Building with Tree Trunks: A Tale of Two Structures

Craft first: a roof built on experience

On a recent family holiday, Buckland’s Managing Director and Chartered Structural Engineer, Robin Nicholson, found himself staring at the ceiling of Ocean Sports Resort in Watamu, on the Kenyan coast, a place he has visited since childhood.

The main building had a new roof, a scissor truss built from gum poles. It’s elegant and its span is genuinely impressive, yet no engineer designed it. According to the hotel owner, it was built by a craftsman who has been building roofs like this for around 30 years, with no structural calcs, just accumulated knowledge and experience.

The roof was created from gum tree trunks, harvested young and used more or less as they come and it’s finished in makuti, panels made from palm fronds, which keeps it light. The joints are simple too, with what looked to Robin like a single M16 bolt through each pair of members (though he stopped short of climbing up to measure it!).

“Turns out you can’t really switch off when you’re an engineer, even on holiday,” says Robin. A few rough numbers in his head suggested it could get close to working on paper. The Kenyan coast has no snow load to design for (a requirement for structural calcs in the UK), and the roof is light, which both help. Looked at with an engineer’s eye, it looks right, and it has been standing perfectly well.

In the UK, a structure like this would likely cost five to ten times as much to build, involve months of engineering sign-off, and probably still cause headaches with building control.

The engineer follows the carpenter?

When Robin shared the roof on LinkedIn, engineers, builders and architects piled in, many with stories of craft leading the way.

One described a set of game lodges in the bush with thatched roofs on gum pole structures. He kept asking to see shop drawings; by his next site visit the roofs were up, looking beautiful and looking right. The builder simply pointed to his head: this is how we do it in the bush. Another, with decades of experience across East Africa, explained that local roofers often fix poles not with nails but with a length of reinforcing bar sharpened to a point and driven into a short pilot hole. He learned that from the roofer, not the other way round.

Robin recognised the same order of things closer to home. “It’s a bit like oak framers in the UK,” he replied. “Often I find the engineer follows what the carpenter does and checks it, rather than specifying what to do. We need to learn more from the craftsmen.”

Historically, that’s how it has always worked. Medieval church builders worked largely by trial and error, and as one commenter pointed out, a craftsman with 30 years behind him has probably learned from a few failures along the way, the ones that don’t stay standing for us to admire. Codes often follow on behind: bamboo, for example, can now be engineered to an international standard, ISO 22156. In many ways, engineering standards are craft experience, tested and written down.

Engineering first: a shed built from the storm

So what happens when the engineering goes first? Over 4,000 miles away in Devon, the Buckland team has just put up a new shed (in the industry sense of a ‘big shed’ rather than somewhere to keep the lawnmower). It spans 15 metres, runs 25 metres in the other direction, and went up in two days.

Any craftsman would recognise its materials. There’s no glulam involved, which is unusual for us, but it does happen. Instead it’s built from trees that blew down in the most recent storm: nearly all spruce, plus one Douglas fir. Every tree came from within about 200 metres of where the shed now stands. They were sawn with a mobile sawmill to sections of up to 400 × 400mm, then taken on a short lorry trip to Buckland’s factory for machining and back. This isn’t really a ’round wood’ structure as we have squared the logs off with a mobile sawmill, but is along the same principles, being a structure made from tree trunks.

But for this project, the engineering came first. Structurally the shed works as a three-pinned arch, with braces providing stability and cross-bracing in the other direction. It was designed to Eurocodes as a green timber structure from the outset, with the appropriate safety factors applied, so there was no waiting for the timber to dry. Green timber shrinks and moves as it dries, so that predicted movement was built into the connection design before a single bolt went in.

In practice that means keeping the bolts relatively close together, so the wood can’t move much over the short distance between them.

What it will be used for hasn’t been decided yet, but the shed stands on the site of the original sawmill run by Bill Blight, one of Buckland’s founders. Building with tree trunks there again feels like coming full circle in a way, and it just shows what is possible – from storm-felled trees to a standing frame in a few months, with no drying time and two days on site. We’ll be taking a closer look at the shed’s costs and carbon footprint in a future post.

Buckland isn’t the only one taking an engineering-first approach to round wood. Just over the border in Dorset, the Architectural Association’s woodland campus at Hooke Park has been pushing the boundaries of round-wood engineering for decades. Its Wood Chip Barn, built by students in 2016, uses 25 forked beech trees from the surrounding forest as its arched roof structure. Each fork was 3D scanned, an optimising script developed with engineers from Arup worked out where each one should go, and a robotic arm milled the joints. The irregular shapes of the trees were kept rather than sawn away, and the trusses went up with help from a local craftsman.

Even at the cutting edge, then, the engineering and the craft end up working together, which is really the point of both stories.

Craft and engineering work best side by side

It’s not that one way is better than the other, and the two structures show why. Craft on its own for construction has limits. Give the Kenyan craftsman a tight brief, a low roof pitch for example, and poles can only go so far. In the UK, unengineered structures simply aren’t an option either, for plenty of obvious reasons, insurance among them.

But, engineering on its own has blind spots too. One structural engineer commenting on Robin’s post warned that graduate engineers can lean too heavily on software outputs without stepping back to ask whether the answer actually looks right, a question that will only matter more as AI finds its way into design.

The shed shows what happens when you don’t have to choose: craft-level simplicity, with local trees used green and up in days, delivered with engineering rigour. Craft knowledge “should at least be part of the mix,” as Robin put it, although we also can’t dismiss the advances that come from the science and engineering side. And having seen the Kenyan roof earlier this year, we’ve been pondering whether gum poles could be engineered to span much further!

Getting the best from any structure

So, the answer to the chicken-and-egg question is neither should come first! Whether it’s a gum pole roof, a shed of green spruce or a complex curved glulam frame, the principle is the same. The best results come from balancing the material, the design, what the building is for, and what it will cost.

That balance is hard to strike without people who understand all of it, from the workshop floor to the calcs, in the same conversation, but bring that expertise in early, while the idea is still flexible, and you can get much closer to the craftsman’s simplicity, with the confidence of a design that’s been properly engineered and signed off.

To discuss your next project, call us on 01363 891 363 or get in touch.