Treehouse construction advice? (photos, video)
September 3, 2007 8:54 PM   Subscribe

Another treehouse question. I'm almost done building mine, and I can't figure out why it's so wobbly. Take a look at photos & video, please, and let me know what I might do to make it more solid. No treehouse experience necessary, this is mostly carpentry 101. Degree of difficulty: I do not want any posts in the ground supporting the treehouse. I want all the support to be in the tree.

I'm done everything but the railing and the floor decking. The plan was basically that I bolted two 2x8x12 beams to the two trunks of the tree, with eight inch lag screws, as primary support for the whole thing. Resting on top of that I put a platform made up of a 12x8 "box" of 2x6s, with 2x6 joists every 15 inches or so end-nailed to the 2x6x12s. T (I intend to add joist hangers) The two joists closest to the two trunks are bolted to them, also with 8" lag screws. (see photos). So basically there is support bolted to the tree longwise (the two 2x8x12s) and then, on top of those and perpendicular to them, the two 2x6 joists.

Then I put six diagonal struts meant to support the outer edges of the platform. They attach to to the tree at a 45 deg angle with the 8" lag screws, and the edge of the platform sits in notches I cut into the diagonal struts. There are two struts each (total of four) supporting the long edges of the platform, centered, about 20" apart at the edge, and then one strut each supporting the short edges of the platform, again centered there. Nothing supporting the corners.

The diagonal struts were meant to solve the wobbling problem, but they don't, (see video, where I'm pulling down on one corner with a rope) mostly I think because the structure no longer sits tightly in the notches, as it did when I put the struts up -- they're still in the notches, but floating there instead of resting in them (?) I guess I could bolt them in there instead of just letting it rest in the notch, but I'm not sure that will totally solve it.

Is the platform just too large to NOT have any support in the corners? Would it help to tie rope or cable from the corners of the platform up into higher branches to support it?

Any thoughts about my specific problem, or any thoughts in general?
posted by stupidsexyFlanders to Home & Garden (16 answers total) 2 users marked this as a favorite
 
Why not put supports on the corners? You could also 'branch' the supports you have currently to the corners, so you don't add more pressure to the tree. It is possible to kill those things buy building treehouses.
posted by delmoi at 8:59 PM on September 3, 2007


I'd say, either drop supports to the tree as you have on the edges, or put a diagonal brace across the corner the spread the load to the better supported sides.
posted by sycophant at 10:07 PM on September 3, 2007


I think your treehouse is really underbuilt, if you are wanting it to not wobble and to support a heavy weight. Your 2x6 box just isn't very rigid at all, as you can see. Your tree may be on the small side to have it be the exclusive support -- there is a reason that photos of impressive treehouses are usually in large trees, and that treehouses in small trees often have supplemental piers for support (often 4x4s or larger timbers, similar to what are used to support decks, sometimes as large as telephone poles if the treehouse is up high). I mean, if you were to call this a "deck" and ask the building inspector to come and take a look, it would never be certified for use. Not all deck standards will really apply to what you are building, but it is worth thinking about this in light of how other common structures are supposed to be built.

And when I say that it looks underbuilt, I mean that it needs both larger pieces of wood, and to have much better connections. The diagonals need to be attached securely top and bottom if they are to function, for example. (And are your single lag screw connections really current best practice in supporting a treehouse? I thought I'd read about treehouse builders using a different sort of attachment, that is supposed to hurt the tree less, but I don't know that for sure.) Those notches are an unusual way to anchor the diagonals -- I'd prefer a straight 45 degree cut and either lag bolts or better an appropriate gusset of some sort.

So yeah, if my kid were to be going up in that treehouse, I'd be happier if it had some chains or cables holding it up at the corners, assuming that there was a really solid anchoring point up above. I'd be a lot happier if the treehouse were disconnected from the tree, and instead supported on a set of piers, honestly. And I'd be totally happy if you had a real life engineer (or experienced structural carpenter) take a look at it, rather than random people on the internet, nice and helpful though we may be.
posted by Forktine at 10:15 PM on September 3, 2007


The lag bolts may seriously damage, or even kill the tree if left in. The lower supports are not rigid enough laterally, and the corners are unsupported. I would remove all lower supports except maybe a ladder, and hang ropes from the top branches to the corners, like this Vancouver building, which you likely remember from MacGyver.
posted by weapons-grade pandemonium at 10:16 PM on September 3, 2007


I agree with the above posters, and I think this tree is much too small to support the platform, and the lag bolts drilled through the tree will seriously injure and weaken the branch and possibly kill the tree.

