ability to adjust saws and cutting patterns to suit the project specifics

● recommended species for lamellae and dowels

● permitted number of knots, visual grading

● moisture content of lamellae and dowels

● sizes of lamellae and dowels

● size and shape of grooves in lamellae

● dowelling method (diagonal or perpendicular)

● jointing of lamellae to achieve required lengths up to 6m (finger jointing?)

● machining options for facing side of lamellae

● finish; compatibility of fire retarding coating (HR prof or similar) with OSMO oil finish

● transport of timber from sawmill to workshop and transport of panels from workshop to site: protection, handling, size of panels

WKW presented their research findings and made their recommendations regarding species choice and Brettstapel production methodology. Certain key technical decisions were made; 15% MC Douglas fir lamellae were the preferred option, joined using ribbed beech dowels inserted at right angles through lamellae.

This decision was made because Sohm Holzbautechnik have a patent application for use of ‘dovetailed’ or angled dowels in Brettstapel panels. Therefore if the patent is granted, any Brettstapel manufactured with dowel angles between 15° and 45° would infringe Sohm’s rights. Because orthogonal dowels were assumed to impart little racking strength to Brettstapel wall panels, 18mm OSB sheathing was specified by the architects to be fixed to the outer non-visual side of these panels. This would also act as an airtightness layer.

Discussions between Pochin and Architype’s preferred sub-contractor were not proving very productive, therefore there was considerable doubt regarding procurement of Brettstapel made with homegrown timber; not because of technical challenges, but rather because of the nature of the contract. Under the selected form of contract (JCT intermediate) it was clear that the subcontractor would be expected to carry all risk associated with using the innovative structural panels.

Where the risks are high then such problems can be removed by the adoption of a ‘partnering’ approach to the delivery of innovative projects, where risks are assessed and allocated on the basis of who is best placed to carry them.

Woodknowledge Wales stated their case:Research carried out by Napier University and WoodKnowledge Wales demonstrates that Brettstapel production is now achievable in Britain using homegrown (HG) timber. Most technical barriers are understood and prototype panels have been produced in Scotland and Wales.

Various options were discussed during June 2012 including use of imported Brettstapel. This was not an attractive option for the client Forestry Commission Wales who wanted to showcase use of Welsh softwoods in an innovative building. WKW were asked by FCW to submit other ‘fallback’ options to use HG timber whilst the architects sought new sub-contractors.

Architype offered the work to Williams Homes of Bala who had been one of the original bidders for the scheme and by July 2012 they had agreed to undertake the project. A meeting was held on August 13th 2012 between the architects, FCW, Pochin and representatives of Williams Homes (and their structural engineer Bob Johnson) at Architype’s Hereford office where final specifications were agreed. For the Brettstapel panels Douglas fir lamellae to be 15% ± 2% MC, dowels 6% ± 2%.

Figure 8: Ribbed Dowel

page five

One difficulty with the public procurement policy was pointed out by Ruth Jenkins of FCW; particular sawmills could not be specified, therefore putting at risk the procurement of the Welsh grown timber required by FCW. Owain Williams raised his concern about weatherproofing of the Brettstapel panels during transport and erection pointing out that wrapping the panels was as problematical as attempting to seal them using suitable oil. It was decided that focussed detailing would be discussed at a further meeting on 23rd August 2012 at Bob Johnson’s office in Shrewsbury.

Nearly two years had passed since Architype were appointed as architects to this project and autumn was already approaching. There had been concerns about timing of construction since the project started and at this point it was clear that the vital timber elements might have to be erected during winter.

Williams Homes started discussions with Pontrilas Group and Woodknowledge Wales regarding kiln drying of homegrown softwoods. The kiln manager at Pontrilas sawmill was not confident about getting good results with larch but was happy to dry Douglas fir or spruce to around 15% MC. This was an unexpected turn of events; most sawmillers (including Pontrilas sawmill) had up to this point rejected the notion of drying spruce below 20% MC as being too risky i.e. too high a reject rate.

