Whole tree structure – first in UK?

If I had known how difficult this project was going to be, would I have still done it in the way I did? – absolutely!

Genesis (J&T) Ltd., owner of a successful pre-school nursery chain in Midlothian asked me to design an extension to their Dalkeith branch. They chose my practice, Halvorsen Architects, having seen a timber treehouse we designed and built with a local P7 class.

Genesis is run by a delightful and energetic family of Greek descent who I find easy to engage with due to our mutual love of the outdoors and aspirations for the children. Genesis have won many awards for sustainable learning and outdoor activities. They asked for a dining room, craft room and performing arts room. The first phase – the dining room and craft room – is now complete.

Halvorsen Architects tries to design ‘honest’ buildings that reveal their structure, and so for these young children I wanted the structure to be not only obvious but also fun. I had seen some branched whole tree constructions in American publications, most notably some of the astonishing structures designed by the Wisconsin-based Whole Trees Architecture and Structures and wanted to design something similar here. The client embraced the concept immediately and I set to work, blissfully unaware of the problems ahead.

The main obstacle I faced, was that unlike in America, British buildings do not use ‘natural’ whole trees and they are not even recognised in design standards used by British engineers. There is no regulation governing their use and and there is a distinct shortage of people qualified to visually grade living whole trees for structural use.

I then embarked on an extensive search which was initially something of a wild goose chase. It involved following leads from experts in, amongst other fields, academia, the Forestry Commission and saw mill owners, up and down the land. The difficulty was finding anyone in the UK who was able to grade living trees with their branch structures still intact. My big breakthrough came when I tracked down James Coulson who has taught timber grading for over 40 years and set up TFT Woodexperts Limited, North Yorkshire, in 1991 to teach both Wood Science and Timber Technology. Jim put me in touch with one of his ex students, Iain Thew of Structural and Civil Consultants, based in Northallerton, but then based in York. Iain was trained by Jim and the two seem to be the only people in the UK able to grade live trees.

I then discovered that the UK hardwood grading rules, BS 5756, only cover a very limited number of tree species – oak and chestnut – while larch is the only UK grown species that can be visually graded to the strength class C24. We were lucky enough to have some natural, branched beech growing in one of my clients nursery grounds that needed to be felled, which we had hoped to use but because they are not covered by the UK regulations we will have to use them internally in the performing arts room for non-structural partitions.

Then the fun part came – sourcing the trees. I contacted several ASHS members and the first to respond positively was Willie Dobie owner and managing director of Abbey Timber. On a glorious sunny day, I set off with Willie around his beautiful woods on a bank overlooking Whiteadder Water near Abbey St Bathans, about a 45-minute drive away in the Scottish Borders. We narrowed our search to forked oak trees that could be used as posts and straight larch that could be used as roof beams for the nursery extension. We were looking for lengths of 3.5 metres for the branched oaks and up to 6.7 metres for the larch, and we wanted 18 in total. Willie was delighted as I was going to be making use of trees that were otherwise redundant due to their bends and branches.

Once the trees had been identified, qualified grader Iain Thew joined us for another glorious day in Spring, armed with a long measuring stick, a tape measure and a pink spray can. He assessed the growing trees and identified which ones had the right load-bearing capacity. This was done visually by assessing the position of the branches – and therefore the knots – and carefully measuring limbs and trunks. A couple were deemed to be too thin but the remainder were approved and given the official pink spray of approval.

The trees were driven to Leslie Winthrop’s builder’s yard in Dewartown, Midlothian and tooled with oversized tongue and groove joints. The entire structure was placed upside down in the workshop with the oak columns hanging from the gantry to check that all the joints fitted. Getting the trees to the site was not easy as all the materials had to be manhandled up and over scaffolding straddling a single story building, as the back garden of the original Georgian house could not be accessed any other way. It took eight men to carry each tree up the ramp and then lowered them by winch onto the site. They weighed approximately half a tonne each.

I also had some fun with smaller, forked parts of the oaks. Two of these were used for the intermediary supports required in a long feature window at low level for the children while one massive one was used as a support the pizza oven.

The trees currently still have their bark and lichen on. Some of the larch beams that had been sitting in the yard for a while have started to shed their bark but most of it is likely to stay on for a while. Again, this was relished by the client. Once all the bark comes off, we intend to get a woodworker to carve some nature-related features such as a tree creeper with its footprints spiralling up the trunk.

Having spoken to many people involved with trees along my journey I have been surprised at how little knowledge there is of the strength of living and, especially, forked trees. In fact it seems that this building may be the first in the UK to use “natural” or “forked/branched” trees. Dan Ridley-Ellis, head of the Centre for Wood Science and Technology, Edinburgh Napier University said “globally we do not understand how to grade round wood as well as we should, making better use of its intrinsic strength, particularly for forked timbers”.

The second phase of the building – a performing arts room – is due to start in July. This will also use whole tree structure and include a roof garden and a small outdoor amphitheatre.

It has been a real pleasure and very satisfying following the trees from woodland source to in-situ column and beam, less than 40 miles away. So far the response has been extremely positive – everyone I have shown around loves the raw trees, especially the children.