Step 1: Installing the Beams
Lag two sets of beams to the tree using a 3/4 diameter x 10" lag screw. The top set of beams will be the floor height of your treehouse. Try to to drill the pilot holes as straight and level as possible. Next install the perimeter beams to the top set o beams and square them up using the 3,4,5 rule. Once the frame is square install your floor joist.
Step 2: Level the Platform
Level your platform by getting the floor somewhat level then mount a long 2x4 on one corner and go to the next corner and level that beam and install another 2x4. The longer the level the better. Now one of the four perimeter beams is level and continue to work your way around all the sides, temporarily installing the 2x4's.
Step 3: Install the Knee Braces
Once the platform is level you are ready to install the permanent braces that will support the treehouse. I used two 2x6's screwed together and cut at 45 degree angles. You can pick up some "hold down" brackets to mount to the tree with another 3/4" lag.
Step 4: Install the Decking
This is the best part of the project because you will be able to stand on the platform when this step is complete. To install the deck boards start at one end and let the boards hang over the edge long ways so you can make a clean cut with a circular saw at the end. Use a carpenters pencil for spacing your boards. When you get to the tree, use a compass with a pencil spaced about two inches out so to give room for the tree to grow and move in the wind.
Step 5: Frame and Install the Walls
Frame your walls on the ground. Account for the thickness of your siding when taking measurements for the walls. Install your siding on the ground at this time as well. Make sure two of your wall siding will overlap the ends of the walls they will be touching. When you are ready to put your walls up, rig a pully high in the tree to pull the walls up to the platform. Get some friends over to help you at this point. The walls will be heavy.
Step 6: Frame the Roof
To get the roof started determine how much of a slope you want on your roof. I used a 4:12 for the rise and run and this was a good slope to stand on while doing the shingles. Blocklayer.com is a great web site for figuring roof rafters and gable overhangs. Once you get the height of the rafters you can put up the center roof beam by making a temporary support to hold it in place until you get the rafters on. When your rafters are all on lay down the plywood for the roof an leave a good 2 and 1/2" space around the tree so it can sway in the wind without hitting the roof. I used shingles cut in half to seal around the tree and shingled the rest of the roof like normal.
Step 7: Add Access Ladder and Doors
I made the door out of t&g pine. For the ladder I used treated wood since it is in contact with the soil. The steps are spaced 10" apart to make it easier for little legs to climb it.
Step 8: Install Railing and Enjoy
The railing is 36" high and the balusters are 4" apart. I made a jig out of a scrap piece of wood cut to 4" to speed up the installing process and to make sure they are all spaced evenly. Start in the middle and just screw in all the tops of the balusters using your jig for proper spacing. Then go back and level out the middle piece and screw it in then you can use the jig off of that for all the bottoms.