Question from Greg, Bellevue, WA:
“Building a treehouse for the kids; but now that my wife has researched a little is freaked out that what I’m doing isn’t safe so I’ll ask for advice on the basic support structure.
Three cedars approx. 50-70 ft tall, 14″-18″ dia. at 12′ off the ground where the main beams are in a triangle roughly 12’x8’x7’. The beams are 4×6 and are attached w/8″ FastenMaster Timberlok screws directly against the trunk (3 staggered at each point approx 2″ apart, and in hindsight from what I’ve read I realize this is not ideal; question ‘do I change it now?’), these screws have a rated sheer factor of 3K lbs and pullout factor of 1,500 lbs ea. with only 2″ of thread embedded. I used these because they are about 3/16″ in dia. rather than drill 1 large hole (and I had them in the shop). The beams were placed on the outside of the trees w/the floor joists attached to 3/4″ ply overlapping the beams, but not attached to them to allow movement effectively making a floating lid over the beams. The main question is, at 12′ off the ground on trees this size are those Timberloks substantial enough (I don’t know the sheer factor of a single 3/4″ or 1” bolt and also in hindsight 1 penetration is better than 3; again ‘do I change it now?’) or do I need to rethink this before going further. We’ve had some good wind and it seems to move just fine, I’ve been in the construction trade for 20 yrs, but I’ve never put 1,500 lbs of structure/people/snow/whatever in a tree before and I was making it up as I went….
I attached a sketch to help clarify“
I’ll tell you what I think would be safe for your treehouse, and then you can decide whether to act on it or not (and what you’re going to tell your wife in reality).
1) The shear rating on a fastener is meaningless unless the load is applied in shear. Due to the softness of a tree’s outer tissues, there is probably some leverage and bending going on which reduces the safety factor. I think that Timberloks are completely insufficient for what you are doing and you’re asking for a disaster… Even though it has held up so far, I have no confidence that it will remain that way over time and as you add weight (2 separate issues). I would recommend larger fasteners, no doubt. An example might be a treehouse bolt such as a 3″x9″ TAB or 6″x9″ TAB. These have the advantage of being backed up or extended if necessary as the tree grows or as you decide to add the 2nd & 3rd stories to your fort. In this case, I would say yes, change it now. (with the right tools, you can temporarily suspend each corner while you change fasteners, so as not to lower what you’ve already built to the ground).
2) I would recommend using a larger beam on a 12′ span. Something more like a 4×10 / 4×12. If there is any doubt about sizing of beams to ensure safety, then please consult a local engineer or your town’s building department for the best advice. I’ve seen treehouses built with undersized beams and you will likely experience higher than normal deflection (bounciness) and it will sag in the middle over time. The 4×6 probably won’t fall down, but why not make it feel safe and remain flat over time? This is one that I may or may not change now, that’s up to you and your building department.
3) Allowing movement is a good idea, so you’re on the right track. I would recommend using a floating bracket on the fasteners, which allows a low friction interface between the beams and the attachment bolts. This is the system that I would use if I was building it for you.
4) Lastly, I see you have 3 beams up there. Chances are, I bet you only need 2 of them. You may need to adjust which side of each tree the two beams are attached to, but for a normal joist layout, it will be just as safe with 2 beams as with three. That is cheaper to buy hardware for and better for your trees since you’re only making 4 attachments instead of 6.
Best Wishes building a safe treehouse, and please reply here and let us know how it goes.