Is My Treehouse Plan Safe?

Question from Greg, Bellevue, WA:

Building a treehouse for the kids; but now that my wife has researched a little is freaked out that what I’m doing isn’t safe so I’ll ask for advice on the basic support structure.

Three cedars approx. 50-70 ft tall, 14″-18″ dia. at 12′ off the ground where the main beams are in a triangle roughly 12’x8’x7’. The beams are 4×6 and are attached w/8″ FastenMaster Timberlok screws directly against the trunk (3 staggered at each point approx 2″ apart, and in hindsight from what I’ve read I realize this is not ideal; question ‘do I change it now?’), these screws have a rated sheer factor of 3K lbs and pullout factor of 1,500 lbs ea. with only 2″ of thread embedded. I used these because they are about 3/16″ in dia. rather than drill 1 large hole (and I had them in the shop). The beams were placed on the outside of the trees w/the floor joists attached to 3/4″ ply overlapping the beams, but not attached to them to allow movement effectively making a floating lid over the beams. The main question is, at 12′ off the ground on trees this size are those Timberloks substantial enough (I don’t know the sheer factor of a single 3/4″ or 1” bolt and also in hindsight 1 penetration is better than 3; again ‘do I change it now?’) or do I need to rethink this before going further. We’ve had some good wind and it seems to move just fine, I’ve been in the construction trade for 20 yrs, but I’ve never put 1,500 lbs of structure/people/snow/whatever in a tree before and I was making it up as I went….

I attached a sketch to help clarify

Plan sent in by Greg to ask if treehouse attachments are safe

Hi Greg,

I’ll tell you what I think would be safe for your treehouse, and then you can decide whether to act on it or not (and what you’re going to tell your wife in reality).

1) The shear rating on a fastener is meaningless unless the load is applied in shear. Due to the softness of a tree’s outer tissues, there is probably some leverage and bending going on which reduces the safety factor. I think that Timberloks are completely insufficient for what you are doing and you’re asking for a disaster… Even though it has held up so far, I have no confidence that it will remain that way over time and as you add weight (2 separate issues). I would recommend larger fasteners, no doubt. An example might be a treehouse bolt such as a 3″x9″ TAB or 6″x9″ TAB. These have the advantage of being backed up or extended if necessary as the tree grows or as you decide to add the 2nd & 3rd stories to your fort. In this case, I would say yes, change it now. (with the right tools, you can temporarily suspend each corner while you change fasteners, so as not to lower what you’ve already built to the ground).

2) I would recommend using a larger beam on a 12′ span. Something more like a 4×10 / 4×12. If there is any doubt about sizing of beams to ensure safety, then please consult a local engineer or your town’s building department for the best advice. I’ve seen treehouses built with undersized beams and you will likely experience higher than normal deflection (bounciness) and it will sag in the middle over time. The 4×6 probably won’t fall down, but why not make it feel safe and remain flat over time? This is one that I may or may not change now, that’s up to you and your building department.

3) Allowing movement is a good idea, so you’re on the right track. I would recommend using a floating bracket on the fasteners, which allows a low friction interface between the beams and the attachment bolts. This is the system that I would use if I was building it for you.

4) Lastly, I see you have 3 beams up there. Chances are, I bet you only need 2 of them. You may need to adjust which side of each tree the two beams are attached to, but for a normal joist layout, it will be just as safe with 2 beams as with three. That is cheaper to buy hardware for and better for your trees since you’re only making 4 attachments instead of 6.

Best Wishes building a safe treehouse, and please reply here and let us know how it goes.

Dan Wright

4 Responses to “Is My Treehouse Plan Safe?”

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  1. Bob Finch says:

    2 years ago we put a treehouse platform on our land. We’re finally ready to build a house on top, but I want to double check things to see if I need to make any changes before proceeding.
    We have three 14″-18″ HemFir’s, each with a 1×6 TAB installed. I put a 6″x6″ post in concrete for a 4th beam support. Two 4″x6″x16′ beams span about 12 feet between supports. I put 2″x6″x12′ joists, 16″oc, between the beams, which are about 10′ apart. So the joists are about 2′ cantilevered on one beam. Also, when I put up the beams, I just couldn’t get myself to pay $100 each for the floating bracket, so I made my own from some gate steel I had. I used an old IKEA nylon cutting board to create friction plates. The trees seem healthy. The TAB collars are somewhat “consumed” by the tree, but everything is sealed around the collars.

