On Treehouse Masters

The Genius of Pete Nelson   

In recent years one of my favourite TV shows has been ‘Tree House Masters’. Sadly I do not believe further episodes are planned. Every week the show followed the exploits of the Nelson Treehouse Company and its eponymous leader Pete Nelson, as they embarked on another building project.

Perhaps Pete Nelson’s greatest genius is convincing a British engineer to enjoy his rather un-British manner. I can’t help it, I find his boundless energy and enthusiasm completely infectious. That said, the thing that a really like, and the reason Pete gets a post devoted to his work, is that despite describing himself as an architect I think he gets to call himself an honorary engineer. Not only are his designs architecturally elegant and beautiful, but the engineering is too. He clearly has a sound understanding of structural load-paths and he unquestionably understands trees and timber.

I rather suspect that Nelson Treehouse has reached the hallowed status that few designers have whereby people want to buy a treehouse with Nelson’s badge on it. They don’t just want to buy a treehouse, they want a Nelson Treehouse.

On that basis Nelson don’t need an endorsement from a blog that nobody reads so I am going to stick to what I know and that is engineering.

Once a grove of suitable trees has been selected to provide columns for the structure it seems to me that the most important factor in treehouse design, at least from the point of view of an engineer, is dealing with movement. 

Trees sway in the wind and like small children have a habit of setting off in different directions at the same time. If this cannot be accommodated then unwelcome stresses will be locked into the construction causing it to become distressed. This is particularly unwelcome if it damages one or more of the supporting trees.

Trees’ ability to sway is important, nature intends them to behave this way so that they can concentrate on growing tall quickly; there is a race to reach sunlight in the forest canopy. Spending their scarce energy reserves growing impossibly large foundations and a thick heavy trunk is undesirable in the race to the top.

The reason this approach is successful highlights the difference between stability, strength and stiffness. Wind blowing on a tree will exert a lateral force, primarily on the canopy, which is the largest part. This force is transferred via the trunk into the roots, which are spread widely like the foundations of a building.

Wind is of course a dynamic load that gusts and billows rather than a constant static force. This is important. As a strong gust catches the tree its trunk will start to yield by swaying. As the structure moves it is not offering resistance to the applied load. Obviously resistance must be achieved at some point or the tree will simply fall over, however swaying buys the tree time while the wind gust diminishes. In this way the bending stresses experienced by the trunk are somewhat reduced i.e. low stiffness attracts low stresses. Or in other words a flexible tree does not need to be as strong as a stiff one to prevent the trunk from snapping. 

Similarly, a flexible tree exerts a smaller overturning force on the root structure, which means that it takes less effort to remain stable than if the tree were stiff i.e. it is less likely to topple the tree.

Another interesting consideration is that tall thin structures tend to be aero elastic. In simple terms this means that when they move in the wind they generate their own eddies and currents that induce further movement in the structure. A feedback loop is set up, which can make them dynamically unstable.

I have a hunch, and it is only hunch, that evergreen trees, which tend to be the tallest trees, retain their covering to help damp out these effects. I realise that someone is going to point me to some very tall deciduous trees; I expect in a rainforest. I haven’t worked out this theory in full, but in my defence I would point to the use of buttresses in large rainforest trees.

Movement is also inherent to the way in which trees age. As they get older they grow taller and fatter. Fortunately trees increase in height from the top and not from the bottom, otherwise treehouses would gradually become higher and the entrance ladder would need to become longer. The bigger issue by far is growth in the size of a tree’s girth. This is really important because of how trees work.

Crudely speaking all of the action happens in the 20 annular rings located closest to the tree’s bark. These rings contain sap wood and they are responsible for conveying the nutrients required by the tree to live. You could think of them like arteries.

If a structure is connected too tightly around a tree’s trunk it will dig into the sapwood as the tree grows and it will begin to choke the tree. There is something all together unpleasant about this. Since the tree is compelled by nature to grow, a tight structure will condemn it to be slowly strangled by that same growth. As this happens it will ooze sap from its wounds. It follows that a less destructive method of connection must be found. 

Nelson have an array of different hangers and connectors that they use to erect tree houses, but they essentially boil down to two types, which have a common method of attachment. A fixed connector that anchors the treehouse and a sliding connector, which permits a treehouse to take vertical support from a tree while allowing the tree to sway horizontally. For each construction a mixture of fixed and sliding connectors must be arranged such that the treehouse is stable while simultaneously being free to move in each direction.

The steel connector attached to the trees is ingenious. The embedded end is of small diameter and has a thread. It is screwed into the tree. The middle of the connector has a larger diameter and is smooth. It is partially embedded in the tree and forms a bearing surface, which increases as the tree grows and envelopes it. The outer section of the connector is also smooth and provides a seating for supporting beams. As the tree grows in diameter beams can slide along the seating without damaging the tree.

There is obviously more to treehouse design than this, but this is the part that interests me the most.

I would highly recommend checking out the Nelson Treehouse Company website or buying one of Pete Nelson’s books. Not because he needs any publicity from me, but because you might just find yourself inspired by one of his fabulous designs.