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Building a safe tree house

Tree House

Tree House

Good Treehousing – Building a safe, and enduring tree house that’s in tune with nature


Thinking of getting away? Far, far, away? Why not leave the ground completely and spend some time up in a tree house? Trees can grow to over 300 feet tall, which I guarantee will change your perspective on life if and when you ever decide to come back down. Such an excursion can be really good for the soul. But there are certain goals that anyone who builds anything in a tree should hold dear. We want to build structures that are in harmony with the trees and the environment, structures that are well supported and safe for all visitors and structures that pass the tests of time. That is what professional tree house builders mean when they say “Good Treehousing.”

Tree houses are as unique as the builders who make them. I know several kids who stole lumber from a construction site over a weekend and dragged it home to create their own landing. At the other extreme are professional custom tree house builders who build platforms and houses to last over 50 years with minimal maintenance. If you go looking, you’ll find a world of difference in the safety, aesthetic beauty, and sustainability of various tree houses. You will find some designs that have killed their host trees within a short time, and others that minimized construction damage to the trees allowing them to continue to thrive. Let’s walk through the steps of building a tree house and cover some of the tips and tricks for building better tree houses that I’ve learned over the years.

The tree is your host

Some builders hold a ceremony to ask the tree’s permission before starting to build their tree houses. I don’t pretend to know whether the tree answers or not, but it is a useful starting point because it reminds us that the tree is alive and therefore has limits and deserves respect. Good treehousing requires that we have an understanding of the relationship between what we do and the health of our trees.

Anything we do in or near a tree interferes with its natural environment. Walking up to a tree compresses the soil that its tiny feeding roots are trying to absorb nutrients from. Climbing a tree causes bark damage which removes the tree’s outer layer of defense against intruding insects or disease. Drilling into a tree interrupts the up-flow of nutrients to the leaves for energy production and the down-flow of starches for storage in the branches, trunk, and roots below. Even if you don’t penetrate the bark, adding weight by sling or clamping will girdle and/or compress tissues and change the growth pattern of the tree. The three main threats to a tree during a tree house construction project are putting fasteners in the trees, pruning off branches, and disturbing or cutting roots.

One common misconception about trees is that they heal, just like we do. When something breaks our skin, we heal from it. We may or may not even have a scar. But trees do not heal; they seal. They seal over wounded areas and keep growing. The tree attempts to compartmentalize the area of the wound and redirect nutrient flow around it. It is instructive to study a branch connection up close. The area around the branch is sometimes called a branch collar, which is the transition between the branch tissues and the trunk tissues. The transition area is thicker because some of the tissues are continuing up the tree and going around the branch tissues, while some tissues are transferring nutrients with the branch. The extra structure in that area helps support the weight of the branch. When pruning a branch, do not cut vertically or flush with the trunk. Rather, cut above the transition area so that you don’t sever trunk tissues. This is very important to help the tree prevent infection of the whole trunk.

You should either hire an arborist for tree house consulting services or make sure you have a solid understanding of tree health before designing your tree house. If you hire an arborist, I recommend a consulting arborist because they have a higher level of certification and typically are more committed to knowledge and consulting than common ISA arborists. You should expect to pay about $250 for a visual assessment and consultation from a local consulting arborist. Further work or assessment techniques may be necessary, and it may not be cheap, but it’s better to know ahead of time, right?

Vision and Design

Your vision is a product of what the trees and their environment give you and what desires you bring to the table. Some people look at their trees and try to imagine what type of structure they are naturally suggesting. Others decide what kind of tree house they want first and then look for a way to support it in the tree. But no matter which perspective you start with, you need to consider both in order to get what you want and make it fit with your trees. You should probably leave specific placement of small tree house options like the classic rope & bucket or a rope ladder until after this stage is complete. Focus now on major decisions like the height, size, layout, and support, and then you will find places to attach your favorite rope swings and hammocks!

Trees with a lot of low branches suggest a more organic tree house design with nooks and meandering walkways. Since it is not a good idea to prune off lower branches, you are better off designing around those branches. Perhaps you should plan for branches to pass through the walls or come up through the roof. It takes a lot longer to build around them and flash them to keep water out and still allow them to sway naturally in the breeze. So if the branches are mostly on one side, consider making that an uncovered deck space where there will be less interference than an enclosed structure. Sometimes, the house can be built off to the side of the tree with a couple of posts to the ground and then the deck space can extend in and around the major branches.

You may object that in order to be a purist tree house, it can’t have support from the ground. If that’s you, then consider responsibly building a smaller treehouse and/or finding multiple sturdy trees to use. Many large tree houses for public use have no options but to use some ground support due to the strict commercial construction codes. Sometimes, even though I tend to be a purist at heart, it is apparent that using one or two ground supports to help will relieve a lot of stress on the trees. It is best to consider all perspectives and make the best overall decision for each specific project.

There are no set artistic rules for designing a tree house. Styles and materials vary in different parts of the world, so browsing the internet will turn up pictures of tree houses that look like futuristic spheres, modern architecture, grass huts, club houses, castles, pirate ships, luxurious getaways, and complexes of bridges, walk ways, and zip lines stretching hundreds of feet across steep gorges. Most tree houses are for personal use, but are a growing trend for eco-tourists to visit. If you’re going to invest in a custom tree house project of your own, then why not visit a few of them first to get a feel for what you like best?

