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I want to install "floating" steps around a tree in a "spiral staircase" design up to a tree house. I will be milling the treads from a 10" diameter log. Each tread will be 24" long and 7"- 10" wide in a tapered design to better conform around the tree. The log will be cut in half so each tread will be a half circle from an end profile.

I do not plan on using a mortise and tenon joint as I do not want to cut out large squares of the tree. So, similar to using tree house anchor bolts, can I use either 1/2" x 24-36" Steel bars or re-bar. If I use 2 per step, set them 12" into the tree and the other end into the step along with an anchor underneath to hold the tread to the tree and stabilize it, will that provide enough support to hold 500 lbs.

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  • What kind of tree is it? How old is the tree and what is the minimum circuference of the trunk? If this kills the tree over time, are you okay with that ?
    – Criggie
    Feb 15, 2023 at 21:28
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    If you do go with anchoring that many things to the tree, treat it like surgery. Sterilize your spikes and clean the bark as well as you can. Otherwise you could introduce fungus and such into the interior of the tree that will make it rot out around your spike.
    – Perkins
    Feb 15, 2023 at 23:07
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    Besides thinking that the only authentic "stairs" to a tree house is a rope ladder, did you consider to build the circular stairs around (instead of into) the tree?
    – U. Windl
    Feb 15, 2023 at 23:15

3 Answers 3

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No. Not even close. They'll fold right over. Even with a smaller kid they'll be a diving board. Also, that many steel rods will probably kill the tree.

The reason tree house anchor bolts work is because the force is primarily applied in shear, parallel with and tight to the tree trunk. You're talking about bending forces here, and with a long lever attached. That's a whole 'nother ballgame.

You need heavy steel angle or lighter steel with diagonal bracing, or you need to devise support for the outer ends.

I'd forego the spiral staircase, though. My kids' suspended, circular treehouse was accessed via a ship's ladder notched into the platform. They were quite happy with that arrangement.

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You'll need much thicker bar than that to support human weight cantilevered out to 600mm. And you'll probably need much deeper anchors.

To see this done Right, look at how the Gloucester Tree's ladder is constructed. Or the similar Diamond Tree:

Diamond Tree rungs
(photo: SeanMack/Wikipedia)

An important feature to notice is the distal ends are wired together, so they are all effectively supported from the top and bottom. That's much stronger than each spike being an independent cantilever.

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    Note that wiring together the ends works on the trees because their "staircase" is only a little off the vertical. The more staircase like you make it, the less effective that will be.
    – Perkins
    Feb 15, 2023 at 22:53
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    If the wire is taut, it will work quite far off vertical - it's a similar principle to wire-rope safety fence, in which the posts are merely supports for the tensioned wire. Feb 16, 2023 at 14:17
  • The bottom end of the wire does need to be securely anchored, of course. Feb 16, 2023 at 14:18
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    Anchoring the bottom will help hold back against the horizontal vector as you flatten out the angle, but you still end up giving the weight on the step more and more leverage over the cable, especially once you pass 45 degrees.
    – Perkins
    Mar 20, 2023 at 19:15
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There are devices made for climbing trees in things like hunting blinds and whatnot. They are called tree steps (Google image it). They go into the tree at a right angle to the tree, then drop and go out again.Some of the tree steps claim 700#+ max.

like

<\\\\===||
        ||
        ||=============

So the force of weight is applied downward, the dropping portion braces that force against the threaded portion screwed in.

I would not trust anything screwed into the tree long term as the wood surrounding the threads would surely die and decay, but on a large healthy tree, through and through, put a nut and washer on other side, and the tree should survive, eventually growing around and absorbing the head it would create, and the steps would remain mechanically sound. (Use non rusting materials!) Note: Only the cambium is alive in the sense you would expect, any other damage is just a wound where parasites, fungus, etc can intrude and decay the wood. Properly sealed the actual damage to the tree is very minimal. Think of old timber they cut into and find fence wire, nails, etc. once sealed in it is entombed, there are products to seal tree wounds just like when you cut a limb off. In fact some people have been known to "spike" trees, with very large nails in old growth timber, to wreak havoc on the would be harvesters of those trees, the trees themselves are damaged little to none.

You could fashion actual steps on top of them. The real determining factor here would be the strength of the steel at the bottom 90, and the outward length.

IF you needed more support in theory you could do an angular brace at the lower end as well if you are building them yourself or having them built.

like

 <\\\\===||
         ||
         ||
         ||
         ||\\
         || \\
         ||  \\
         ||==================

But to be honest, I would not trust my life to it unless an engineer looked it over, and double checked my angles and material composition. :-)

Since this topic has my mind curious now, and someone was kind enough below to offer good suggestions to my engineering, it also occurred to me... If one were to fashion it out of angle iron rather than straight rods, put the wide end against the tree and turn the other upward toward the step, that would give twice the contact surface with the tree and step, and thus help with rotation. A good welder could cut and join that without the obvious brace as well, as the two sides of the angle iron would technically form that same mechanical structure twice over. The result would probably make that strong enough to lift an engine, much less hold a person.

Lets see if we can visualize that in ASCII! Ummm, not so great...

    |\
    | \
<\\\|==|=|
    |  |
    |  |
    |\-|----------/
    |_\__________/

But you can get the idea. The angle iron would be stronger than a round support, and the way they would fit over one another when welded would form two of the angular braces while giving a straight 90 degree profile. Kind of making me want to fabricate one of these and test it.

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    +1, but that angular brace you suggest at the end looks like a massive tripping hazard. Better extend the vertical segment a bit below the tread and put the brace down there.
    – TooTea
    Feb 16, 2023 at 9:43
  • Agreed possible, depending on how far out it came, the tree steps do not even have one, but the extension is relatively short. To properly brace that above or below would only take an inch or so and likely be concealed inside the step. But your suggestion is very valid from an engineering perspective as the compression forces would strengthen them differently and arguably better than the tension force.
    – Sabre
    Feb 16, 2023 at 14:24
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    Well actually tension is better as long as we're talking about steel. A brace in tension can hold a whole lot since it won't ever buckle, so you have to really rip it in half or make one of the attachments fail. In contrast to that, in compression buckling is always the #1 concern. But yeah, this is going way off topic for the purposes of OP's question.
    – TooTea
    Feb 16, 2023 at 15:40

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