The Treehouse - Leveling and attaching main supports
With both supports lashed to the trees, and boards laying across them, I was able to use a carpenter's bubble level to set the two boards exactly level between the trees and in relation to each other. The lashings hold them tightly, but allow slight adjustments.
Now I was ready to drill the holes and set the bolts. My plan called for 5/8 inch bolts and the trees are between 10 and 18 inches thick. So I asked my friend, Bob, if I could borrow his hammer drill. He went one better, coming over to drill the holes and pound the bolts through. (You'll find, when you are building a treehouse, that other guys are very willing to help. It's fun!)
THIS IS IMPORTANT! When you attach treehouse supports across trees. You can't bolt the boards to both trees. Trees move relative to each other. And since they're so thick and so tall, the leverage from even a couple of inches of movement at the top can generate enough sheer force to break a thick steel bolt. So you need to bolt one side of the board directly to one tree and hang the other end from a cable (see details on the cable page) which is attached to a bolt through the other tree. This allows for movement between the two trees.
So, in the picture, the near end of the closest board will be bolted to the Southwest tree (closest to the camera). But the far end will be hung from a cable attached to a bolt through the Southeast tree, about a foot and a half above the board.
The farther board (the one on the left, closest to Bob) will be hung from a bolt in the thick limb where Bob is drilling. The far side of that board will be bolted directly to the Northeast tree. (farthest away in the picture)