Professional Tree Attachment Points

Once you have a conceptual plan for your treehouse, it is time to design it structurally. This will be an easier task if you have built decks or houses for several years professionally so that thinking about joist orientation and sizing of beams and girders comes naturally. Thinking backwards from a floor to the certain points where load is transferred into the tree and down to the ground is a task for a skilled carpenter or engineer. If any part of the floor is not supported well enough, it will be likely to sag over time, bounce when walked on, and possibly cause movement in other parts of the platform. Either way, follow building codes and good building practices to ensure safety and prevent future repairs.

All attachment points cause some kind of damage to the tree. The best attachments minimize that damage and provide the maximum strength per attachment. The first point to learn is that it is far better to use one large fastener than to use a dozen smaller ones. If fasteners are placed too close together, then the tree may compartmentalize the whole area as if it was one wound. Eventually, the wood will decay and the small fasteners will become loose and the beam will fall down. One very large lag bolt is a better choice than 10 nails.

Another important good treehousing principal that only tree house construction experts seem to understand is the theory of perching loads on top of strong fasteners rather than pinning beams to the tree by screwing a lag bolt through the beam and into the tree. I first heard the phrase “Perch, don’t pin,” from Jake Jacob, another tree house builder and friend. When a beam is pinned to a tree, it can’t move. If each end of the beam is pinned to a separate tree or branch, then the beam may be pulled apart or crushed during a storm. Another bad outcome is that the forces on a pinned beam may cause it to wiggle, keeping the wound in the tree open. The bolt will eventually wiggle loose or break and fall down. The other good reason to perch a beam rather than to pin it is to give the tree many more years to grow before it starts to touch the structure. Depending on the species and age of the tree, a properly attached treehouse may last 20-50 years before major maintenance is needed to allow the tree to continue to grow uninhibited.

Perching the beams makes it necessary to have much stronger fasteners because of the leverage introduced. The best ones are a series of tree house fasteners called artificial limb systems. Most artificial limbs are 5/4” 4140 steel rod about 13-24 inches in length with a 3” diameter shoulder pressed onto it. The rod is sometimes heat treated, and the whole assembly is often powder coated. The original artificial limb was called the Garnier Limb or “GL,” named in honor of treehouse pioneer, Michael Garnier, of Takilma, OR. Michael, tree house engineer Charley Greenwood and arborist Jonathan Fairoaks among others all contributed to the original development of the Garnier limb. The modern versions are more refined and generally bigger than the original, but the concept is the same. Artificial limb systems can safely support anywhere from 3,000 to 40,000 pounds in the trees. These artificial limbs and associated hardware can be expensive and hard to find, but are the best on the market. Since the manufacturing and installation of these fasteners is an unusual process, you could consider attending a treehouse workshop to learn about them and tree house building in general. Alternatively, if your treehouse is very small or just a platform with no house on top, you might have under 1000 pounds of lumber and only room for 7-8 people up there. In that case, if you are spreading the load over 3-4 points, it may make more sense to use a 1” or 1.25” diameter lag bolt instead because they make smaller holes in the trees. It is important to make the fewest and the smallest holes necessary to support the tree house safely.

Most tree houses are supported conventionally from beneath. This means that the beams rest on top of the fasteners or have knee brace style supports that connect to the trunk below the treehouse floor. Alternatively, bearing points can be supported with cables attached to the tree above. Using cabling to support tree houses is somewhat controversial. Scott Baker, registered consulting arborist and tree house consultant, of Tree Solutions in Seattle typically does not recommend this practice because the higher portions of trees are the least suitable for placing loads. The trees move more up there and so the tree house will be less stable. Also, from an engineering perspective, the tree is a vertical cantilever, and so the further up the tree you go, the less the tree can support the downward angled tension loads suspended by long cables. Some situations to consider using cables are when trees are leaning, when a little more movement is desired or okay, where the area beneath the support point can not be blocked with a knee brace or post, or in really large trees where the tree can take it and the cables will not interfere with the floor plan or walking space of the tree house.

In the spirit of “Good treehousing,” we want all tree houses everywhere to be built safely and in ways that do not over stress trees. Building a superior tree house takes a commitment to study attachment points and plan for the future growth of the tree. Please do your due diligence before starting. For more ambitious tree houses, expert consultants such as engineers, consulting arborists, or even professional treehouse builders can save you a lot of time & money in the long run or prevent unnecessary damage being done to your trees. Some of them will even offer to answer your specific tree house questions for reasonable consulting rates. Trees are hard or even impossible to replace, so be responsible with the ones you have.

Tree houses are truly amazing structures and are a lot of fun for kids of all ages. Learn as much as you can and rely on experts when it’s prudent. Good luck, and to all you treehouse travelers, dwellers, and builders, keep it safe out there!

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2 Comments

  1. Catherine July 3, 2009 at 6:00 am

    I sure wish I had known all of this before I built my tree house 6 years ago! LOL! I made most of the mistakes mentioned above. The tree is still looking good, but I think my platform needs a little TLC before I let anyone go up there.

  2. vinothkumar December 11, 2011 at 12:17 am

    sent me witch trees good for growing in house sounding ?

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