Dangerous things to avoid
As you will no doubt realise, 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 - they must be of suitable quality and size to sustain the load of a treehouse. 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 - 6"x2", 3-7 feet - 8"x2", 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 peoples lives depending on it.
Don't bolt beams directly between thick trunks
Large trees can generate immense pressure near the ground. If you bolt a beam between two trees like this, you run the risk of the bolts snapping in strong wind. It doesn't matter how many you use, they can simply shear off. Fit a simple flexible joint at one end of the beam to avoid pressure build up.
Use bolts instead of nails for supports
To minimise 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". 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. If you can't find bolts locally, you can order them online from suppliers such as Bolt Depot, or Amazon.