Progress Report

Well the base supports, floor joists and floor have been installed. The entire structure sits 10 feet off of the ground.
All the corners were braced with a section of 4" x 4" post. There are 3/4" carriage bolts with split washer, washer and nut on each. there are two bolts per side. Where the supports contacted the tree, 3/4" x 6" lag screws were used. I tried to put at least two in everywhere that contacted the tree to alleviate any cantalever.
http://s229.photobucket.com/albums/ee206/theredhills/Treehouse/?action=view&current=DSC00351.jpg
We have had several bad wind storm and nothing moved. Lots of creaks and moans from the tree/supports, but no damage.

Once the entire base support system was installed, I began spacing out the floor joists using standard joist hangers rated for treated lumber. The base support systems is mainly constructed with 2" x 6" boards, but there are two 2" x 12" supports next to the tree. All of the floor joists were treated 2" x 6" lumber. Rated 10d nails were used to attach the joist hangers and the joists. Oh, and I did add some 2" x 6" diagonal supports to the 4" x 4" posts. It was impossible to bury the posts due to the amount of roots around the tree, but the posts are set on concrete pads to disperse some of the weight.
http://s229.photobucket.com/albums/ee206/theredhills/Treehouse/?action=view&current=DSC00350.jpg

http://s229.photobucket.com/albums/ee206/theredhills/Treehouse/?action=view&current=DSC00348.jpg

Once all of the floor joists were in place, 3/4" plywood decking was used for the floor base. This will be covered at a later time with something a little more decrative and functional.
http://s229.photobucket.com/albums/ee206/theredhills/Treehouse/?action=view&current=DSC00357.jpg
Of course my girls had to try out the floor with me!