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.