Looking at the way in which you've designed and engineered the platform, it looks like you've built an upside-down pyramid with the tree poking through what would be the top (or, in your case, the bottom) of the pyramid, and I think this is the flaw that's causing the instability. The pyramid shape wants to live as, well, a pyramid, with the large flat area on the bottom and the point at the top. Imagine building a small pyramid from cardboard and then trying to balance it on the point--it's going to want to tip over, and that's what's happening with your design.

If you decide to go ahead with this tree (which I do not recommend) you're going to need some supports from each corner of the deck straight down to the ground (like you're building a giant table) and set them in concrete footings. Alternatively, you could cut a 48" length of 5/8" rebar and weld a bracket to one end, then pound the rebar into the ground and secure the support to that.

You'll also want to add some gussets--cut some 18" x 18" squares of 1/2" plywood and cut them diagonally, then toenail these triangles (hammer the nails in at an angle) into all the corners where the supports will attach. The shearing of the platform will probably not be an issue once you nail in the plywood floor.
posted by fandango_matt at 11:53 PM on September 3, 2007


When you put flooring in the frame you've built, the weight will be too much for the supports, and the tree's not big enough. Dittoing the calls for posts to the ground.
posted by anadem at 11:58 PM on September 3, 2007


posted by stupidsexyFlanders Degree of difficulty: I do not want any posts in the ground supporting the treehouse. I want all the support to be in the tree.

I meant to add that I saw this design constraint, but it's just not going to be possible with this tree--the tree is simply too small and there's not enough of it above or below to support the treehouse. I'd rework the design so it's essentially a giant table with a tree growing in the middle of it--the current design is just not going to support weight, and you're going to injure or kill the tree.
posted by fandango_matt at 12:36 AM on September 4, 2007


One last note--the wobble in the video is due to the fact your diagonal joists from the tree trunks are attaching in the middle of each edge of the platform instead of at each corners.
posted by fandango_matt at 1:19 AM on September 4, 2007


This is essentially a mechanical engeneering 101 question! There are 2 important principles that make your construction wobble.

1. All connections (even bolted) can rotate to a degree. Even small rotations can lead to visible deformations. Think of a simple square construction. If you push it from the side, you can see that it will shear because of this rotation. How to make it stronger? In approximate descending order of strength: You can add crossbeams, side plating, gussets or steel connectionplates. As a rule of thumb, triangles rotate less then rectangles, so begin by moving your side supports to the corners (making triangles), then add gussets or side plates to all rectangular parts.

2. Some connections and construction pieces can't have tensile force. Think of each part as a rubber band. If you put a load on one part, is there another part which will stretch? then it's under tensile force. For instance, in your video, while pulling the left corner, it's the rightmost support. Even if you put an endplate at the connection to the tree, it is a very weak connection. Again, you should move your side supports. That way, you remove the cantilever which was responsible for the tensile force.

As a final piece of advice, make your construction twice as strong as the maximum load it will bear. In this case, think of three jumping kids in one corner. Their total weight is 100 kg, the jumping doubles the load, and you want to double that load as a safety. That makes 400 kg as a maximum tolerable load, ie it is allowed to collapse under that weight. Test it, by hanging 200 kg on the weakest place. Slowly add weight, until your construction starts to make a lot of noise. If the final weight is more than 300 kgs, you can congratulate yourself on a solid treehouse!
posted by Psychnic at 3:37 AM on September 4, 2007


Agreeing with all who say it is underbuilt; the tree is way too small; you will kill the tree; and, supports need to go to the corners.

Why not start over? Take it all down. If this is the largest available tree, abandon the idea of tree house, and build a freestanding elevated structure. For instance.
posted by beagle at 6:30 AM on September 4, 2007


For reference, I wouldn't try any of that testing on this particular treehouse...

Or, at least, don't stand under it when you do.

The size of the bolts through the trunk are way too big for the size of tree - you very much risk killing it. The size/number of bolts ratio for the size of treehouse is way too small.

Basically, the project is too ambitious.

I'd downsize it, and very much learn a lot more about strength in wood. The 45degree joints are very, very weak from a wood point of view - grain is the strength in wood, and you have a tiny amount of it actually supporting the weight - something like 25% of the support will actually do anything, so it's large size is misleading.

The tree needs to live, so I would support the platform with the ground supports, as mentioned, and find a much nicer way of attaching it to the tree itself - even if it is just cosmetic. The most important thing is the safety of the structure.