Figure 9: tongue and groove profiles machined into lamellae

The following decisions were made at the 23rd August meeting:

● Douglas fir and larch were the preferred options for lamellae.

● Four sawmills were shortlisted to supply the softwood.

● Final lamellae thickness to be 50mm with a 2mm shadow gap to be revealed, lamellae to be machined with a tongue and groove profile.

● Panel sizes to be based on 1.2m width to fit with OSB panel size.

● Batches to be manufactured and installed as required.

● Panels to be finished with HR Prof fire treatment and Danish oil (Osmo is incompatible with HR Prof).

● Panel to panel joints to be half-lapped with over- sailing OSB and cross-screwed fixings.

● Short sole and head binder plates to be fixed at time of manufacture at same width as wall panels with tongue and groove joints between lamellae ends and plates. The whole panel to be fixed to in situ soleplate at time of erection. Header rails to be fixed along length of wall over binder rails where necessary.

Figure 10: Mixed Douglas fir and spruce panel showing shadow gaps between lamellae

Figure 11: A typical internal Brettstapel wall showing binder rails, soleplates and header plates

page six

53.200 FFL

DPC detail (first floor level)1101

Sto Superlit insulated render system as NBS M21 to 150mm below finished ground level

250mm Sto Therm Classic K EPS thermal insulation with taped joints bonded to RIW waterproofing layer

350mm in situ concrete retaining wall to SE details

Brettstapel dowelled massive timber floor cassettes

175

175

215

Sto Silco render system as NBS M21

RIW Structureseal as NBS J40

Finishedground level

Protektor 9044 perforated angle

Vertical board on board timber cladding on horizontal battens

300mm Larsen truss fully filled with Warmcell insulation

140mm Brettstapel massive timber wall, sheathed with 18mm OSB (airtightness layer). OSB to have taped joints and to have overlap over soleplate. Brettstapel panel to be fixed to soleplate from external side using screw fixings to SE specification

15mm Bitroc sheathing board with bitumen sealed joints

OSB sealed to concrete slab using airtightness tape

Timber soleplate (species to match brettstapel wall), fixed to concrete wall using fixings to SE specification

denotes airtight seal throughout all following drawings: floor, walls & roof

140 18 300 15 20 20

350

2mm Nora Ultra Grip rubber flooring on 18mm OSB on 100 x 50 acoustic battens voids fully filled with semi rigid insulation

50

215

min

756

Project

Dwg No.

Project Dwg Title

Dwg No.

Scales Date

Revision

Checked by

Client

Drawn by

Coed y Brenin

Forestry Commission Wales

GM

DD110

1:5 at A3

DPC details - Sheet 5

- 18.04.2012

CONSTRUCTION ISSUE

C1

Architype’s detailing of the junction between Brettstapel panels and concrete wall (image: courtesy Architype)

page seven

Figure 12: First Brettstapel panels at Williams Homes’ factory in Bala, November 2012

Figure 13: Brettstapel floor being installed in situ

By mid-September Tony Hughes of Williams Homes had ordered a mixed batch of spruce and Douglas fir from Sharon Poynton of Pontrilas Group with a specified MC of 15%. This was delivered in October and the first Brettstapel panels were made in early November.

As the Brettstapel panels were being manufactured through November and December 2012, Wales was entering its third wettest winter season ever recorded (Met Office, n.d.). When erection of the glulam frame and the first Brettstapel walls commenced during January 2013 the site was saturated.

Williams Homes made the decision to install the Brettstapel floor in situ; partly because the site conditions precluded the option of using a crane to install heavy floor panels (which were specified at 215mm thickness) and also because of the difficult shape of the floor plan. Prefabricated panels would have been more problematical (and possibly more costly) to design, manufacture and install.

