Types of support | Flexible supports | Rigid framed supports | Fixtures and fastenings | Metal brackets | Cables
Knee braces | Dangerous things to avoid | Non-flat surfaces | Improving stability | Building without trees
Dangerous things to avoid
As you will no doubt know, building a treehouse presents various situations in which you can hurt yourself or others. As the builder you are ultimately responsible for the safety of anyone working on, playing in or occupying the treehouse.
The following are some not so obvious things to know about treehouses and the specific dangers they can present. All of these details need to be addressed to bring your treehouse to the minimum acceptable safety level. The fact that treehouses are not legally regulated like normal houses is one of the important aspects of the freedom they represent, but this does not mean you can neglect to take steps to keep people safe.
Don't skimp on new wood
The support beams are structural and of critical importance—they must be of suitable quality and size to sustain the load of a treehouse, which can easily exceed 1000lb for the building alone. Avoid knotty pieces and ask someone if you aren't sure if the beam you want to use is strong enough. A rough guide for spans between trees would look something like; 0-3 feet: 2x6, 3-7 feet: 2x8, anything greater depends on the treehouse and will probably need additional support along its length.
Be careful with reclaimed wood
Properly processed reclaimed wood can be an environmentally conscious choice for your treehouse. However, if you are simply recycling old pieces of wood that you have found from a demolition, etc, you must be very careful that your wood is going to be strong enough. Check the beam in detail for signs of rot or decay, insect attack or splits/cracks caused during removal from its original position. Ask someone with experience if you are in doubt—dangerous wood could collapse with people's lives depending on it.
Don't bolt beams directly between thick trunks
Large trees can generate immense pressure near the ground when the wind moves the branches higher up. The large movement at height is translated into small but very powerful movements near ground level. If you bolt a treehouse support beam between two trees like this, you run the risk of the bolts snapping in strong wind. It doesn't matter how many fixtures you use, they can simply shear off. Fit a simple flexible joint at one end of the beam to avoid this pressure build up.
Use bolts instead of nails for supports
To reduce tree damage, you should only use one bolt at each attachment point. It therefore makes sense to use large diameter bolts at each point. Use ¾" as a minimum, but preferably 1" or greater. The bolts should be at least 8" long to allow plenty of grip within the tree. It is worth the extra money for large bolts to ensure the safety of the structure. Lag bolts (which don't pass all the way through the tree) avoid damage to the opposite side of the trunk. The best fixture is a Treehouse Attachment Bolt, specifically designed to support heavy loads in trees. If you can't find any of these bolts locally, you can order them online—see the Tools page for a list of suppliers.