This step is laying out the platform. Most decking comes in 12′ long boards so that is what we are using, cut the decking in half, and lay it out, this should make the tree house 6′ long. Cut two 2 x 6 boards the same length as the decking. Then set the remaining four to the width of your decking, less the thickness of two of the 2 x 6′s. This should make it 1 1/2″ each. Remember to leave a 1/2″ gap between boards, providing a drainage solution for future weather.
Step two is building the platform. Gather your 3″ deck screws and attach the four 2 x 6′s perpendicular to one of the other 2 x 6′s, spacing them as you go so that once they are in the tree the perpendicular joists will miss the tree. Now get that buddy I recommended in the tips section and put the platform up into the tree. Now you can center it, making sure to tie it down really well.
To finish and square the platform using a tool like the Empire Level 3990 12″ Heavy-Duty Magnum Rafter Square, screw the remaining 2×6 to the other end of the platform. It should be centered and squared before you proceed.
Begin to attach the platform to the supports, this is done with the rafter ties, attach the platform to the 2 x 10′s that are currently screwed to the tree. Add the joist hangers and use galvanized nails to attach.
This step is bracing the platform. Add diagonal bracing made of 2 x 4′s using a single lag screw (long) to attach both to the tree. Note, to make it easier cut a 45 degree angle into the 2 x 4. If you do this use an 8″ lag screw to make up for the thickness of the two pieces of lumber.
Hanging a pulley will help bring up needed tools throughout the next steps and can be kept for the kids to play with later. Hang your rope on a tree branch with a hefty climbing carabiner at the end of the rope. If you need to make one, use a lag screw and a 2 x 4 between the two trees protruding far enough to hang the pulley.