What does “don’t pin a tree house beam” mean?

Question from George, Glen Ridge, NJ:

“In your list of things not to do, you say not to “pin” a beam. What exactly does that mean?”

Well, “pinning” would mean any way of fastening a tree house beam directly against the bark of the trunk, such that the beam will not move independently from the trunk. This is typically done by buying the biggest lag bolts at the local hardware store (usually 1/2″), predrilling, and lagging right through the beam and into the tree. Some tree house builders in Japan used to pin two parallel beams to a tree by clamping them together with all-thread, washers, and nuts. They squeezed the tree so hard that it would actually hold some weight before slipping down. I still consider those beams pinned because they can’t move, and the tree must decide whether to push them away or grow around them – neither of which is good for the tree or the tree house.

Take a look at what is happening here to a tree house built by a “do it yourselfer” in Pennsylvania.

Unsafe method of attaching a tree house beam

This tree house was 3 years old at the time of the photo and one of the main beams has been pushed out over the head of the lag bolt about 7/8″ or more than half way off the 2×12. When will it fall down? Probably in another 3 years or so. Also note that these two yellow poplar trees are not looking very happy either, and it’s only been 3 years.

The better method is to “perch“, or set the beam on top of a much larger fastener some inches away from the trunk so that the tree can grow without constriction or rubbing due to wind movement. Thank you for the question… –Dan Wright

2 Responses to “What does “don’t pin a tree house beam” mean?”

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  1. Jeff - Columbia, MO says:

    Hi Dan,

    I have a question related to the pinning issue mentioned by Glen.

    After reading a different book on treehouse building (not yours), I used some plans included in the book to start a treehouse on a red oak (54.5″ circumference). I put a lower support beam (2x8x8) into the tree using a 3/4″ x9″ lag bolt. I planned poorly and didn’t drill the pilot hole deep enough, causing one of the lag bolts to break when putting it in. I hastily moved forward and put another lag bolt right above the broken one (spaced out only a couple of inches). Now that I have the support beam lag bolt inserted so close to the original hole below it, I am worried that the integrity of this portion of the tree may be compromised due to a damaged cambium.

    1) Do you have any recommendation on the safety of the new lag bolt attachment right above the broken lag bolt? Could it still be viable?

    2) If the pinned lag bolt is compromised, do you think it’d be okay to adjust the rotation/height of the treehouse and start over using the 8′ treehouse kit from your website? Or, am I better off switching to a TAB system for connection?

    Many thanks for any insight you have. Thank you!



    • Jeff, thanks for reaching out. Treehouse building is fun and exciting, but there are some unique considerations when you begin attaching your support structures to trees. I don’t recommend putting lag screws through beams for several reasons but, most importantly, because using the lag screw in this configuration provides no clearance between the beam and the tree, which increases the amount and area of pressure on the cambium and sapwood, which will ultimately degrade the health of the tree, all the more so as the tree grows; it will begin growing around the beam which will increase the contact area and result in further damage to the outer layers of the tree. Additionally, any long-term damage (rot) to the tree that occurs at the site of your two lag screws will be impossible to see, as it will be covered by the board.

      Not to worry, there are several good options to continue your build while minimizing damage to the tree. For all but the smallest treehouses and platforms, I would always recommend using TABs to secure your support beams. The 3” collars distribute the load over a much wider area, engaging more of the natural strength of the tree. The 6, 9 or 12-inch shafts provide ample clearance between your beam and the tree, allowing for years of unrestricted growth. Finally, the TAB and pipe bracket combination secure to your beam from the bottom, ensuring that the full depth of the beam is used to support the rest of your tree structure. For smaller builds, the 1 1/4″ lag screws and pipe brackets that come with our 8×8 treehouse hardware kits provide sufficient support (also from the bottom of the beam, just like a TAB), while still allowing several inches of clearance between the tree and the beam. If you’re able to raise your deck height a few inches, I’d recommend extracting the unbroken 3/4″ lag screw and using the hole as the pilot for the TAB or 1 1/4″ lag and cutting the sheared lag screw off as close to the bark as possible, to avoid further damage to the tree.


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