Tree House

— UPDATE 12/28/2013¬† — Scroll down to read back story

Lots of great comments, both here and on Facebook. I’ve come up with a much better design for the base which is so simple, I’m sorry I didn’t think of it myself. I found an article in Popular Mechanics where someone with more space than me built an amazing house. His used a grid floor that will be a lot stiffer and also resist the torsional loads better. box floor 2013 12 28 v3

Those will be 2◊8’s on the bottom and 2◊6 for the floor joists. Should do the trick. I’ll cut the end off one of them for the trap door. There will also be corner pieces going to a lower place on the tree, but I didn’t draw those.

Lots of comments suggest that I shouldn’t kill the tree. There are a lot of good arguments on that side. The most convincing is that dead wood will get termites and rot (I can attest to that given that I spent a bundle of money repairing that very problem when I moved in). The flip side, though, is that I may not care… I bet it will take 10 years to weaken the tree¬† much, by which time my then-high-school aged kids may not care. If it happens rapidly, it will be easy to stabilize the house with some steel supports.

The other big argument against keeping the tree is a design one. There is a living branch that is sticking out that could probably keep the tree aliveliving branchThe problem is that it is 10′ up… so giving it a little space, that would put the base of the house 11′ up the tree. This may surprise you to hear that I’m not actually bothered by having the kids climb 11′ up a rope ladder to get to their house. I am, however, a bit worried about the aesthetic of a 6′ high house 11′ up a tree… it would pretty much dominate the yard… in fact, it would be taller than most of my house. Not so good. There is another live branch going straight up that might be able to go through the roof. However, it’s small enough that it will grow pretty rapidly and we’ll have to adjust the roof to accommodate it.

— Original post 12/27/2013 —

I’ve got this ivy tree in the back yard. I don’t actually know what kind of tree lives underneath it, but all I see now is ivy. About 30′ tall, and really ugly. As part of the backyard redo, this is coming down.
Now, one of the first complaints we heard from the girls about the new house was “where is the hammock going to go?” which is a legit question. The hammock was one of their favorite places to play back in Friday Harbor. People have suggested that we buy one of those little hammock stands, but those people don’t understand that when my kids talk about “swinging in the hammock” they mean pile 5-6 friends in and see if they can get someone to push them all the way over the top. This worked at FHL where we merely chose the closest pair of 75′ fir trees which were used to surviving gale-force winds. A cheap steel stand would last less than an hour and might hurt someone as it fell apart.

This led me to think that cutting the tree all the way down might be a missed opportunity. There isn’t a matched tree, of course, but there is a cinder block wall that seems sturdy enough for a big anchor bolt.

So thinking about chopping down the tree, but leaving the stump got me thinking: what if I topped it about 8′ up? That would leave me a tree that maybe I could put a tree house on? We had a landscape designer take a look at our yard, and when I threw out this idea, she loved it; the drawings thus include a tree house in the yard, I just have to figure out how to make it happen. Of course, the designer was a little skeved out about trusting a dead tree to hold kids, so she drew in a load bearing steel frame. Um, that doesn’t sound like a tree-house to me.

This is presenting some interesting design challenges that I’d love some help thinking through. The trunk is pretty stout, probably 24+” at the bottom, and maybe 16″ at a height of 8′. It’s a little hard to tell because there’s about 4″ of thick ivy vines wrapped all the way around it. I can see a few live branches sticking out at the top of the tree, so I assume it’s all live wood.

trunk close up

trunk close up

I’ve sketched up a quick idea of what I’m thinking it might look like:
v2 perpective
For scale, the sassy-looking model is about 4’8″… I’m thinking an 8’x8′ platform with a height of about 6′ at the front with a sloping roof to 5′ at the back. I had a lot of ideas about a peaked roof, but a flat one seems more within my skill set. One thing that is awesome in the bay area are these recycling building products yards. I can buy casement windows for about $20 if I’m not too concerned about the fit or double-panedness. I saw a stack of domed skylights sitting in the corner that weren’t being treated like they were valuable, so I’m hoping I can get one of those for cheap, too. I’m even thinking I can get all the wood this way: they have stuff like old redwood studs for $0.50 / foot.

The big question in my head is how to attach this thing to the tree. My first thought is to put a ring around the tree and stick some supports out from it. Then I started thinking it might need some braces going down lower for when kids are sitting way out at the corners… I’ve only shown one on here because they are a pain to draw… but imagine a brace from each corner going 3-4′ down the tree.

v2 undercarriage

The other issue is that we get some serious wind through here… 15-25 mph is a standard afternoon… because the houses are so closely spaced, it’s less in the yard, but still.

Looking at this diagram from the bottom, though, it occurs to me that the joints at the ring will be a weak point… would I be better to notch the top of the trunk so I could have a brace go straight from one corner to the other?

Then there is the matter of the walls and roof. I’m assuming I don’t need 2◊4 studs at 16″ for a 5′ x 8′ wall, but what do I need? I’ve been thinking the platform will be 2◊6 around the edge with 3/4″ plywood over the top. That doesn’t give me much to nail studs into at the bottom, so I’d need a base plate?

I was also thinking of just having a 2◊6 – 2◊4 post in each corner (like on Eliza’s bunk bed for those who’ve seen that).

v2 pillar

I’m guessing 4 of those posts could easily support the roof, then the walls would just need the odd stud to keep them from bowing (and these could just get tied into the plywood without a base plate).

I’ve got no idea on the roof other than a rim of 2x4s with some joists and then some plywood and tar paper. I don’t know how much it needs to attach to the walls as long as it’s tight enough that it won’t lift off in the wind? I’m also assuming it don’t care if there is space between the walls and the roof.

Anyway, I’d love to hear any and all thoughts. Give me a call, shoot me an email or leave comments below.

4 thoughts on “Tree House

  1. tim

    For the floor, I think that you would be better off with two skins, top and bottom, of plywood or OSB, glued to the floor joists with exterior wood glue and screwed every four to six inches. I would start with gluing and screwing the bottom skin on first, drilling some drain holes, and then painting the interior. Bottom first, because you would get better paint cover and any standing water in the floor will collect on the bottom.

    Do you think the radial joists would work better than a regular grid? I think that a semi-monocoque floor with wood skins would be plenty stiff. If you screw your wall studs into the floor, they will also make the structure very stiff. I think your problematic load will be the torsional load at the floor/tree connection.

  2. Sean

    Can you plan to keep the tree longer (enough) to span through your planned roof and recreate the planned floor for a roof and thus added stability? It would be fixed at an upper and lower point on the tree. You lose some interior space though and then cap the top of the tree with a decorative weathervane?

  3. mooseo Post author

    Tim, the monocoque floor would definitely stiffen it up, but I’m not sure it’s needed. The whole box ought to be plenty stiff… especially if I add some cross braces to the corners. My idea with the radial floor was to help transfer the torque from the house box to the tree… that’s totally where I see the problems cropping up.

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