I am wanting to span between 2 trees with 12 to 15 foot beams. I will have one beam on one side of the trees and another beam on the other side. The beams will be supported 1/3 in from one end and 1/3 in from the other. They will rest on metal plates on that will be about 4" long screwed into their undersides. The plates are mounted on tree bolts. So those will be the support points. I believe I have found that this wood will support 1200lbs each, but that is one long span. This is more like 3 spans. The beams will be about 20" apart (the width of each tree). I want to then build an 8x10 or 10x10 standard floor joist system and build a treehouse on top of that.

Including people, snow load, and the weight of all the wood and materials, it will be under 2500lbs. Will my 2 beams be enough? Could I use 2x8s? Do I need to double them up?

The drawing below is close to what I am wanting to do. My trees are straight and the overhang on the left and right is more. There will be around 1/3 of the beam extending beyond each mounting point and the floor will extend out there too.

tree house beam structure

  • I'm unclear, are you asking about the two beams between the trees, or the joists that will be supported on them?
    – Ack
    Apr 2 '20 at 0:25
  • 2
    Aren't those connectors hurricane clips? Not meant for holding up a load, just to better connect trusses with top plates on walls. You should consider using joist hangers. Apr 2 '20 at 0:48
  • 1
    What about the side loading? Put 100 lbs 4’ to the side and the load would be off center, without triangle bracing I would say no.
    – Ed Beal
    Apr 2 '20 at 2:05
  • The picture was copied from another website. I would use a different method to hold the platform to the beams. One idea was to make the beams and platform all the same. The 2 beams form the center 2 joists and I build a frame around them. Does that also address the side loading to a degree? I will probably have cables from above or posts below at the corners to provide cantilever support.
    – Fred
    Apr 2 '20 at 19:19

There’s several issues with your plan: 1) support 2500#, 2) connect platform to beams, 3) differential loading on cantilever beams.

1) Yes, 2 - 2x8’s spanning 4’ or 5’ can easily support 2500# depending on the species and grade of lumber. However, a 4’ or 5’ cantilever is excessive for either a 2x8 or 2x10 and will eventually fail depending on how much of the 2500# load is on the cantilever beams.

2) The platform could exert a force of over 600 lbs. due to rotation on each H-1 connector by having the live load and dead load on the cantilever. The Simpson H-1 clip is only rated for a resistance of 400# to 500#, depending on the species and grade of the joists and beams. H-1 clips are not suitable for this installation.

See: https://www.strongtie.com/resources/product-installers-guide/h1-installation

3) Structures like this will not have uniform loads distributed throughout the structure evenly. A more detailed design and structural analysis should be done by a structural engineer (not civil engineer) or architect if you decide to stay with the extremely large cantilever design.

Btw, there are many videos about how to connect beams to a growing tree for a tree fort.

  • Thank you Lee. As mentioned above, does making the platform part of the 2 beams as one large floor with joist hangers solve part of the problem? If so, he remaining problem is the lever action of all the weight on one side of a beam or the other. Then the light side would want to lift up. The special garnier limb mount will hold the weight, but when lifting up, the lag bolts holding the brackets on the bottom of the beams might not. Oddly enough, this drawing came from a tree building site! I'll look up a CE here.
    – Fred
    Apr 2 '20 at 19:36
  • No, no...you want a SE or architect, not a CE. A CE can size a sewer pipe or layout a perfect parking lot, but cannot help you with this problem: a) differential load on middle third of beams with significant cantilever and affect of rotation on cantilever, and b) buckling of beam due to significant rotation load upon middle of beam...especially significant in 2x10.
    – Lee Sam
    Apr 2 '20 at 21:18
  • Got it, I meant SE. Thank you :)
    – Fred
    Apr 12 '20 at 1:26

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