The K-Zone: My treehouse diary

Under construction

This page describes my experiences of building a small (8' x 8' floor) treehouse/hide in a wood. Because the wood is rather remote from my house, this is a long-term project -- I started in September 2005, and expect to finish some time in 2007. I have to combine the time available to work on the treehouse with other forestry work and, consequently, the actual amount of time I've spent working on the treehouse is relatively small -- about seven days so far.

Legal and safety issues

Before starting work on a treehouse, it's as well to be aware of some legal and safety considerations.
      First, if you aren't working on your own land, you need to be sure that you are legally entitled to build in the tree where you intend to site your construction. Trespassory tree dwelling as a form of environmental protest is very popular these days, but that isn't what this article is about.
      Second, construction of a treehouse may well require planning consent. In the UK, you should consult your local authority's planning officer for more details. In general, a construction the size, shape, and type of most treehouses would likely be classified as a `temporary building' if it stood on the ground. But it is by no means obvious that the works required to elevate the structure would fall into the same class. What's more, recent UK court decisions have held that the height of a treehouse should be measured from the ground, not from the point of attachment to the tree; consequently many treehouses will no longer benefit from the planning exemption that applies to garden sheds, because their highest points will be more than 3m above the ground. My treehouse is used for the storage of forestry equipment, and other forestry works, and therefore is exempted from planning approval under provisions related to forestry land.
      Third, as the treehouse builder you will bear the legal responsibility for the safety of people who enter the treehouse with your permission. You will even be responsible, to some extent, for the safety of people who enter without your permission. You must therefore make sure either that the treehouse is reasonably safe, or is inaccessible without your consent, or both. If the treehouse will be used by children, as mine will, the height of the construction must be carefully considered. The height above which a fall is likely to be life-threatening depends on the ground surface. For a hard surface such as sand or gravel, a fall onto the head from more than about five feet is likely to be very serious. Children older than about four years of age normally have sufficient coordination to put their arms out if they fall head-first, but that doesn't prevent broken bones elsewhere. Unless your treehouse is completely fall-proof, or your children are extremely closely supervised, you probably shouldn't build your treehouse more than about six feet off the ground.
      My treehouse is about twelve feet above the ground, and therefore presents a significant safety hazard. To reduce the danger, I have not provided the treehouse with a permanent ladder or rope -- I use a folding ladder which is kept locked when not in use. Because my treehouse is in a commercial timber wood, the trees have no low branches (they are all pruned off), and without a ladder getting access to the treehouse is extremely difficult -- probably impossible for a child. In addition, the treehouse building will be completely enclosed up to child height, making a fall from the building itself very unlikely. Nevertheless, children will need to be very careful supervised when ascending and descending.

Design constraints

The constructional methods used for the treehouse will, of course, depend on the trees available. It is generally easier to build the platform of the treehouse between three or four stout trees than in a single tree; if only one or two trees are available, trees with low side branches are easier to build in than straight trunks. If you do build between multiple trees, be aware that trees -- even very stout trees -- move in the wind, so if your treehouse is rigidly fastened to the trees, it needs to be strong enough to withstand the forces between the trees. If you build low down in a tall tree, these forces are likely to be considerable.
      Therefore, ideally, a treehouse will rest on the supporting branches of the tree(s), and be secured with ropes, rather than screws or nails. The weight of the treehouse will usually be sufficient to prevent it moving around too much. However, my treehouse is in a timber plantation. No substantial tree is closer than about twelve feet to any other tree, and no tree has side branches, as these branches are pruned off to maximise the thickness of the main trunk. This necessitates a single-trunk design, with bracing from the ends of the treehouse platform to the trunk. This is not optimal, and I wouldn't have chosen to build this way if there had been an alternative.
      Where attachment directly to the trunk is unavoidable, I think it is probably both healthier for the tree, and safer for the treehouse occupants, to attach using a small number of very large screws or bolts than a large number of small ones.
      When I started on this project, I had little idea how much bracing would be necessary to stabilize the treehouse platform on a single trunk. I had hoped to get away with only four attachment points between the treehouse and the trunk, two on each side of the trunk. My plan was to start with a pair of `gallows' braces, one on each side of the trunk, and add bracing until the structure became stable enough for me to jump up and down on it without it moving (much). It turned out that I needed five attachment points, and a total of six braces from the treehouse platform back to the trunk. Even then I had to prop up the end of the treehouse platform furthest from the trunk, as explained below.

