Monthly Archives: September 2014

A Gentle and Strong Tree Fort

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As a child I built several tree forts, which had the problems most tree forts face:

  • fasteners. Nails and bolts can injure the tree, and their rigid nature causes them to weaken and loosen if the structure moves in relation to the tree. Exposed fasteners present a danger to climbers. Bolted metal cables are stronger and more flexible, but are hard on the tree bark and can easily cut off circulation to the limb they rest on.
  • rigid structure. A wooden platform cannot bend or sway with the tree, and will either be weakened by the tree’s movement and growth, or will weaken the tree.
  • weight. Wooden platforms can create a great deal of weight upon the tree, which makes them difficult to maintain and can damage the tree. Larger structures must rely on additional pillars driven into the ground, and some tree houses are simply elevated structures built around the tree, and not resting upon it at all.
  • handholds. Wooden platforms provide little in the way of hand- or foot-holds, which must be explicitly planned and built in.

I thought for some time on how I might build a tree fort for my daughter, before it finally occurred to me to apply my experiences as a commercial fisherman and build nets between the branches. This provides a structure with a number of advantages:

  • fasteners. I use rolled nylon webbing to attach the net at key locations, making sure not to constrict the limb. Where available I will simply run the net’s lead-line (frame) through a fork. This allows the bark to grow and function, and the connection points to adjust with the tree’s movement and growth.
  • flexible structure. A net platform moves with the tree, and can change its shape or be adjusted easily as the tree grows. There is redundancy in the design of the nets, and the fishing-net design is meant to hold thousands of pounds with no single point of failure.
  • weight. The fort pictured above uses 30-40 pounds of rope and webbing. It is much lighter than a wooden structure, and there is almost no stress on the tree when not in use.
  • handholds. The entire net structure is handholds. It is easy and safe to climb. If at any time one feels unstable they can simply hang into the net and be held lightly.
  • material. I use three-strand twisted nylon. It weathers and handles abrasion better than natural materials, is reasonably soft and doesn’t get scratchy like polypropylene. Knots hold better and it is less expensive than braids or core/sheath rope, it can be spliced easily and it provides a firmer grip than a smooth sheathed line.
  • knots. The lead-line of the net is made with a loop of larger rope with a small eye-splice on one end, the other end is attached there with a double sheet bend. This is secure, easily untied and adjusted, and strong. The net is tied using cow- or clove hitches to the lead-line, and a fisherman’s net-knot (basically a sheet bend) on the interior. All lines are ended with extra material and a larger eye-splice, which the children use in creative ways, and allows for later adjustments to the net.

I built the structure pictured above for Sage Hamilton’s program, which my daughter Tesla attends. It took about $400 in materials and a little over twenty hours of my time. If I were to bill for something similar the total cost would be around $1300. Sage was very instrumental in the design, and I continue to collect feedback from the children.

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