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I'm working on my first treehouse as an adult 😂 I've built plenty out of scraps as a kid, but nothing of this magnitude. I'm mostly concerned about the structural integrity of the platform with this post. I'll layout my general plan, post some pictures, and ask some pointed questions at the end.

I'm planning an 8x12ft platform where 8x8ft is enclosed with a roof and 4x8ft is a deck with railings. Right now this is built around 2 trees that have a diameter of ~12in and sit ~5.5ft apart. I'm using two long 1/2in stainless steel tubes/bolts that go completely through the trees to hold the main beams. The beams essentially rest on the bolts and sandwich the two trees. On top of those beams the floor joists rest perpendicular. All wood is pressure treated.

Pictures of where I'm at (click to enlarge):

tube_bolt platform

Questions:

  • As you might be able to see, everything is currently built using 2x6in boards. I think the floor joists will be fine; I'm not dry walling or installing a fridge 😆 That said, I'm second guessing the beams. Should I double up the 2x6in beams or swap them for 2x8s or even 2x10s? I'm leaning towards doubling up the 2x6s since they're sitting on top of the bolts and I have the full 6in of vertical support.
  • Obviously I need to add some knee braces. My initial plan was to run 2 up on the outside of each tree parallel to the joists for a total of 4. However, I'm thinking maybe I should shift those 4 to the inside of the trees and then run 4 more out to each corner for a total of 8. Thoughts?
  • Last question, am I missing anything else obvious? 😁
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    What's the spacing between each 2x6 joist? I foresee trouble from those trees increasing in diameter, it could cause problems with your main beans. Also, next time do your structural calculations before you start building, not midway through. – whatsisname Aug 7 at 5:12
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    The joists are 16in apart. I've been thinking about growth with the beams too...I'll probably back them away from the tree a couple inches when I make the recommended changes to them. I regret having to pause and reassess midway, but you live and you learn, better safe than sorry, etc. etc. right? – djdmbrwsk Aug 7 at 11:57
  • What are the bolt's size? Did you drill completely thru the trees to place the bolts? Those seem questionable to me. I would consider adding pressure treated 6x6 posts for support on both inside and outside of the trees. Did you use pressure treated lumber? It is difficult to tell from the photos. Lastly, how much weight do you anticipate the platform supporting? We have to know this to answer the question about increasing size/doubling up... and yes, to four corner posts. – peinal Aug 7 at 12:36
  • As was mentioned, your concern isn't the deck structure (which is fine), but the stability of the thing as the trees sway in the wind and grow. Don't underestimate the forces involved or their ability to tear things apart. Generally you don't fasten so tightly to the trunk, which means that your cantilever is a bit of a problem. – isherwood Aug 7 at 14:57
  • I personally don't see the doomsday stuff others see. If the trees start pulling apart or together you may need to retie in the future. I don't see a need to do it "perfect" as perfect may have the same issues. On tree growing wider... wow. That's a slow process which is remedied in 20 mins. You cut out 2x6 in the middle by tree and sister it. This is not rocket science. – DMoore Aug 7 at 16:59
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Just notes:

  • 2x6s... plenty big. Weight is a factor unless you will reinforce more. You do not want it too heavy and these trees aren't that big.

  • see no issue with having boards a couple inches from trees. As commented you can always re-evaluate and cut/sister these in 10 years. Having a tree house means your structure needs to move with the tree(s).

  • you do need cross bracing

  • I don't see a a good way to get IN the treehouse. Is the plan really to climb 15 feet then turn your shoulders so you can squeeze through 14"?

  • really the only thing I don't like is the resting on the bolts. I am not against this method but I would like to see an "eye" or a "C/D" anchor that holds these into place (installed underneath). It doesn't have to lock but I think your current design flaw is this thing rolling to one side.

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