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Question:

How much weight can a 3/4" x 8" lag bolt hold when drilled in half way? What about other lengths and thickness, and percent drilled in? Is there some sort of formula to calculate?

Optional Background:

I'm planning on building a modest treehouse, on a tree whose trunk is oblong and whose long sides are probably not exactly parallel to each other. Because of this, I had the idea where instead of using lag bolts to screw in the joists on those oblong sides, I want to screw the lag bolts independently into the tree, and have the joists sit on top of them. This way I could have a perfectly square floor even if the sides of the tree are not parallel - it gives me some flexibility. It's a similar idea to a TAB (Treehouse Attachment Bolt), but a much cheaper version, as TABS can cost from $200 - $400 each! I want to make sure the bolts can hold enough weight when screwed in partially. Here's a screenshot of a similar plan where they're using TABs (highlighted in the screenshot - I will be using lag bolts instead of TABs - and I'll probably have 2 on each side):

enter image description here

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  • I don't quite see the issue - you can attach parallel boards to any reasonably round tree, oblong or not, at any orientation you choose. Every round shape has an infinite number of pairs of points where the tangents are parallel.
    – Nuclear Hoagie
    May 6, 2020 at 16:43
  • see things like: calcresource.com/statics-simple-beam.html
    – Solar Mike
    May 6, 2020 at 16:49
  • @NuclearWang - I don't understand how you could - here's a crude slightly exaggerated drawing of what the shape of the tree may be: snipboard.io/2dEsnT.jpg -- given that, how could you attach a board to side A and side B and the boards be parallel?
    – jbyrd
    May 6, 2020 at 16:55
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    @jbyrd Attach the board at the bottom-most point of the image where the curve is parallel to side A. You'll only have one attachment point for side B, but that's the only place you could put a lag bolt in your proposed plan anyway. Either way, you'll wind up with two parallel board that are both touching the tree - just drill through them where they touch the tree, no?
    – Nuclear Hoagie
    May 6, 2020 at 17:16
  • "i will be doing something similar, but using cheaper lag bolts instead" and you or your kids are going to be in this tree house ??
    – Alaska Man
    May 6, 2020 at 17:51

3 Answers 3

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How much weight can a 3/4" x 8" lag bolt hold when drilled in half way?

It's not just about weight but where and how that weight is applied. In your case, it would seem that the how is purely vertical, that is, no tension and applied to the perpendicular grain of the tree. The where is still open. Where on the bolt is the load applied? The further it is away the less it can carry, and the design problem changes from purely shear to a moment (tension / compression) situation.

What about other lengths and thickness, and percent drilled in?

There is far too much math and variables to answer this question yet. You need to lower the variables first, aka settle on a design and work out some of the details first

Is there some sort of formula to calculate?

Yes, many of them but these are outside the scope of this website

The description and everything else aside, this all comes down to loads and resistance based on things like material properties and the specifics of your design. Since you yourself are not sure about the details then it's not possible to answer this question yet

In general, lag screws (they are -not- bolts) can be used in place of the tree house bolts as there is little fundamental difference between them and the tree house bolts other than dimensions and maybe steel properties (but that probably may not control design).

But to specifically answer your question, you'd have to hire an engineer, sorry

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I would counter that this is very much not what lag bolts are for. As comments suggested, drill through a full board tangent to the tree.

There might be a reason :-) that TreeHouse bolts are that expensive - they're designed to carry significant weight.

Otherwise, I would consider using triangles of 4x4 and run two long lagbolts (each) thru them into the tree.

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    There is no reason that lag bolts can't be used this way. The THBs are bolts modified to better address the needs but both them and the lag screws (they are -not- bolts) are based on exactly the same principles, factors and material properties
    – Ack
    May 6, 2020 at 18:45
  • @Carl Witthoft - Are you saying to still use the lag bolts for attaching the boards, but just put them through the boards instead of having the boards sit on top of them? If so, I'd think the difference in downward load bearing strength would be negligible, no? You're still having the same amount of weight bearing down on the bolts.
    – jbyrd
    May 6, 2020 at 18:46
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Why not use carriage bolts ? They would be much easier to thread nuts onto and there would be no concern for stripping threads out of the wood. They would be strength of the bolts. Or threaded rod with nuts if you need some odd lengths. Even straight through the tree.

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  • The only way I could use those is if I did in fact go all the way through the tree - that would be a pretty long carriage bolt (the tree's at least a foot thick I'd say), and I wonder if that would harm the tree to go all the way through?
    – jbyrd
    May 6, 2020 at 20:35
  • Threaded bar would be easier to get thane carriage bolts. I put thread rod through at least 12 in. of an Elm tree to strengthen a crotch. I was going to remove it in a couple years but bark grew over the nuts. I have lost a few hooks in trees, they keep growing when you are not looking.
    – blacksmith37
    May 6, 2020 at 21:01

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