I wrote a post a few weeks ago asking about the safety of a treehouse we are having done for our kids. I wanted to share some updated pics after talking to the builders, and get your feedback and thoughts. Anything look questionable? What else do I need to be asking or having them do?
First, nice treehouse.
When you first posted this a few weeks ago while it was mid-construction, I had some issues with the way the joists were in the air. But since, your carpenter added a cross-beam between the two posts, so that the joists rest on this. I don't love the idea that the deck is being held by the shear strength of the bolts attaching the cross-beam to the posts, as opposed to the compressive strength of the wood. I would have notched the posts instead.
Others have commented on the bolt in the tree, but this is a very common way of attaching treehouses to trees. To be a treehouse, a tree has to be somehow involved, otherwise I would call it an elevated deck that happens to be close to a tree.
The diagonal braces are also very common with this type of design, to control the deck from swaying around the tree.
I would also have preferred to not face-nail the steps on the ladder, but rather put the steps between the 2x4s, broad side up, resting on a piece of wood nailed to the sides of the 2x4s.
But it's still OK and playground equipment, including treehouses, are exempted from code.
I like the design the tree can absorb the threaded rod and nuts and the hole in the tree is fairly minimal. Over time as it grows the tree has room before it expands to the width of the joists.
You can spend a $1200 and get hardware specifically designed for a treehouse.
The one thing pointed out that resonates with me is the single point of failure on the beam through the tree. I'd be tempted to repeat the pattern but lower in the tree. I'd go 24" off the ground and put another small set of beams in place perpendicular with your existing ones using the same threaded rod technique. I'd then put posts between the bottom beams and the top beams. The bottom beams then help support the top beam. The holes through the tree are distance separated to avoid weakening the tree as it resists lateral loads.
Your diagonal supports could then also tie into the perpendicular beam instead of the tree and you could enlarge the fasteners. They currently look a little mickey mouse though I can't see what size the fasteners are. I hope they are structural screws.
