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I'm planning to build a 2-tree treehouse and will be constructing tri-beam supports as seen in this video (or more accurately each side will be braced to the tree separately due to slight lean, but the same idea applies for the purposes of this question).

I was wondering what the best way is to screw the lags in to secure the brace to the top spreader beam? The video says to do them perpendicular to the top spreader beam but I wanted to check if anyone has different thoughts (such as perpendicular to the braces for example)

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I'm not an engineer, but I prefer to equalize the relative angles of the fasteners with respect to the structural members. Therefore I'd split the difference between perpendicular to the beam and to the lower face of the brace. If the braces are at 45° angles, set the screws at 22½°.

Penetration is probably the more critical factor, though. You want adequate pass-through depth on the brace, to avoid splits and other failure, and you want near full thread depth in the beam, where pullout strain is greatest. By leaning the screws a bit into the beam you make greater depth available, also.

Another consideration is pilot hole size and depth. Make sure you understand how to do that correctly for your screws.

Short answer: Find a nice compromise and don't fret too much. Your fasteners should be adequately sized anyway, such that a little variation or less-than-optimal orientation aren't catastrophic.

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  • If going with an angle like that where the lag screws are not perpendicular to the brace beams, are there any issues when the head meets the wood (or technically the washer in this case) with it being not at a flush angle? Like perhaps reduced strength or something. Sorry if this is a silly question but there will be a good amount of load on these and I'm far from experienced so want to make sure. Thanks
    – g491
    Commented Sep 1, 2022 at 23:31
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    I would usually countersink for the washer anyway, so that sets the angle. If you don't want to do that, definitely go perpendicular to that surface.
    – isherwood
    Commented Sep 2, 2022 at 12:42
  • What do you use to get the angle right? A drill press that angles? A "mitre box" for drills? Not sure what's out there. I wouldn't do this often so preferably there's something cheap. I'll check more online too but wanted to get your thoughts. Thanks
    – g491
    Commented Sep 2, 2022 at 17:32
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    It's a tree house. Use thy eyeball. The wood will compress somewhat and erase minor error. It can help to sketch the lines on the lumber.
    – isherwood
    Commented Sep 2, 2022 at 17:33

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