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

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