Tree Fort


When I was a kid we had some big trees in the back yard. My Dad built us a really big neat tree fort in one, and then my brother and I each built one of our own. There was a rope bridge, and a zipline, and a tire swing... It was the best.

But, years later, I observed some problems with the design. The tree kept growing, but the tree fort didn't. The floor got crushed, boards pulled apart, and generally the tree wrecked the structure inside of ten years.

We moved, and built another tree fort or two. This time the design was a bit more flexible. If we loosened the bolts out every year or so the trees wouldn't crush the boards against the bolts. But even so, eventually the trees tore it down, especially as they swayed in the wind. I grew up, went to college, and kept dreaming of tree forts. At some point I heard about Free Spirit Spheres, and something clicked. Tension support! Of course! I knew what to do.

Eventually I bought a house. It has a big tree in the back. A Chestnut tree. It has seven trunks, and lots of potential. I just had to put a tree fort in it!


The plan is to make a three story tree fort, likeaso.

Each level will be a floor supported by ropes. The ropes will pull laterally on lag bolts anchored in the tree. In this way, the whole platform can adjust to the growth of the tree by backing out the lag bolts periodically. I ran some calcs with an engineering buddy, and it seems that 1/2" lag bolts 3.5" long are the right choice. I'm planning on using 3/8" rope, which should be plenty strong.

The railings will also be ropes, probably run through eyebolts. I'm planning on putting a removable tarp up as a roof, but we'll see how far it gets. It's not going to be a very "safe" tree fort, but it should be light, cheap, very low maintenance, and have a minimal impact on the tree.


We found some palates in the dumpster at work. They seem ideal! Excellent!

I bought a hundred feet of 3/8" polyester rope in expectation of a tree fort project. It was just laying around. Good enough!

I've got eye bolts and thin cord from the same previous project. May need to buy more at some point.

Finally, I bought some hardware. 1/2" lag bolts, washers, and metal thimbles to ease the strain on the rope and reduce wear.

The bolts have a full 1/2" shank, which will keep the tree from "filling in" the space where the threads will grip. This won't be a problem initially, but years down the road it will reduce the grip of the bolts. We can solve the problem by machining down the shank to the minor diameter.

Once it's done, the machined section looks like this.

But why didn't you machine down the whole shank?" you may very well ask. "Well," I would reply, "because of this!"

The un-machined part will never enter the tree, and it gives me a good visual "depth" for turning in the bolts. Plus it would have taken twice as long to machine! Madness!

Putting it Together

Nearly done! Now we've just got to... actually build the tree fort.

Level One : Foundation

The first level is the largest, but also the lowest. Most importantly, I can get up to it with my ladder! Not sure how I'm going to do the higher levels, but no point worrying about that now.


First off I used small eyebolts to test out the position of the platform and the supports. If I have to move the support locations, they won't leave huge gaping holes like the lag bolts. I'm not putting any load on these platforms, even though they could probably take it. Load tests will wait for the final fasteners.

The main lines support the platform, while the stay lines keep it from swinging and bumping against the tree.

You can see that there are four anchor locations, but only three ropes. This is intentional. With four separate fixed-length lines, the platform would tend to "wobble" like an overly rigid four leg table. With one support line wrapping underneath and allowed to slide, the platform will self-level, while still getting enough support to (hopefully) not tip over when standing on the edge. The small lines underneath (one is hidden, off to the right) keep the platform "down", and resist lateral movement. Together, all the lines should keep the platform well anchored without any hard parts touching the tree.

You can see here the two fixed support terminations. The rope wraps around grooves I put in the 2x4s using a half-round file. The rounded groove should reduce wear and "kink" in the rope. I probably could have left the 2x4s out entirely and pulled directly against the plywood. However, this solution distributes the load much better, and makes the engineer in me happy.

Here's a close up of the grooves I made for the second platform. Should keep the rope well in place and in good shape.

And then, some painting! I put a good coat of exterior paint on the platforms. It should keep them from warping and rotting quite as fast as they would otherwise. I should probably paint the place where the ropes touch as well, since they will be quite wet... maybe later. For now the undersides are raw.

Full Load Anchors

And now it's time to install the actual anchors that will carry the load of the floor.

Before driving the lag bolts, I drilled pilot holes; If the hole is too small you risk splitting the wood (and the bolts are hard to turn). Too big, and the threads won't have enough engagement. 3/8" holes for a 1/2" bolt turned out just about right. It's as if they were designed that way!

And finally, the fully assembled rope anchor. I'm not going to take close-ups of every one of these, but they all basically look the same. There will be three or four of these per platform (maybe only two higher up the tree!) which should give us a rediculous load margin on top of our already triple design factor.

