Choosing the trees and the first beam goes up!

Not that there were so many options in our backyard, but I wanted to 1) try and keep the treehouse completely off the ground (no posts), and 2) use more than 1 tree. I ended up choosing a triplet which was roughly isosceles (two sides equal length) and included a sturdy Oak, which is great for treehouse building, and two hemlocks (not so great as most here are being killed off by a wooly adelgid bug and so i'll have to keep an eye on it and spray them every other year or so).

I took lots of measurements, planned out the platform support structure, which was doubled up 2x10 PT lumber, and put up the first beam in August 2014. I used a fixed anchor with a 10" long and 3/4" thick galvanized lag bolt (thickest I could find locally) driven deep into the oak tree. A special long auger drill bit, slightly smaller diameter than the bolt, was needed to drill into the tree first before wrenching and pounding the lag bolt in. Galvanized washers were placed on either side of the beams and it felt pretty solid once it was in place...