Alternatively, don't downsize, but make the tree a feature rather than the primary support. You seem to be applying mechanical knowledge that equates the behaviour of steel rather than that of wood. The way to maximise the strength of a wooden structure requires very different techniques to that of steel, and your design looks like a steel girder structure to me. Maybe use natural wooden posts as the vertical supports and bracing and retain some of the character/concept by using a more natural look?
posted by Brockles at 6:43 AM on September 4, 2007


By the way, Treehouseguide is a very comprehensive site on treehouse construction.
posted by beagle at 7:51 AM on September 4, 2007


...the wobble in the video is due to the fact your diagonal joists from the tree trunks are attaching in the middle of each edge of the platform instead of at each corners.
posted by fandango_matt


No, it's not. Visualize cutting off the four corners so the joists are supporting the corners of a new, smaller square. It would still wobble. The problem is there is no lateral support, because the angled joists are attempting to do the job of the sturdy branches of a larger tree. Here's the construction of a platform I built with my kids in my cherry tree. There are no nails or bolts in the tree--it's all locked in by being "woven" through the branches. Easy for me to say, right? Realistically, this is the size of tree you need for your platform.
posted by weapons-grade pandemonium at 10:24 AM on September 4, 2007


The wobble is from at least four things:

-- The 2x6 platform isn't rigid, so it will wobble or at least flex almost no matter how many diagonal braces you add.

-- The braces in the middle of each side leave long cantilevers at the corners, giving a lot of leverage to downward pressure at the corners.

-- The weirdly notched braces are not attached to the platform (only gravity is holding the platform in the notches), so a downward pull on the northeast corner allows the southwest corner to lift.

-- Because of how everything is attached to the tree, there is little or no resistance to twisting or torsional movement (whatever the precise term might be). Imagine attaching ropes to each corner, and then having a bunch of May Day maidens run in a circle around the tree while pulling on the ropes (so that they are trying to twist the treehouse like a twist-off bottle cap). I predict that the whole thing will crumple and twist with that kind of load.

And since each of these odd loadings can be sorely tested by a group of rambunctious neighborhood children (imagine an entire little league team rushing to one corner and jumping up and down in time with the ice cream truck song), I am joining the "redesign and rebuild" set of voices.
posted by Forktine at 11:12 AM on September 4, 2007


Hard to pick favorites, every question was thoughtful and informative. It's interesting that beagle pointed me to Treehouseguide; I used a lot of that material in my design for this -- he talks a lot about knee braces, which I used, and about using lag screws in the tree -- from his perspective, what I've done should not have been harmful to this size tree, but everyone here seems to disagree. I've contacted the owner of THG and pointed them to my photos and this thread for his comments.

Agreed that the corners need support -- whether I'm going to use posts or try to hang them from rope or cable anchored further up in the tree, I don't know yet.

thanks again!
posted by stupidsexyFlanders at 11:14 AM on September 4, 2007


The wobble is from at least four things:

-- The 2x6 platform isn't rigid, so it will wobble or at least flex almost no matter how many diagonal braces you add.


What would make it rigid? Would the plywood flooring do it?

-- The braces in the middle of each side leave long cantilevers at the corners, giving a lot of leverage to downward pressure at the corners.


Right, the video shows that.

-- The weirdly notched braces are not attached to the platform (only gravity is holding the platform in the notches), so a downward pull on the northeast corner allows the southwest corner to lift.


I'll bolt those joints instead of floating them and see what happens.

-- Because of how everything is attached to the tree, there is little or no resistance to twisting or torsional movement (whatever the precise term might be). Imagine attaching ropes to each corner, and then having a bunch of May Day maidens run in a circle around the tree while pulling on the ropes (so that they are trying to twist the treehouse like a twist-off bottle cap). I predict that the whole thing will crumple and twist with that kind of load.


Can you elaborate on "because of how everything is attached to the tree?" Is it because all the connections to the tree are in the center of the platform? Or, said another way, because that platform extends so far beyond the primary connections to the tree?

And since each of these odd loadings can be sorely tested by a group of rambunctious neighborhood children (imagine an entire little league team rushing to one corner and jumping up and down in time with the ice cream truck song), I am joining the "redesign and rebuild" set of voices.

I'm looking for a solution that fixes what's there, as opposed to tearing it all down, as you can imagine. Certainly I'll do what's necessary to make it safe, but you will understand my not wanting to throw away a lot of work/materials if I don't absolutely have to.
posted by stupidsexyFlanders at 11:27 AM on September 4, 2007


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