Woodknowledge Wales held an event at the Coed y Brenin visitor centre on January 25th as the Brettstapel floor was being installed onto the glulam frame concrete retaining walls. Attendees were able to witness an extreme rainfall episode and the consequent effect on the proceeding work; it was necessary to sweep pooling water off the temporary Visqueen weatherproofing and even so water was seen to be penetrating it and dripping between lamellae through the completed Brettstapel floor.

In mid-February Pochin’s site manager Stuart Gaylard-Rees was expressing concerns over the moisture content (MC) of the Brettstapel panels. Out of 324 moisture readings, 4 were over 30% MC, 56 were over 21% MC and there were 28 instances of 21% MC. Moisture content in the building now became an issue.

Woodknowledge Wales advised Williams Homes to increase air flow throughout the building with large fans to avoid pockets of high humidity building up in areas where natural airflow was restricted. The building was not closed off until several weeks later so there was little point in using dehumidifiers. WKW continued to monitor moisture content over the next three months using a hammer probe with long pins to ensure readings were taken within the core of the lamellae.

In the last week of March, there were severe snowstorms and snow cover persisted over the first week of April. Brettstapel panel moisture content in the problem areas did not start to decrease until later in April by which time the building was closed off and solar gain allowed dehumidifiers to function efficiently.

The highest moisture content readings were found in Brettstapel wall panels where water had been swept off the roof in late January.

When readings were taken through the external layer of OSB in these zones, over 30% MC was encountered. The decision was taken to remove the Larsen trusses and OSB to examine the underlying lamellae.

Fortunately it was found that the saturated OSB had swollen to a thickness whereby the moisture meter probe pins were measuring MC of the OSB; the lamellae underneath were at safe moisture levels under 20%. New OSB was fixed to the wall panels and Larsen trusses replaced. By early summer 2013 all moisture content readings were below 20% and mostly below 16%. The main concern had been that above 20% MC it is possible for fungal infection to take hold, especially in open structures such as Brettstapel where lamellae are separated by narrow air gaps of generally less than 1mm width.

Interstices such as these allow water to be transported deep into panels by capillary action. However, no signs of staining or growth of fungal mycelia were observed on any Brettstapel panels despite the fact that they had been erected through some of the wettest winter conditions on record.

page eight

Figure 15: extreme weather conditions in early April 2013

Figure 14: Site conditions at Coed y Brenin in January 2013

page nine

Figure 16: View from the south; homegrown Douglas fir rails, posts and decking stacked on right

Figure 17: Finished interior, looking south through the curtain walling

By August 2013 moisture content of Brettstapel panels was averaging 15% and gaps between lamellae had not opened significantly. Homegrown Sitka spruce and Douglas fir has been used to successfully manufacture Brettstapel panels. Untreated Douglas fir has been used in external handrails and decking; all of these applications could make useful case studies.

It has become clear that Williams Homes valued highly the experience of working on such an interesting project. They along with WKW, would however, like to see greater clarity on risk-sharing. In this respect it would

undoubtedly have been better for all parties to the contract, if risks had been properly evaluated and allocated from the outset by adopting a ‘Partnering’ contract.

It is therefore felt to be essential that all parties work in collaboration on the pragmatic development of any project and the resolution of its technical challenges.

The fair allocation of risk-sharing needs to embedded in all projects, but especially in those of an innovative nature.

Lessons learnt:

At the start of this project there was considerable doubt surrounding the technical feasibility of using most of our homegrown softwoods for precision solid wood panel manufacture. However, these doubts tended to arise from normative thinking, old assumptions or insufficient scientific data.

Initial assumptions regarding (low) moisture content specifications in Brettstapel lamellae were incorrect; Germans firms use softwoods at up to 18% MC (Cheret,et al., 2000).

During 2011, Dr Barry Gardiner of Forest Research was still warning about the difficulty of drying Sitka spruce. In the same year Edinburgh Napier University’s Dr John Moore’s authoritative research report on Sitka spruce was published; up to that date our technical knowledge of the species was at best patchy and limited (Moore, 2011).

We do not have complete, authoritative technical reports on any other homegrown softwood species; giving space for normative thinking and assumptions.

