Tensegrity Treehouse

When we bought our house, I had decided already that the huge, twin-trunk pine tree overhanging our sidewalk desperately needed a treehouse. This was a little premature as my wife was only pregnant with our first at the time, but it means I did spend some years idly contemplating designs. When my daughter was two or three and showing interest in climbing, I started sourcing materials, including some driftwood my dad and I harvested from the beaches I grew up near. But still the project languished, unstarted. What broke it loose was losing my job and falling into depression, idling my days away while my dad was dying of cancer (see my previous post). I realized that I needed a project to keep my sanity, something I could pour my heart and soul into and remind myself that I could still accomplish things. 

My dad built me a treehouse many years ago; it was in a many-armed willow tree where my mom had originally built a platform for my much older brother. My dad was a carpenter, as were many of his friends and neighbors and it was not uncommon to go to a barn raising or such where they would all get together for a day to help with someone's big project. My dad had never taken his turn, so he finally decided to invite them all over to build me a treehouse. It turned out to be pretty amazing, as should be no doubt with a whole troop of carpenters fulfilling their childhood dreams: it had a huge slide and eventually walls, windows and a second story, all without a single plumb line or square corner.

My kid's treehouse was not going to be quite as epic, but I wanted it to be unique and exciting nonetheless. I also took it as a challenge to make a treehouse that was minimally invasive by having relatively few anchors into the tree so as to keep it as healthy as possible. Eventually I was inspired by a baby toy we had: a stretchy icosahedron held open by six orthogonal sticks.

My treehouse design was inspired by this baby toy. The twin tree trunks took the place of the purple and yellow sticks. 

Another design goal for this treehouse was to be safe while looking dangerous, because that's what makes it fun! The final design is a tensegrity structure, with four pieces of driftwood acting as floating compressive members and the whole thing hanging from the tree by four giant eyebolts, with a symmetric four eyebolts at the bottom providing tension. The main ropes form most of the edges of the icosahedron and then the entire thing is wrapped in netting to make a complete mesh ball with a single tube opening at the bottom. 

In order to build it, I attached an extremely minimal scaffold to the tree using only about eight screws. It was just enough space to stand on, with a couple of boards at chest height I could stand between and lean on. The most terrifying part was leaning out over one of those boards about twenty feet above my sidewalk with a gigantic angle drill to make the holes for the eyebolts. Definitely makes you remember you're alive!

Night shot of the treehouse under construction. All the boards pictured are the temporary scaffold.

It was quite a process getting the skeleton hung up. I ended up assembling the top three beams and their ropes on the ground, then heaving them up and balancing the top beam on my scaffold while I strung the ropes through the eyebolts and knotted and tightened them. There are actually only four main ropes, top-left, top-right, bottom-left and bottom-right, encircling two triangles each. I cut slots in the ends of each beam to pass these 7/8" ropes through and around, and by using fewer, longer ropes it was easier to adjust everything because key junctions can slide when the rope is slack. Then I just had to tie on the lower ropes to hold the lower beam and then tension it all down to the lower eyebolts with trucker's hitches. Everything is held together with knots, though I did add zip ties to keep the ropes from sliding through the eyebolts. 

The driftwood beams are seven feet long, so four-foot-wide netting fit nicely across the width of the triangles. I managed to cover it with five sections: one long one going around between the trunks and two small ones on each side. I used another spool of fine rope to whipstitch around each of the main ropes to keep the net from sliding and between the net sections to make them continuous. 

At this point I removed the scaffold since I could now safely stand inside it. However, it was a bit cavernous for my four-year-old daughter, so I started building interior hammocks from the extra netting. The main one was strung between the central beams, and then another V-shaped one below connecting to the lower beam. This makes three levels of hammocks, counting the bottom of the ball, and with the addition of some black ropes with handle loops formed with alpine butterfly knots, the whole thing became quite climbable. 

My daughter demonstrating she can climb above the top hammock.

Apparently one of the worst things to do to a tree is nail a bunch of ladder rungs to it, so I built a ladder that only attaches to the tree at the top with two screws, and then attaches to stakes in the ground at the bottom. This tree is not girdled anywhere, so it has plenty of room to grow; even the eyebolts have about an inch of shank exposed. It's flexible enough not to work itself apart as the tree moves in the wind. And it has wildly high structural margins; it weights very little but the ropes and eyebolts can carry something like 10,000 pounds each. The netting also adds a lot of redundancy to the structure. I used marine grade polyester ropes to make sure it doesn't degrade in the sun, and I go up in it every year to inspect and make sure it has no problem supporting my weight. 

I'm almost eye-level with the middle beams when I stand on our porch.

With the way the tree leans over the retaining wall, it's really quite high up.

My daughter was a bit scared of it at first, but before long she was climbing up and around to the very top. My son is excited about it too and has been venturing slowly higher; he hasn't quite made it to the top hammock yet, but he's only three. Pre-pandemic I've seen at least four kids climbing around in it concurrently. 

I don't know if you have any need for a minimally-invasive tensegrity treehouse, but if this in any way inspires you, I'd love to see what you build.