Apart from the limitation of using only a single trunk for support, the design had to accomodate the fact that I had to build the treehouse single-handed, and away from any source of mains electricity. While battery-operated tools are OK up to a point, the battery charge doesn't last all that long under heavy load, and it was a long way to the nearest mains outlet. So the design had to accomodate construction entirely with hand tools. In fact, there are few things you can do with power tools that you really can't do with hand tools -- it just takes much longer.

The design also had to accomodate the fact that the largest piece of timber used in the construction had to fit into my car. In particular, while it would have been helpful to be able to use 6"x2" or 8"x2" timbers, the largest size that I could fit in the car was 4"x2" or 3"x3".

Day zero -- timber preparation

Let's face it -- when your treehouse is built you aren't going to want to spend ages going over it with wood preserver. You might not even be able to, since much of the wood will be inaccessible. Since the safety of the treehouse depends on the integrity of the structural timbers, I decided that as far as possible I would prepare them before assembly, even though I was using pressure-treated external timber. Conventional wisdom has it that the cut ends of timbers are most prone to decay and rot; clearly this will be the case if the ends are buried in the soil. So I measured out wood preserver in small pots and stood each structural member in a pot for several days. For this process to be effective, ideally the timber needs to be cut to length before preparation. This is only partly possible, for reasons discussed above. So you can't really expect to do all the timber preparation before assembly -- you're still going to have to go over the whole thing with wood preserver after it's finished.
In practice, to make use of the available daylight, I sometimes found that I had more time available than I had prepared timber; and then I had to buy pressure-treated timber locally and install it immediately. So a certain amount of post-assembly treatment was inevitable.

Timber preparation station. A treehouse consumes a huge amount of timber, so you might as well prepare it on an assmbly line basis. Note the ends stood in pots of wood preserver to soak. I used special camoflauge wood preserver for our treehouse; you can buy it from Wickes. No, really -- go in and ask for some. I dare you.

To get the camoflauge effect, I used a mixture of different colours of wood preserver -- various shades of green and brown. The colours will blend together nicely if you pick exactly the right time to wait between applications. If the first colour application is very wet when the second goes one, all the preserver will run together into a uniform browny-grey; if the first application is completely dry, the colours will completely separate and you'll end up with an effect like the Airfix models of tanks you made as a kid.

Day one -- raising the platform support beams

The first substantial job was to start the construction of the treehouse platform by mounting the first pair of horizontal platform support beams in the tree. Of course, these being only 4x2s -- the largest size I can fit in my car -- they won't even support the weight of the structure, let alone make it rigid. But because they're relatively light it is tolerably easy to make them perfectly level in all directions. My hope was that, with this basic structure being as perfect as it could possibly be, it would be easy to build upon it to make a platform that was both perfectly level and strong.
The support beams are mounted to the tree using 19x200 carriage screws -- the largest size I could buy. Even when the screw holes are pre-drilled, it is a devil of a job to screw them in to the trunk of an oak tree. If I were doing it again, I would get a longer spanner, to apply a greater amount of oomph.

The first support beams go up. I put the horizontal bars up first, then positioned the diagonal braces with a spirit level strapped to the the horizontals

The fixing of the beams to the tree is extremely strong, and I am confident that it would take ten times the load I actually envisage. The weak points are the holes in the beams and the diagonal struts where the carriage screws pass through. As the beams are eight feet long, a relatively small amount of force at the far end translates into a large force between the wood of the beams and and the screw shafts -- enough to weaken the wood eventually. This problem will be reduced by adding additional cross-bracing to prevent the ends of the beams moving laterally and stressing the wood at the tree end.

Day two -- doubling the support beams and laying the joists

The next job was to add a second tier of horizontal support beams, and above that the floor joists. In practice, it turned out that I was only able to get a few of the joists in place on day two -- it's much more difficult than it first appears, because at this stage the structure isn't really stable enough to lean a ladder against. Thus on several occasions I found myself drilling sideways with my left hand while dangling precariously from the structure with my right. Some sort of scaffolding or work platform would have made this job much easier, but a second pair of hands would have been even better.