Of course, heavy duty mounting doesn't keep you from falling out of the fort, but it does keep the fort from falling out of the tree.

After a few hours of work, I managed to get both platforms mounted and fairly level. It holds my weight! Level one victory!

Both platforms were a bit "wobbly" by themselves, and tend to tip to the side (even with the guy lines), so I screwed a few boards across both to keep them steady. The boards don't touch the tree, and just keep the platforms from tilting crazily as you walk around on them.

I'll have to keep this in mind for level two, as I was only planning on using one platform up there. Maybe I'll just put a long beam on cross-wise and tie the anti-wobble cables to that.

Rope Ladder

Wherein I secure access to the tree fort.

At this point the tree fort is usable. I sat up in it and read for a bit. Lay on my back and gazed up through the branches. Good times. But, I'm not done yet. It's really a pain to haul out the ladder every time I want to get up in the fort. Clearly, I need some sort of access.

Normally, a tree fort is accessed via boards nailed to the trunk of the tree. But that's silly. Tension structures are the way to go! And we've already got a lovely place to anchor it. An hour or two of work, trial, error, and perseverance, and the ladder is in place.

I trimmed the chestnut tree before beginning this project, so there were lots of perfect limbs just laying around the yard. The knot is a "Rolling Hitch" which can be tied in the bight (really helpful when dealing with ten meters of rope). They are tied opposing each-other, so that the rung won't tend to "roll" when you stand on it. The knots aren't super secure, and tend to slip if they aren't tight. I've found if you stand on the loop at the bottom before ascent it pulls the knots quite snug. I'ts kind of odd, but I haven't had any problems since adopting this procedure.

Finishing up Level One

I was going to put up railing, but that's fallen by the wayside for now. Perhaps at a later date.

I'd like to put up a spiral staircase, but probably after I do the railing. I don't see this happening any time soon. Probably will be the last thing I do.

I was up in level one with a saw taking down some dead limbs. The platform sure makes it easier to work up in the tree. No more fiddling with ladders. Then I saw the neighborhood gang (four middle-school boys, with high freetime-to-father ratios) skulking up the back alley. They didn't see me (no one looks up in the trees (I learned this in college. You can lay on a limb right over a walkway and be practically invisible)) so I put on my friendliest demeanor, waved exuberantly, and yelled "Ahoy! Hi there!" For whatever reason they acted like they were doing something wrong, and slunk back the way they came. Sheesh, can't a guy be friendly to his neighbors nowdays?

So far the fort has been useful for reading books, daydreaming, trimming branches, and frightening children. Best tree fort ever.

Also, the rope ladder used up the last of my 3/8" rope. So... I guess this is it! Level one is complete. On to level two!
(And buying some more rope)

Level Two : Foothold

I'll be honest here, I'm still in the process of mastering my childhood fear of heights. The thought of putting up level two makes my palms sweat.


First, need to be able to get up there and work on stuff. I'm thinking of setting up a temporary scaffold of some sort. Hmm...

So, it turns out that ladders are really handy. Just, kind of, set it up on the platform and hey presto! I can reach any place I want on the second level!

Still really high up though. Taking some getting used to. I've found that thinking about the immediate environment instead of the location of that environment helps quite a bit. Wouldn't want to get too comfortable though, but that doesn't seem to be happening any time soon. To help assuage my terror, I've taken to tying the ladder to the trunk against which it rests, just in case it wants to fall over. This has proven quite useful on several occasions where I would have otherwise been stranded way up in a tree. Probably could have slid down the trunk, but that is a less-than-optimal scenario.

I also put that big yellow rope up. It doesn't really do anything, but I can reach it from any of the three trunks and it's useful for hauling things up from the ground.


Getting the platform in place, anchors in, and everything secured.

While going through some paint left by the former owners, I discovered that we have a decent amount of "no-skid" paint, which is basically just paint with sand in it. Figured slipping and falling from level two would be an even more unhappy event than on level one. I might add this paint to level one as well... at some point.

Another shot of the paint, along with some reference for how high up this platform is.

And here's the platform all up in the tree! I decided to put these anchors in higher up than on level one, since the angles are smaller and it's easier to stabilize. These are about at eye level, where level one has anchors at about waist level.

Oh, and it's spring here now. Harder to get a clear shot with the camera.


Lesson learned from level one, these things tend to be wobbly.

I had to re-work the ropes on this end to make it less likely to flip over suddenly and send anyone on the platform to their certain injury and/or death. The ropes still tend to "cam" over the thimbles, so I've tied them off. The other one is just like this one, only on the other side.