One of the biggest perceived obstacles at the start of this project, kiln drying of Sitka spruce below20% MC, was achieved relatively easily whenindustry overcame their own inertia and took up the challenge.

Many of the perceived problems in making Brettstapel were resolved by seeing continental practice and discussing technical issues with researchers or other specialists during the ‘Limesnet’ field trip to Germany, Austria and Switzerland.

Public procurement policy in the UK tends to act against using locally grown timber in public projects.

●

●

●

●

●

●

●

page ten

We do not have complete, authoritative technical reports on any other homegrown softwood species; giving space for normative thinking and assumptions.

One of the biggest perceived obstacles at the start of this project, kiln drying of Sitka spruce below20% MC, was achieved relatively easily whenindustry overcame their own inertia and took up the challenge.

Many of the perceived problems in making Brettstapel were resolved by seeing continental practice and discussing technical issues with researchers or other specialists during the ‘Limesnet’ field trip to Germany, Austria and Switzerland.

Public procurement policy in the UK tends to act against using locally grown timber in public projects.

Normal construction industry contractual arrangements can be adversarial and act against innovation. Partnering contracts offer a ready-made solution.

We are still learning the limits of what can be done with our homegrown softwoods.

Working through one of the wettest winters on record, a successful Brettstapel building utilising home-grown softwoods was constructed in a challenging upland location.

From start to completion the project took five years, demonstrating the difficulty in driving innovation through the bureaucratic process. It was however ultimately well worth the effort with technical solutions now available for wider application within the industry.

Figure 18: The original visitor centre on the left and the new extension on the right of picture

●

●

●

●

page eleven

page twelve

Figure 19: View from south of finished structure

Conclusion

This project demonstrates the challenges to implementation of innovative techniques in construction utilising home-grown timbers. Ironically the construction phase, despite extreme weather conditions, accounted for only 10% of the time taken from inception to completion of the project.

Most importantly, the project demonstrated that Brettstapel construction can be successfully undertaken in the wet climate of western Britain. Primarily however, this project demonstrated that with proper collaboration, innovative projects of high quality can be delivered using home grown timber. This offers great hope for the future of the industry in Wales and the UK.

page thirteen

Figure 19: View from south of finished structure

Bibliography

Cheret, P. et al., 2000. Holzbau handbuch. [Online]

[Accessed March 2012].

Holladay, M., 2011. All About Larsen Trusses. [Online]

Available at: http://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-larsen-trusses

[Accessed 15th Setember 2013].

Kaufmann, P., 2012. Moisture content of lamellae for Brettstapel [Interview] (5th March 2012).

Merkle GmbH, 2013. Stacked plank construction. [Online]

Available at: http://www.merkle-holzbau.de/00engl/home/index_home.htm

[Accessed 4th October 2013].

Met. Office, n.d. Statistics for December and 2012 - is the UK getting wetter? [Online]

Available at: http://www.metoffice.gov.uk/news/releases/archive/2013/2012-weather-statistics

[Accessed 15th September 2013].

Moore, J., 2011. Wood properties and uses of Sitka spruce in Britain, Edinburgh: Forestry Commission.

Schwaner, K., 2012. Brettstapel construction [Interview] (5th March 2012).

Smith, S., 2013. Solid Wood Solutions [Interview] (23 February 2013).

TRADA, 1999. Section 2/3 sheet 6. In: WOODinformation. High Wycombe: TRADA technology Ltd.

Contact Details:

Architype email: george.mikurkik@architype.co.uk

Dainis Dauksta email: dainis.dauksta@woodknowledgewales.co.uk

Forestry Commission Wales(now Natural Resources Wales) email: kim.burnham@cyfoethnaturiolcymru.gov.uk

Pochin’s PLC web: www.pochins.plc.uk

Pontrilas Group email: sharonp@pontrilastimber.co.uk

Williams Homes and Timber Frames email: owain@williams-homes.co.uk

Woodknowledge WalesOctober 2013