Tree House Design – Part I

Designing and building a proper tree house is definitely one of the things on my bucket list. I mean, who wouldn’t want to live in a tree house similar to those inhabited by Ewoks in Star Wars? Inspired by these furry little guys, this post kicks off a two-part series discovering that magical world. No, not the Ewoks, but the world of tree houses.

In this first part we’ll look at how to pick the right trees how that selection effects the tree house layout. Then we’ll focus on the support beams that hold everything together, and how they are fixed in a way that causes minimal interfere with the trees natural growth and movement. In Part II, we’ll cover the design of the actual tree house shell itself.

FIGURE 1 – Finished Tree House

DESIGN INTENT

As Einstein once put it ‘Imagination is more important than knowledge’. And that certainly applies here, because a tree house should be anything you want it to be – no matter how unconventional or unpractical. It’s about channeling your inner child and letting those creative juices run free. It’s about creating your own place for escaping the busy world below. It also needs to be something that’s part of the bigger picture. Part of all that calmness and isolation of the forest, rather than just some raised cabin with some trees in the background.

DESIGN PRINCIPLES

The most important thing to remember here is that trees are not still, static supports – they move about in the wind, all independent of each other. They are living organisms that grow over time, in girth and in height. These are all factors that need to be taken into consideration when designing a tree house.

It would be harmful to the tree’s health trying to prevent these natural forces, not to mention it can have catastrophic effects on the tree house’s structural integrity. So the main challenge then becomes figuring out how to design a structurally sound support that allows for free tree movement.

1. TREE & SITE SELECTION

Before we can start designing the support structure, we obviously need to find some trees. We’re not going design a tree house first, and then go out looking for trees to match that design. Instead, we need to find suitable trees first and let them dictate the type of  tree house we can build. Not all trees are created equal, so there are few things to look out for. As a rule of thumb, spanning multiple trees with beams is easier than trying to build a large structure in a single tree.

FIGURE 2 – Tree Selection

We want to pick the strong and healthy trees. The bigger the better really, as they’re generally stronger and move less. Also, the better we understand the environment where we want to build, the better chances we have of completing the project successfully. So it might be worth calling an arborist, or a tree surgeon, who can come and verify the health of the tree(s) you’re planning to use. They’ll be able to confirm if that particular tree, or trees, are structurally sound to build with. As a guide, some of the ideal (and not so ideal) trees are listed in the table below.

TREE---Tree-Selection-Table
FIGURE 3 – Ideal, Acceptable & Bad Trees (source)

Another site consideration is to figure out just how practical it is to build on. Remember that there has to be access to the site, to unload and move materials, and to be able to work at heights. We also need to think about how the finished tree house will interact with its surroundings. For instance, any possible neighbors may not be so ecstatic about a backyard tree house if it ends up blocking their evening sun.

So we need to ask ourselves which views do we want to capture? Can we make use of the sunlight to keep the tree house warm? Or do we want to use shade from the surrounding trees to keep it cool?

2. TREE HOUSE LAYOUT

Next step is to draw up the tree house layout. It all starts with a rough sketch of all the trees and their major branches on site. Then we’ll take out the tape measure (or preferably a laser measure) and add measurements as we work our way through. The dimensions required include distances between the trees, as well as the tree trunk diameters. The final sketch will look something like the drawing below.

FIGURE 4 – Tree Layout Sketch

These distances between the trees will tell us how long our support beams need to be. There are literally endless variations you can fix these beams to the trees – as long as they provide enough support for the tree house, that is.

So what about the height? Well, we could build our tree house at almost any height, but just remember that the higher you go the more the trees will move. As a general guide, 3 to 6 meters (or 10 to 20 feet) is considered a safer and more enjoyable height to build a tree house. Although working at any height, safety should always be your priority.

3. TREE GROWTH

Trees grow in two directions: upward and outward. Upward tree growth only happens at the tops and tips of the trees. Meaning that a tree house will stay at the same height even when the trees grow taller. Which leaves us with the outward growth – the increase in tree girth. So how do we make sure the tree doesn’t just push our support beams out as it grows over time?

FIGURE 5 – Tree House Attachment Bolt – or TAB (shown in inches)

At this point, let me introduce you to a piece of hardware called the tree house attachment bolt, or TAB. These TABs are designed to allow for increases in tree girth. As the tree grows, it will grow around the TAB, rather than pushing the piece of hardware of out the tree.

So eventually the tree will swallow up the TAB and start pushing the beam outward along the shaft. This swallowing up process is also known as growing reaction wood around the perimeter of the boss. This is the tree’s own natural defense mechanism as it seals off the possibilities of diseases and fungus infiltrating the tree.

TREE---Tree-Growth-3.jpg
FIGURE 6 – Tree Growth (source)
4. TREE MOVEMENT

The TABs will let the trees grow in girth, but what about the swaying in the wind? All trees move to their own , regardless of how other trees are moving around them. So we need to allow for this independent tree movement and not try to rigidly fix trees together with beams.

TREE-Attachment-Bolts.jpg
FIGURE 7 – Static & Dynamic Uplift Arrestor

As you might expect, there is a specific piece of hardware designed for this very purpose – called the dynamic uplift arrestor. This fixing will allow us to safely connect a beam to more than one tree while still allowing for lateral movement. It also prevents vertical movement of the beam. The purpose of these fixings is that they allow the beam to stay pretty stationary while the trees move in the wind.

The TAB will slide back and forth within the slot, but the dynamic uplift arrestor stays still. This is because there is a piece of UHMW fixed within the slot. UHMW is basically a plastic pad that allows the beam to slide on top of the steel bolts.

FIGURE 8 – Tree Movement (source)

And remember, the higher we build, the more tree movement we need to account for. The maximum movement allowed by a large dynamic uplift arrestor is 14 inches (roughly 350 mm). If too much tree movement becomes an issue, TABs can also be used to hang loads.

5. THE DYNAMIC TRIANGLE

Finally, we can add some extra strength to the fixings by introducing a cable suspension assembly to the end of the TAB. This combination is also known as the dynamic triangle – and is shown in Figure 9 below. First, a K-nut is threaded onto the end of the TAB after the timber beams have been installed. Then a turnbuckle is used to connect the K-nut and a cable sling, which is then anchored into the tree with a lag screw. The slack from the cable sling is then removed by turning the turnbuckle by hand.

TREE----Dynamic-Triangle
FIGURE 9 – The Dynamic Triangle (source)

CONCLUSION

So that’s it for the first part of the tutorial. You should now know the basics of mapping out a tree house layout, and be familiar with the different beam fixtures. Allowing for tree moment is the main lesson to remember here. Over time, the beam positions with respect to each other will change so that’s why the beams can’t be rigidly fixed. Otherwise somethings got to give – and it’s usually the tree house structure!

If you want to learn more about the topic, I’d recommend getting your hands on Pete Nelson’s Be in a TreehouseIt was by far the most useful practical guide I came across while researching for this project. There are plenty of good books that have cool projects for inspiration, some of which I’ve linked in the references. You can also check out some TAB installation tips on YouTube.

In the next next post we’ll look at designing the tree house itself, with its rope ladder and all. In the meantime, if you have designed and built a tree house before, or have any questions or comments, please let me know in the comments below.

REFERENCES & FURTHER READING

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