The main beams are now 8x2, comprised of two 4x2s held together by wooden brackets which also hold the floor joists. Note that only two of the joists are fastened to the beams at this point -- because the structure does not have enough lateral strength to lean a ladder against, putting the joists up is rather difficult. However, once the joist that will hold the diagonal bracing stuts is in place, the structure can be braced more fully (see below), making it much easier to attach the remainder of the joists.

The joists are, like the rest of the structure, 4"x2" 8-foot timbers. As well as being fastened to the main horizontal beams, the joists will be attached to secondary horizontal beams at their ends. This will help to distribute the load when a point mass (i.e., a person) is placed on the end of one joist.

Detail of one of the brackets that holds the 4x2 support beams and 4x2 joists tied together. The brackets are just pieces of 4x2 cut and drilled. They need to be substantial, because they will carry a large force in tension when the platform is under load

Day three -- bracing the structure, and more joists; laying the floor

The plan calls for bracing struts between the secondary beams at the joist ends, and the tree trunk (or near the trunk -- we don't want to drill the trunk itself too much). These struts will improve the lateral stability of the platform structure, and make it much easier to lean a ladder against it safely for the rest of the construction.
However, to fasten these bracing struts there has to be joists in place to fasten them to, and getting the joists up without leaning a ladder on the side of the structure is a challenge, to say the least. So I fastened only two of the joists to the beams before adding the diagonal bracing.
The bracing struts meet the joists at their upper end at an awkward angle to cut. It's approximately a cut of 45 degrees across the timber, then another cut of 45 degrees along the first cut. However, it's not exactly 45 degrees, and in practice I was unable to make the two pieces of timber butt together perfectly. This is a result of my inadequate wordworking skills, rather tha a deficiency in the basic idea, I think.

Addition diagonal struts refer forces from the joist ends back to the trunk. However, in order to avoid drilling the trunk more than necessary, these struts are bolted to the main diagonal struts just above the trunk, not to the trunk itself

The joists are spaced at 16 inches, which I think is probably adequate, given the thickness of the floor. I spaced the joists with the plywood floor panels laying loosely on top of them, to ensure that the ends of the panels would lie exactly on the joists -- it's quite difficult to measure the joist separation precisely when their ends are not yet fastened to anything.
But with a least some of the joists fastened to the beams, it is possible to get at least a few floor panels securely fastened, which means that it is possible to get onto the platform to work. At this stage the structure is quite strong enough to work on, although it still has a small amount of lateral wiggle. The floor comprises of eight 8'x2' panels of 19mm marine plywood.

Day four -- even more bracing; finishing the floor

Because the diagonal struts that form the gallows braces meet the platform about five feet from the trunk, the trunk end of the platform is not very well supported. So, with some reluctance, I added some more diagonal braces from the joist ends at the trunk end of the platform to the trunk. Even with this additional support, I found that if I stood on the platform at the end furthest from the trunk, and jumped up and down (carefully -- it's a long way up), the platform did deflect an inch or so. With hindsight this was almost inevitable -- the tree trunk is not completely inelastic, and a deflection of the carriage screws in the trunk of a few millimetres would allow the unsupported end of the platform to move an inch or more. I toyed with the idea of living with this slight instability, but while I was working on the platform it became obvious that it was a safety hazard. At this point the structure had no walls -- not even a hand rail -- and it felt very uncomfortable having the end of the plaftorm moving under my feet while walking around.
So I decided to prop up the platform at the end furthest from the trunk. As it happens, a small oak tree had recently blown down in the wind, and the trunk was just about the right size to act as a prop. I dug a hole about two feet deep and cemented the fallen trunk into it, then attached the platform using carriage screws.

Additional load-bearing support provided by the trunk of a fallen oak tree

Day five -- framing the house

With the platform in place, and fairly solid, it was time to start work on the house itself. Again, it would have been very helpful to have an elevated work platform so that I could have worked on the structure from the outside; but lacking such a think (it wouldn't fit in the car), I had to work on the platform itself. Since there was nothing to stop me falling off the platform, and it's a long way up, this was a rather disconcerting experience. I really couldn't recommend working this way unless there is absolutely no alternative.

The skeleton of the treehouse is made from 3"x3" fence posts, and 1"x5" planks. A certain amount of diagonal cross-bracing is require to make it stable enough to lean against without deflection.

The skeleton of the house structure in place. The uprights are 3" fencing posts