A cottage industry of treehouse contractors and hobbyists convenes in Seattle each fall. I was a rigging instructor at the World Treehouse Association convention for 5 years.
The Global Treehouse Symposium will be hosted by Peter Nelson, author of Treehouses of the World, on September 23-25, 2011. Here's a photo of Robert Oxman in a treehouse built by Pete at Treehouse Point in Fall City, Washington. No need to click on the attachment, this photo is it.
A treehouse involves a method of attachment to living wood. Space between the tree & the beams allows for future growth. A rigid foundation & floor with attached walls and roof must be cradled by the connection to a single tree, or even multiple trees, as seen in the photo. Hybrid treehouses add posts to the mix.
Lag screws are available with an extended head that allows the tree to grow into an airspace without shoving the treehouse off of the tree. Brackets slide over the head of the lag bolt, and move freely, creating a metal on metal 'suspension' that absorbs flex and growth. This connection gets stronger over time as the bolt is supported by new wood added by annual growth rings.
These 'artificial limbs' are designed and sold by Michael Garnier from his website,
www.treehouses.com. They are known as GL's. GL stands for Garnier Limb.
A special jig attaches to a drill to position the various attachment points in plumb with each other. This allows the foundation of the treehouse to use as few screws as possible to lessen impact on the biology of the tree. Minimizing this impact gives the advantage of a design that allows the tree to flex.
A variety of sizes & styles of GL's are available that support different amounts of weight. The GL head can even be extended further after years have passed, and the tree has grown to cover the exposed hardware. This gives a longer project lifespan that can be worth a considerable initial financial investment.
Many of the GL styles combine a pair of 2 screws in vertical alignment. One of the GL's gives support by either propping up its twin from below, or suspending it from above. Longer beams carrying heavier loads can span greater gaps in tree architecture, providing spacious treehouse floorplans.
The custom built treehouse can be a temporary structure built by amateurs, or a permanent structure built by a professional treehouse designer. A client needs to be informed if the treehouse will begin degrading from the day it is installed, or continue to be an asset to his property.
The treehouse construction industry is in its infancy, and will benefit from quality products, or suffer from experiments that break down after a short time. I recommend attending the 4 day class by Treehouse Workshop in Seattle May 18-22 to get a better idea of the design & construction process.
Here's a fun video of a treehouse being built.
