Structural Engineering for Treehouses

• Christopher Imbeau
• Marketing Director
• Rafn Company

Many Rafn craftsmen have built treehouses for their kids over the years, but as a company, we have never had the opportunity to build one like you see on TV. Our friends at Swenson Say Faget (SSF) structural engineering, however, have gotten the opportunity to work on 15 high-end treehouses throughout the US. I sat down and talked to SSF’s Project Engineer Mat Johnson, PE about his work on these projects.

What’s unique about designing structures in trees?

The nature of it; no pun intended. The fact that they are not on the ground and they have living foundations. It’s a moving structure so a lot of thought goes into that dynamic. Also, no two trees are the same. Different species, different growth rates, and various distances apart make each project unique. Even similar projects are vastly different from each other.

What are the various construction-related challenges that you’ve run into on these projects and how were they resolved?

The first construction challenge is that the workers are hanging from trees and are tied off while they build. Second, during construction we often get calls that the roots of the tree(s) are in different spots than anticipated. The braces that we have designed then need on-the-fly adjustments to miss. If they need to move more than a foot or so we may need to redesign the system. And finally, when we design, the trees are just circles on paper. The construction team may need to adjust to work around branches that interfere with the design. Often they will determine a proposed solution and run it by us.

What is your favorite aspect of this type of project?

I like that it is a very unconventional approach to a conventional building type (a simple house). It adds a cool dynamic that requires us to think outside the box. I come from a large commercial structure background that requires specialized design with complex solutions so this type of project fits well with my skillset. And I like the back and forth, and give and take, that comes with working with the architect and craftsmen on these types of projects.

Is there something different in this type of project that you haven’t done yet?

The short answer is that I don’t know! The great thing is that you never know what the next one will bring. Sometimes it’s one tree, sometimes four. You can reuse the same type of thinking, but everything is different from one project to the next. That being said, my favorite is the single tree because it typically doesn’t have any other supports; it’s a true “tree house”.

What is the highest treehouse you’ve ever worked on?

The highest treehouse that I’ve designed was 35 feet up. It was on a 5 and a half foot diameter spruce tree in Oregon and was a single tree structure that the owner lives in full-time.

The difference when you get up that high is there is wind to deal with and the tree’s diameter gets smaller. Putting the mass just off the ground is no problem, the tree doesn’t even notice it. But putting the mass up that high creates a reverse pendulum effect which is more susceptible to oscillation in wind and seismic events.

How many treehouses has SSF designed?

We have completed 15 full treehouse designs, of which I have worked on 13. We’ve also consulted on several others which didn’t require full engineering.

Have you slept in a treehouse?

As a kid, but not in a modern one.

Are you afraid of heights?

No. I grew up riding roller coasters and mountaineering. I’ve trained myself not to be afraid.

Examples of SSF's treehouse projects can be found on their website: ssfengineers.com/project/treehouses

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