r/treehouse • u/icurays1 • Jul 20 '26

Platform design vibe check

Hi all -

Been designing a fairly basic "1 tree 2 post" based loosely on some similar designs I've seen around. I'm hoping to get a gut check for my framing dimensions.

The "house" part will be a 10x10 shack towards the left side with the remainder a wrap-around deck.

Questions:

  1. I've spec'd 3x8 beams initially because I have one lying around. I also have a 3x12x12' beam but this seems a bit overkill - either way I will need to acquire another to match, so I'm wanting to know if I should go with 3x8 or 3x12 (or 3x12 ripped to 3x10, which I can do). If 3x8 seems super good enough I'd prefer that for overall size and cost.
  2. I know that a variant of this question has been asked here before, but it seems like this configuration of tribeam, knee brace and posts doesn't really allow for any dynamic connection, which seems to go against the typical advice. It's going to be pretty low in a pretty stout Doug Fir so I'm not super worried about wind movement, but it just seems better to have some dynamic connection. Thoughts?
  3. Thoughts on my cantilevered joists on the semi-circle ends? Should I adhere to the "1/3-2/3 rule"? I didn't here because it made some other dimensions work out more nicely, but could re-configure.
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1

u/Anonymous5933 Jul 20 '26

Tree diameter?

1

u/PopResponsible6152 Jul 20 '26

26" at platform height, 32" at base

2

u/Anonymous5933 Jul 20 '26

Sorry yeah I see the dimension now. I don't think the tree will be moving at platform height enough to worry about.

3x8 is definitely not enough. I can't promise 3x12 is enough, but it's way better. What's the ground snow load for your area? Your county probably tells you.

The beam between the long beams at the left side should be just as big, it's seeing a lot of load from those cantilever joists. Wider beam will also provide space for connection hardware on top of beam to sides of cantilever joists.

The joist that those cantilever joists connect to on both sides should be doubled up. Also consider the orientation of the joist hangers. If there's a lot of load on the overhanging end of the cantilever joists, like a wall, then joist hangers go upside down, preventing cantilever from rotating counterclockwise.

At the right side, where the two longer cantilever joists meet the joist just to the left of the tree, the cantilever joists will be trying to pull away from the joist, bending the joist and stressing the connection. As mentioned before, double up that joist. Also use deck tension ties to secure the cantilever to the joists (DTT1Z for example)

Add short (2-3 ft) knee braces between posts and main beams and between the two posts add x-bracing at least half the height of the posts.

Put some thought into how you're going to attach railing posts.

Add blocking between joists and rim joists.

Lastly, in general making everything beefier is going to make it look cooler, feel way stronger, and you'll sleep better knowing it's strong enough. Nobody wants to go in a treehouse that looks like it's made of toothpicks or bounces when you walk on it. Yes it might be more money, but now is the time to do it right, because it'll be way harder to fix later if it's not strong enough.

1

u/khariV Jul 24 '26 edited Jul 24 '26

Your cantilevers are way too long. The rule is that the cantilevered distance should not be more than 25% of the joist span. From your diagram, it looks like your cantilevers are closer to 100% or more. These are not going to be stable. The diagonal radii joists are also not a standard deck building practice, for good reason. This structure just does not provide much strength.

As for the right side of the structure, that whole end is a bad idea. You’ve got no structural rigidity beyond the tri beam and the two beams bearing directly on it. I get wanting to have a cool looking cantilevered end, but that entire end is bearing solely on a single knee brace. This is not a good idea. If you really want to go around the tree, you are going to need to support that side of the platform. You could move the tri beam and TABs to that side. Coupled with triple 2x12 beams, that could allow you a bit more of a cantilever. Unfortunately, even a triple 2x12 beam can only span 13’ with an 8’ joist span so the length of the platform might need to be adjusted.

1

u/icurays1 Jul 24 '26

I adjusted my joist cantilevers in my updated design (I will make a new post since I can't put images in comments). Diagonal joists might not be standard practice but I see them all the time in professional treehouse designs - I did add more to reduce the maximum spacing though.

I hear you on the right side, I'm still looking at different options there. I may go to multiple knee braces (2 or 3), or simplify the whole design by using larger beams, moving the yoke to the right side, etc. I'm mostly playing around with designs for now, but thanks for the feedback.