Need Tree Platform Build Advice

Question

First time here and I am hoping someone can help me with my issue.
I have been planning platforms for the past couple weeks for the kids and am starting with the most basic one.

Rough sketch is attached below..not exactly to scale.

I know the basics and have some prior training building. I am NOT a math head and that is my weakness.

Dimensions are 8' x 16'. See diagrams for more specifics. Roughly about 6' off the ground. Two trees with 1' diameter.

I have two different designs for this making a platform, the main concern is the 8' span between trees. I need it to support an active weight of say 500lb around the middle. 2 Adult + 2 kids

I know traditional route is 2 beams parallel to the trees, however I am concerned the one tree is leaning a bit. I might not be able to make it a straight line, it may be over 5 degrees, perhaps 10.

In the first design I have:

2 beams 2x8x16 - Joists are 2x6x8

Hence build number 2 going with 4 horizontal beams and following up with 16' joists. The main question dogging me, will plan number 2 work.

4 beams are 2x8x8 - Joists are 2x6x16

Specifications:

Douglas Fir #2 Kiln Dried for all structure.
Beams 2x8's
Joists 2x6's
Plan on spacing joists 16" apart.
Traditional V support on each tree.
3/4 8" Hot Dipped Galv Lag Screws
3/4 10" HDG Lags for the V's
galvanized joist hangers
galvanized rafter ties

Was planning on using 1 x 6 x 8 boards for the deck across.
Not sure what wood to use, this is the most expensive part.
Options I have been thinking about.

T1-11 Plywood or OSB floorboard.
Whitewood Common / 1" doug fir boards, treated with wood sealer / preservatives.
Redwood / cypress is out of my price range, wife is allergic to cedar.
I am open to ANY and all suggestions.

Trying to keep this under 300-400.
I do not have the budget for expensive things like TABS,
I am trying to preserve the tree as best I can with single lags.

Thank you for your time.

Answer 1

At the time of this post you haven't asked a clear question, so I'll offer some general advice. Your design is reasonable except for a few key things:

1. Rigidly attaching such a structure to two separate trees is a sure to result in its rapid destruction. Movement of the trees with respect to one another transfers huge amounts of force to the framing. No lumber or fastener will last long.

2. Joist hangers serve no purpose in your design. As the common joists rest on the beams, and the rim joists carry little weight, there's no point in using them. Screw through the rim joists into the common joists with coated 3" screws. Toenail through the commons into the beams with same.

3. Fastening directly to the tree fails to account for tree growth. Rather, suspend the deck by cables anchored up a ways (from large branches near the trunk, or from brackets bolted to the trunk) in an A configuration. Rather than expensive purpose-built hardware, I'd use some simple steel plates drilled for several lags and a cable eye.
   
   Keep the primary framing several inches from the trunks, and leave a gap between the decking and the trunk to allow for several years' growth. You can always run a jigsaw around the tree if you need to later. If any of this is unclear, please ask.

4. OSB isn't an exterior product. Period, end of story. Unless you intend to fully roof and side this thing, don't use it anywhere.

5. A solid floor is a bad idea anyway. Think of your floor as a deck, and use decking with drainage gaps. Even if you install the deck boards tight together, the floor will drain and you'll have much less mold and rot.
   
   To save cost, watch Craigslist and similar for used pressure-treated decking. Ask around for anyone doing a deck rebuild. Often decking that's being discarded is perfectly serviceable once flipped over. I build a round treehouse very much like your plan with some beautiful 5/4 redwood that was headed for the landfill.

That's all I got for now. I'll update if more comes to mind.