I took 4x4 cedar posts and notched them (see technique 3) a half inch to help support the girders (the 2" x 10" pressure-treated (PT) boards around the outside). I notched the posts so the floor of the platform is about 4' off the ground in front and 3' in the back (the ground slopes). Digging the holes for the posts was rather difficult, given the rocky (and rooty) condition of the soil. I mostly used a posthole digger, but at one point rented a two-man gas powered auger. I wasn't able to make too much progress with it due to the rocks. I got down to 27" for one hole and about 18" for the others. I put about 4" of gravel in the bottom of each hole and then put in the post and poured in concrete, using a level and one of the girders to ensure that the posts were properly spaced and the notches were level with each other. This was made more difficult by the fact that I built this platform on a slope. I had to trust the level instead of my eyes, because while the posts and boards were level, they did not look like it.
See how the vertical girders rest in a notch and are partially supported by the post
Before I set the posts, I applied these post collars at ground level. You can see them in the photo above right above the concrete - they are dark brown. They are supposed to protect this vulnerable part of the post against moisture, decay, and physical damage. I treated the tops, bottoms, and notches of the cedar posts with copper wood preservative, which I was actually unable to find in stores near me - I bought it at a Menards in North Dakota when I was up there visiting.
After I set each of the 2nd-4th posts, I attached a girder. I used two 4" fasteners on each side of each girder. These bolts are supposedly easy to drive and require no drilling, but in fact they were very hard to screw in all the way. I had to use my power drill like a screw driver, turning it manually while holding the trigger - that is probably bad for the drill. I spaced the posts six feet apart.
Once I finished the four posts, I attached two 6' 2" x 10" PT joists from the front girder to the back girder to help support the floor. I spaced each one about 5" away from the tree, figuring that it will take a good while for the tree to get that much wider. I used metal joist hangers to attach them.
The joists and a joist hanger (at right where boards meet) viewed from under the floor.
For the floor, I used 5/4" x 6" PT decking cut to 77" long to give a bit of overlap past the girders. I notched the first board at each end around the corner posts. I cut the middle boards to go around the tree leaving a space of about 1.5" - close enough so little feet don't fall in, but far enough to allow a few years' growth of the tree. I figure that, as the tree gets bigger, I can remove these boards and trim them as needed. I attached the floor boards with 2.5" deck screws (the ones with a star drive head - they are easiest to screw in).
I nailed PT 2" x 4" rails around the platform at the top of the posts to prevent falls. I put a second rail on the front (high) side. I left the back side open - that is where I attached a 2" x 4" to the posts a foot off the ground to serve as a step.
In a couple of weeks, I will apply some sort of sealant to the wood (at least the PT stuff - not sure about the cedar posts).
The oak tree around which this platform is built has long, thin branches that hang low to the ground. I asked the kids if they wanted me to trim them back so they could see, but they like the way the branches create a canopy over the platform, so I left them alone.
The first modification I made at the kids' request was to pound some nails into the joists underneath the floor for them to hang their pots and pans. The first use they came up with for the platform was as a campsite, so they built a pretend campfire on the ground in front of it. This may be because I told them we could camp on the platform some night - perhaps this weekend. Lucy also wants me to put a partial roof and some partial walls on it; I may try to do that at another time - I need a break!
I am interested to see what uses the kids come up with for the platform. So far, it seems like they spend as much time under it as they do on top of it.