    We want to build a 9’x12′ house, with one of the 12′ sides sitting on top of one of the beams. It will be 9′ high on one 12′ side, sloping to 11′ high on the other 12′ side. It will be a very basic structure with lapped cedar siding, rough on the inside, a metal shed roof with transom windows on the high side, and a lot of opening windows and probably some type of french door to open to the outside.

    I’m wondering if the 4″x6″ beams are too undersized for this application. I haven’t blocked the joists yet, and the structure (currently with old 5/4″ decking) feels a little “wonky”. Do you think it is safe? Not knowing how heavy the structure will be, can you make any guesses on deflection? Would it help if I scabbed a 2″x12″ to each of the beams? Or…? How about anything else you’ve read. My wife just wants to get the house going, but I’m feeling like anything we add is potentially something we’ll have to redo, until I feel good about the underlying structure.

    Thanks!

    • thb98il1Tr3e says:

      Hi Bob,

      You’ve done a pretty good job explaining the issues, but I do wish I had a diagram showing beam and joist layout, and a couple photos. In general, it sounds like the beams and joists are slightly undersized. “Wonky” is generally not a term that inspires confidence. I suggest having an adult bounce in different spots on the floor that you have built while you watch from the ground and see what members are deflecting the most – I suspect at this point that it is the beams. Wonky doesn’t mean that you have a looming catastrophic failure – wood will bend, sag, etc… for a while before it breaks. And let’s face it, backyard treehouses don’t always need to have the same deflection standards that permanent ground homes do.

      Joists: The 2×6 joists probably only should span 9′ or 9’6″ for just a deck. With a house on them, they may need more strength depending on how well the load transfers directly to the beams. Normally, houses are kept further from the tree trunk than the beams are… Also, there is a strength difference between #1 and #2 lumber which may be a material factor in this case.

      Beams: the 4×6 probably is not sufficient to span 12′. We use a lot of treated 4×6, but don’t usually span more than 6′ unless the loads are lighter than normal. You may want to try a double 2×12. I’m a big fan of paralam and PT glu-lam beams for the high stability and strength ratios, but they will cost more money.

      I hope this helps as you consider what to do,
      -Dan

      • Bob Finch says:

        Hi Dan,
        Thanks for the response. As I read it, I went back out and found that some of my measurements were incorrectly written. First of all, my beams are 4×8 treated #2, not 4×6. Sorry. Still probably not big enough, but thought I’d mention it. Also, the diameter of the trees at the point of the TAB’s is 10″, 12″, and 13″, respectively.

        Since the span between the beams is 10′, I’m a little shy with 2×6 joists, based on your notes. So, 2×8 joists would get me the span that I need. If I go to a longer 16′ 2×8 joist, would a 3′ cantilever on each side be good?

        Question 1 – does the larger (4×8) size of the beams make any difference, or is it still undersized? How about once I change the joists to 2×8?

        Question 2 – If I go to a 4×12 beam (4x lumber is cheaper in the WA area than double 2x’s), with the 2×8 joists, are the tree and tree TAB combination, strong enough to support that weight, and the added weight of the treehouse?

        Thanks again. Great site!

        Bob

        • thb98il1Tr3e says:

          Hello Bob,

          Since you already have the 2x6s, you might try adding more – say 12″ on center. Also, you could add solid bridging to reduce deflection without increasing joist size.

          On the beams, a 4×8 is a lot better and depending on the size of what you are building on top, might be sufficient. Be advised that various species of wood will have different load ratings per dimension.

          I’m sorry to hide behind uncertainties, but this is about as far as I can help you in a general sense on a public forum. Your questions about lumber sizes are generally answered with authority by an engineer licensed in your state. My opinion from Pennsylvania and without seeing drawings is not to be relied upon in the same way. Short of hiring an engineer, you may be able to ask your lumber yard for span tables to help you make a decision in accordance with local standards. Alternatively, we do have an engineer who has a license path in Washington, if you are looking for those services – but unfortunately he won’t start a review without an engagement letter and deposit. So, did you try the “bounce test” I suggested? 🙂

          Regards,
          -Dan

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