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Beam Spam for One-Tree Treehouse, No Knees
(43) #1
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Subject: Beam Spam for One-Tree Treehouse, No Knees
I'm building a one-tree treehouse next month in a very large Silver Maple in my backyard in Illinois. This will be my second treehouse of this sort. My first treehouse is at my mother-in-law's in Pennsylvania, an open deck structure built on the main trunk with 8' long 2x8 beams supported with 2x6 knees.

This time I'd like to avoid knees, because I want the treehouse to be as close to the ground as possible, accessible by 3-4 steps so that my 2-year-old can play in it safely.

The diameter of the tree is 3'. So I'd like to use the longest beams possible (I'm thinking 12') in order to have a decent area on the deck. I should add that I've also built a large deck, and a substantial 2-deck play area (my previous house did not have a treehouse tree), so I've got some experience.

My first thought was to use 2x12's, or perhaps even doubled 2x12s. Bolted in the middle, they would stick out 5.5' (assuming one foot in the middle where the lag bolts are) in either direction.

If it's not possible to avoid knees, I'll raise the treehouse somewhat to accommodate them. But I won't be able to go much more than about 4 feet from the ground; so even if I use knee braces, there will be some cantilever.

I'd be grateful for any practical advice on this, as I haven't been able to find out anything much about cantilever loads for treated pine beams.
(1) #2
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It's not recommended to cantilever so far with only one pivot point like this for a couple of reasons. First, if several people were all at one side of the treehouse, the force on the bolts would be substantial and could easily bend the bolts and/or twist them through the wood of the tree. Second, you should only use one bolt for each attachment point to the tree to avoid compartmentalization problems (where the wood between the bolt holes rots away). A single bolt would make a cantilever impossible by itself.

Without space for braces, you could use cables to support the beams from higher in the tree. These can either run from the end of the beams to a point above the treehouse, or can be routed through the walls first, with a strut at the top of the wall holding the cable out from the tree. Cables require quite a bit of hardware but are very strong and are widely used in commercial treehouses.

Another, perhaps simpler, method is to use posts at the end of the beams, although this does detract from the treehouse look. If you offset most of the weight of the house to one side which has a post, the other side could then be cantilevered as it would have a counterweight. This method could be used to disguise or hide the posts and give the impression the house is floating.
(43) #3
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Thanks a million for the advice.  I wasn't aware of the problem of "compartmentalization," though come to think of it, I've seen it in my Pennsylvania treehouse.

I think I'll break down and use bracer, putting the first deck higher than I'd planned. But how about braces placed only partway  along the beam? Maybe I could still cantilever each side a foot a two?  that way I could minimize the number of feet that I have to go  up.
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