Any Treehouse Experts?

[MikeF]

I have started building a treehouse for my boys. It is suspended between two large pine trees. The platform is approx 16' x 8'. The trees are about 8ft apart and the house sits on the platform between the trees.

I got the PT lumber from the BORG, and based on their specs, the whole thing should weigh around 2500lbs. (Not sure if they calculate that weight wet or dry).

So here is my concern. I used 5/8 by 8" galvanized Lag screws (5 on each tree) to support the whole thing. 2500lbs is a lot of weight and now I'm second guessing myself if the lag screws are sufficient for that weight. I could support the structure with 6x6s to be sure, but then it would be more like a deck than a treehouse. I would appreciate any thoughts about if the lag screws are enough to support the treehouse.

Thanks

-mike

 

[scsmith42]

A 2,500 lb tree house??? Holy Cow, that's a Tree Castle, not a tree house!

2500 lbs divided across 10 lag bolts = 250 lbs per lag bolt, and some of this weight will be transferred by the friction between the board and the tree.

The bolt safe working load (full bolt shear) on a 5/8" grade 2 bolt is 2,300 lbs. The breaking strength on a grade 2 5/8" bolt is 16,700 lbs

I think that you're probably ok from a dead weight perspective...

However, there is the issue of wind loading on the side of the treehouse and how much additional force that will impart upon the bolts. Considering the location and the fact that it's blocked by forest in the background, you're probably ok here too.

That's a way cool tree house too!

Scott

 

[Mike Davis]

I would be concerned about wind moving those tall trees about and loosening the bolts over time. Just check it every year to be sure the bolts are tight and none are broken or pulled through the 2 bys.

I don't think I have ever seen a tree house bolted to pines like that. Looks cool just hanging there.

 

[DaveO]

I would be most concerned about tree growth. I install deer fencing and we use lag eye-bolts in trees to hang the fence from. I got called out to an installation done a few years back, and the tree had already grown over the eye-bolt. The trees had put on 2-3" extra girth in such a short time period. Pines are fast growing trees. The tree's growth probably won't loosen up the screws but could cause alignment issues with your decking and support system.

Dave

 

[MikeF]

I would be concerned about wind moving those tall trees about and loosening the bolts over time. Just check it every year to be sure the bolts are tight and none are broken or pulled through the 2 bys.

I don't think I have ever seen a tree house bolted to pines like that. Looks cool just hanging there.

The two trees are not actually connected by the platform. On the right side, the platform is bolted to the tree, but on the left side the platform just slides on the supports so the movement of the trees won't pull the structure apart (Hopefully!!)

 

[Mike Davis]

Brilliant! :icon_cheers

 

[MikeF]

The bolt safe working load (full bolt shear) on a 5/8" grade 2 bolt is 2,300 lbs. The breaking strength on a grade 2 5/8" bolt is 16,700 lbs

Scott

Thanks Scott. Can you elaborate a little more the difference between working load and breaking strength?

-mike

 

[scsmith42]

Thanks Scott. Can you elaborate a little more the difference between working load and breaking strength?

-mike

Mike, safe working load is typically a percentage of the ultimate breaking strength of an item. Usually working load is somewhere between 10% - 20% of the amount of force required to break the item.

Engineers calculate loads accordingly so as to have extra capacity available to take care of unforeseen loads that may arise.

 

[McRabbet]

My precaution would be related more to the source and quality of your lag screws. If they are typical "BORG" quality, then I'd be concerned. They import some very soft cast bolts and I've broken 1/2" ones torquing them down. If you can find some old telephone or power pole hardware, they are as strong as strong can be. And you should be able to find through bolts that would allow you to run them straight through the tree. As the trees grow, you should be able to adjust them accordingly. Check with a scrap or salvage yard to buy some by the pound or at a utility company yard to see if they might free some up for you. They have excellent galvanizing as well.