Treehouse between four trees. Fastening?

[joa_bj]

Time for building a hut. I have four sturdy pine trees (about 50+ cm in diameter) that I plan to use as corners for the hut. It should be mentioned that when the hut is outgrown, it's likely that the material will become firewood.
The idea is to attach a longitudinal beam on the outside of each pair of trees and attach the short side beams to the longitudinal ones.
I plan to use 45x145 beams and attach them to the trunks with stainless steel hex head screws with washers on the outside. I'm also thinking of attaching a longitudinal beam in the middle so the floor (pressure-treated 22x120) gets proper support. It will be about CC80. That beam is fastened with joist hangers in the beams on the short sides.
I plan to replicate the solution for the roof but make it slightly slanted for possible water drainage.

My considerations:
Does it sound reasonable?
Is the dimension of the beams overkill or outright too weak?
Is it unnecessary to use pressure-treated beams? They are not in contact with the ground?
Size of the hex head screws and how many per tree?
Other reflections?

 

[KnockOnWood]

My thoughts: Does it sound reasonable?

Absolutely! I fondly remember the treehouse we built in a pine grove on a friend's property when I was 13 years old. Though it was built in three pines, which stood about 2.5 and 1.5 meters apart. It was just over three meters above the ground.

1. Is the size of the beams overkill or just too flimsy?
2. Is it unnecessary to have treated beams? They're not in contact with the ground?
3. Size of lag screws and how many per tree?

1. Hard to say when you haven't mentioned the size of the treehouse, that is, the length of the floor beams. But I think it seems generous.
2. Yes, I think that's unnecessary, unless the treehouse is meant to last for hundreds of years.
3. When we built the treehouse I mentioned above, we just nailed up the framework. Galvanized nails, maybe 5x150 mm. Say three in each beam end.

 

[topmount]

Isn't part of the point of a treehouse that it should be built from wobbly scrap wood and be somewhat dangerous to climb up into?

I think it will be good as you're planning, if the platform is self-supporting, then not every fastening is so critical, which is nice since the trees move.

 

[joa_bj]

Absolutely!
I fondly remember the fort we built in a pine grove on a friend's property when I was 13 years old.
Though it was built in three pines, which were about 2.5 and 1.5 m apart.
It was just over three meters above the ground.

1. Is the dimension of the beams overkill or simply too weak?
2. Is it unnecessary to use pressure-treated beams? They're not in contact with the ground?
3. Size of lag screws and how many per tree?

1. Hard to know since you haven't mentioned the size of the fort, that is, the length of the floor beams.
   But I think it seems quite sufficient.
2. Yes, I think it's unnecessary, unless the fort is meant to last for hundreds of years.
3. When we built the fort I mentioned above, we just nailed up the framework.
   Galvanized nails, maybe 5x150 mm. Say three in each beam end.

Ah, forgot to mention the size. The longitudinal beams will be 3.6 meters. On the short sides 1.4.

 

[Andreas Lundgren]

For attachment, I have seen that some people drill a thick hole, sometimes all the way through, and insert an (untreated) dowel. Then it grows into the tree. But there are of course special bolts for this purpose. ;-) https://www.google.com/search?q=tree+house+attachment+bolts

For dimensioning, you can check here, then halve since it is a treehouse. https://www.traguiden.se/konstrukti...uktionsvirke-i-ett-fack/?previousState=001000

 

[myrstack]

Did something similar 3-4 years ago and it has held up well. However, I completely failed by using random demolition wood and created something totally dangerous for the kids...

In my case, the fort is between 3 pines, the framework material is 45x145 fastened with presumably 6x140 or similar into the pines (which seem to be fine). My biggest concern was essentially that the trees' natural movement would tear apart the frame, but the only cracks I see are those caused by the wood's normal movement/drying.

The only thing I did with the pines beforehand was to smooth out the rough bark a bit and then screwed in. No leakage of resin or the like and each pine has 4 screws in it. With a span of around 3 meters between the pines, I would have used thicker decking than 28mm, otherwise, I would have skipped the rule running along the middle.