And even then, the other end tends to tip uncontrollably. At this point a number of guests came over including several children. I warned them that the fort was dangerous... something about burrying them at the foot of the tree if they died. Apparently that wasn't enough deterrent, though no one died, so I guess it all turned out. Afterwards I added these guy-lines to reduce the tendency of the platform to uncontrollably flip over.


At this point in the project I sat for a bit on the level two platform. It's small enough that I can place my back against one trunk, and my feet against the two trunks at the other end. This resulted in a number of experiments into the natural frequency of the stems, which turn out to be about the same (around 0.8 Hertz), probably owing to their similar lengths. I can set up pretty decent oscillations, just by pushing slightly with the balls of my feet. Would be more fun if I wasn't riding an undercurrent of terror the whole time. Maybe I should add some rails to this level? Nah.


Things wear out apparently.

A Couple Years Later

Here's an overview of the fort two years after construction began.

The main difficulties so far have been keeping the platforms free of razor sharp chestnut husks, and keeping the support bolts loosened frequently. Oh, and the rope ladder rungs tend to rot (and break!). Backing the bolts out every few months has been really easy, though the eye-bolts tend to leave larger scars than I would like. No major scarring on the trunks as far as I can see, so the general principles seem to be working well.

We were in Japan last summer, so I didn't get to enjoy the tree fort the second summer we've owned the house. Before we left, I backed the bolts way out to make room for the growth during the summer, and it seems to have worked pretty well. Spring came early this year, and we've been out in the yard, which of course spurred me to resume work.


Level one is pretty easy to reach and brush off, but level two is a bit more formidable. I would need to hold on to the rope ladder with one hand, while brushing the debris off with the other... not my favorite. I got the job done by loosing the stabilizing ties while standing on Level 1 and tilting the whole platform sideways. Then I got one of those long apple-wood shoots that we saved and scraped the debris off. Worked like a charm!

The ropes are starting to grow a bit of green. Moss or something?


I put up a single line of railing around Level 1. It's not really enough to keep you from falling, but it helps psychologically, and gives your hand something to balance against.

Level 2 has (predictably) two lines of railing.

Again, not enough to stop someone from falling, but it gives tactile clues as to when you're getting close to the edge.

I also added another couple stabilizing lines to Level 2. Wouldn't want to rely on single points of failure.


The rope ladder up to level 1 works pretty well, especially as there's a branch behind to grab on to. The rope ladder to Level 2, however, is less satisfactory. It's a harrowing process every time, both to clamber from the ladder to the platform, and the reverse. In addition, a few of the rungs were made of Chestnut (instead of apple) and they began to show signs of rot after just one summer. I replaced the bad rungs, but the last thing I want is to have a rung break when I'm twenty feet off the ground.

With this in view, I purchased a batch of pole steps. (From Great Britain... reclaimed from telegraph poles... because apparently they don't make them like this in the states.)

And mounted them to the tree trunk.

They are not tightened down, so the tree can still grow under them.

I'll have to back these out over time as well, but it beats flailing around on a rope ladder.

3D Model

On my return from Japan, I purchased a laser measuring tape. It's proven quite useful, among other things, for building a 3d model of the tree and the fort therein.

You can download the model here (.stl format).

One of the things I'd like to do is make Level 2 larger and more stable. For this, I'm considering adding wings, or maybe just re-designing and re-building the whole platform. Either way, I'm planning on modeling it first, to avoid improv-at-height.

Level Three : Roost

Every tree has a top


The plan from the beginning (see the top) was for the final stage of the fort to be high in the crown of the tree.

So that's where I'm headed now. I climb up and survey the site... it turns out it's not super convenient to climb, but not too difficult either.


So, after looking around I thought a bit. I had planned on putting some sort of platform up there, but none of the crest branches are stout enough for anchor confidence. So, instead of putting a whole platform up (or, at least for right now) I'm just putting a small seat in the crown.

In order to get it to seat without teetering, I carved out the bit of 2x4 to match the tree, a kind of tree saddle. Of course, I didn't get it right the first time, so I carved a bit, climbed up the whole tree, and pressed it into place, leaving marks on the underside of the saddle.

And then I climbed all the way back down the tree, carved out a bit more, and repeated the process until the saddle fit without too much gap.

Resulting in a seat and/or step that doesn't tip too much or fall over.

After that, I bolted a slab of 2x6 on top of the saddle to give a little more height (I was getting squeezed from sitting on the shorter one) and carved it to both fit the tree and be comfortable, and the roost is complete.


Eventually, I will make it a bit safer and easier to relax.

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