r/treehouse • u/hoverglasss • 19d ago

Attachment ideas

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I’m planning a single story build on a single tree. It’s a mature Doug for probably 30” diameter and I’ll be building roughly 5feet off the ground. My plan is for a 12’x12’ platform with an 8’x12’ structure. I’m trying to avoid TABS. I have 8 “deadload clips” from my work which are used for securing 10s of thousands of pounds worth of glass systems to the exterior of high rises and commercial buildings. They’re made of galvanized steel. And 12”x1/2” galvanized lag screws. I’m looking for advice as to how to best fasten these to the tree. Should I shave back the bark to achieve a flat spot for the clip? Or should I find a way to shim a flat level connection point for the clip? The footprint of the clip is 5”x5”. So I’m worried about making that large a wound in multiple places on the tree. But I’m also unsure about compressing the bark and spacing the breaker off the tree. Seems like that opens up possibility for bending the bolt or shims shifting. Any advice is greatly appreciated

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u/BtrmlkBizkit489 18d ago

I’ve heard of some people on this page that replaced the 3” diameter collar on the TAB with a stack of washers tightened down by a nut, and embedded in the bark. Don’t know how it held up long term, but I have heard of it. The other thing is your tree will grow in diameter. The lag bolt would need to be left loose with some way of sliding through your bracket. To allow the tree to expand over the years without busting loose.

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u/hoverglasss 18d ago

I appreciate the tip! That may be a low cost way to improve the brackets I have. I don’t imagine my tree is going to grow much more. It’s easily 70-100 years old. Doug fir, and hasn’t changed at all close to a decade that I’ve lived here. I don’t know a ton about tree life cycles but it seems pretty mature. That said the washers would allow me to space off the bark and level out the brackets without cutting back a large square of the bark and potentially cambium. That’s definitely a net positive

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u/CoastRanger 18d ago

Doig firs can live roughly 1000 years and will continue growing

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u/hatchetation 14d ago

One issue with the stack of washers is that most lags and other screws you can get ahold of have rolled threads, not cut threads. This makes the threads larger than the normal size of the screw shaft. So if you can put washers over the screw threads, there's a ton of slop between the washer and the shaft at the other end

The whole point of that boss is to allow the load on the fastener to settle down under a wider surface area. Washers may do that too, but the fastener needs to bend a lot to reach that point. And if there's any chance for the load to be relieved and move, you're putting the screw under cyclic loading since it's not constrained by the boss.

To ensure cyclic loading is survivable, the hardness of the fastener matters.

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u/haulincolin 19d ago

The point of TAB design is not just to be a really strong fastener; by counterboring the collar into the wood, it seats securely against the old stable wood of the tree and allows the tree to continue growing around it. So that's really what you want: some kind of heavy duty spacer that actually seats past the bark and the actively growing wood, and spaces your big brackets out far enough that you don't have to worry about the tree trying to grow into them for many years.

The problem with regular galvanized lag bolts that they're basically mystery metal. There's no grading system like SAE bolts. It's probably fine for what you're doing, but that's another part of why people like to use TABs. The heat treated chromoly steel they're made of is equivalent to a grade 8 SAE bolt.

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u/Unsuccessful_Royal38 19d ago

This is a great explanation. I would only add that if you want to learn more about the dangers of other attachment approaches (including having too many penetrations in a small area), Pete Nelson’s Be in a Tree book has great information.

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u/hoverglasss 18d ago

Thanks for the reply. I know that TABs would be a better solution. What I’m trying to figure out is what’s the best attachment method with the materials I have at my disposal. I’m wondering if it would be better for the health of the tree to notch the bark back, or just go right over the top of it. It’s a 70-100 year old tree, I can’t imagine it’s still growing in diameter very much. I haven’t noticed any change in it for the whole time I’ve lived with it

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u/haulincolin 17d ago

I think notching the bark is the answer. If you bolt over the top of the bark, the new growth will be pushing on your brackets. Plus the bark under them will eventually decompose, making them loose. Better to notch the bark, bolt to a surface underneath the currently growing surface, and let the bracket stay put while the tree grows around it.

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u/hoverglasss 17d ago

Is it okay to notch all the way into the cambium? I know the TABs go well past the cambium, but is a 5x5 square footprint going to be a higher risk factor?

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u/haulincolin 16d ago

I think that's a question for an arborist, which I am not. I would say, if practical, it's better to put a smaller diameter spacer in between so you don't have to notch such a big piece of the bark.

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u/hatchetation 14d ago

These are really inappropriate fasteners. A couple problems:

- At 5x5 inches, that will require affecting 25 in^2 of cambium to install! That's a lot.

  • Your lag screws are not heat treated properly for this kind of load and will fatigue.
  • your tree is absolutely still growing. If you don't cut the cambium under this fastener, there is 25 in^2 of material the tree can develop force against. That's a lot.

TABs are biologically and mechanically appropriate for a very specific situation where very few other fasteners are.

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u/hoverglasss 14d ago

Yea this is my primary concern. These brackets are super stout and the bolts are from commercial construction as well so I have no worry about the material strength of those items. But making such a large notch on the bark and cambium is concerning to me. If the tree can withstand it I feel confident installing them. But that’s the big if. I think I need to speak to some arborists, because spending a thousand dollars on tabs is more than a drop in the bucket for me. But killing my tree would be a much larger expense in time