So you want to install a Garnier limb?

Author’s note: This post was originally published in late 2008, and for those of you about to try your hand at treehouse construction and setting Garnier limbs, I hope this gives you a good start. My treehouse business has since suspended operation, but I leave this post here because it still seems to help people find the knowledge and the courage to build a dream or two. For allowing me to be of that service, I thank you all. And remember, when doing any project, enjoy and stay safe.

One of the most common questions I get asked is, “How do you install a Garnier limb?”

My answer always starts with one word:

Carefully.

Garnier limbs are the foundation of the modern treehouse movement, and come in a variety of sizes to address different needs. Make sure you have the right one, and the right tools to go along with it.

The good news is, if you’re taking the time to look this up, you probably already understand that.

You see, the Garnier limb is, quite literally, the foundation of it all. The high-strength special steel bolt developed by treehouse pioneer Michael Garnier and engineer Charley Greenwood is the thing that makes the modern treehouse movement possible. Because of its inherent strength, and the way it works with the inherent strength of the tree, the Garnier limb (commonly called a “GL”) allows for sizable loads to be reliably supported by a wide range of healthy hosts. Properly installed, GLs can safely support a stunning array of treehouse designs while keeping harm to the tree to an absolute minimum.

“Properly installed” being the key phrase in that sentence.

No honest treehouse builder will tell you that what we do causes no injury to the tree. Raising a treehouse means shifting the burden of gravity to a living thing that’s already wrestling with Newton’s favorite force. Setting any connection strong enough to tackle that task means taking a bite out of the tree. What’s important here is that the bite be as gentle as possible, that there be as few of them as possible, and that the tree be given the best chance to recover from the intrusion and grow stronger to handle the load. GLs make that possible, that is, if they’re installed …

Carefully.

To do that, you need to do three things – pick the right tree, develop the right plan, and install the GL the right way. Skip the first two and no matter how well you pull off the last one you’re headed for trouble. So before we get to drills and wrenches, let’s take a quick walk through the early stages of the process.

Finding a tree

First, as it should be, it’s all about the tree. Never walk out in your yard or the woods with a particular site, a certain tree or a set design in mind. Do that, and you’re already off on the wrong foot. Find the right tree, and then let the tree build the treehouse.

Although black willow trees can make challenging hosts, careful planning and good building techniques can make the best of this situation.

Now, not every tree can be the right tree. Some types are just not suited for the job. Like Darwin’s mockingbirds, different trees have evolved different techniques for fitting into a wide range of niches. Some adaptations make for a strong, stable host. Others mean trouble for any long-term guest.
In spite of the name, some common “softwood” trees such as pines and firs can make great hosts, if you don’t mind a little sap. Before getting to far into the process, make sure to take a good look up and keep a careful eye out for branches just waiting to fall. Many pines, such as the eastern white pine, can grow 100 feet or beyond, constantly adding mass to the top and shedding shaded branches underneath. No problem for the tree. Not always easy on whatever is underneath. Be careful of spruces, their roots rarely run very deep and they’re a poor choice for a single-tree treehouse.

Classic “hardwood” species are not all the same.

Some have wonderfully strong wood, but make poor hosts. Black walnuts and butternuts are tough, but their branches are brittle. No one wants to wear a hard hat to relax in the treehouse. Black locust – the hardest native hardwood in North America – tends to grow in muddy or boggy soil that’s not always a good place to build. The water loving black willow grows to massive size, but drops large branches the way middle-aged men shed hair and never seems to mind just tipping over for a good drink. For a range of reasons, elms, cottonwoods and sycamores are rarely good choices. All these trees, however, are wonderful to look at, so just find a good tree nearby and be sure to add a window or porch for the view.

It’s never a bad idea to have a certified arborist take a look at your tree. Careful pruning of dead branches increases safety and lightens the load on your future host.

Maples are excellent hosts, with the native sugar maple and imported Norway maple among the pros’ favorites. Oaks do a wonderful job as well, and it’s hard not to find one in any patch of woods. Apples are smaller than many trees, but strong wood and sweet blossoms make them a great host for smaller treehouses. Ash trees have rock-hard wood, but much of the country is battling a blight that kills or severely damages these wonders so be careful. If you’re looking for the “hard” in hardwood, find a healthy hickory; just be prepared for the workout of your life when it comes time to attach those GLs. Once you’re in, though, you’re in for the long haul.

OK, let’s say you have a few trees in mind (and it’s always best to keep your options wide open at this stage). What next?

Well, even if they’re the right kind, keep your eyes open for a few other problems.

Small and very young trees are fun to work around, but simply do not have the strength of their bigger siblings. Those young ones are very hard to snap, but they’ll try just as hard to shed the load you add by bending out of the way – usually with poor results for the tree and anything else hoping to stay upright. If your ambitions are big enough to consider a GL, then don’t give a second glance to trunks with less than a 12-inch diameter at the height you plan to work.

This tree was rejected as a host because an arborist was concerned woodpecker holes indicated several trunks were compromised. Months later, three of the trunks snapped in a late-winter wind storm.

In addition to avoiding young trees, watch carefully for sick or damaged trees. The first time I headed into the woods with Treehouse Workshop founder Jake Jacob, he brought an old ash baseball bat. He’d rap the bat on the trunk of a potential host and let the sound tell him how solid the tree was. A good arborist is worth more than a few swings of the bat here, but if you don’t have one just be sure to give the tree a good once-over. Is the root head, the area surrounding the base of the trunk, healthy, stable and clear, or are there exposed and rotting roots with soil packed down like concrete? Are there splits or open wounds in the trunk that hold moisture or show decay? Is the leaf canopy full and healthy? Did you see a carpenter ant or a termite anywhere? How about fungus? Are similar trees in the area healthy and sound? What kind of things in your neck of the woods present challenges to this type of tree? If you have any doubts, call an arborist or talk to the folks at the local agricultural extension office. An ounce of caution here is worth tons of peace of mind later on.

All right. You have the right tree. It’s healthy. It’s in a great spot. Great.

Time to listen.

One of the reasons companies don’t mass-produce treehouses in third-world factories and sell them at big-box stores is that every tree is unique. It grows according to the interplay of its evolutionary programming and the conditions it experiences as it rises up. Take a good look. Walk around and look again. Grab a ladder or a rope and get up into the tree a bit (be careful, of course). Where are the available open spaces? Is there good room somewhere near the height you prefer? If you’re lucky, there is. If not, just try to imagine the tree suddenly buried 8 or 10 or 15 feet in dirt. Where would the good spaces be? How do they connect? Where could you walk with enough elbowroom for a few friends? Can a straight piece of wood fit between and around the places you want to be? Try to stay balanced over the main trunk and find that platform the tree wants to offer. The goal here is as little damage to the tree as possible; so don’t cheat by chopping branches in your mind. Work with what the tree can give. If you can’t, you likely have the wrong tree.

Tread with the lightest foot

So you found a tree that’s offering a good home. OK, now it’s your turn to ante up a little mental energy.

The deal we make with the tree is simple: if it will carry us, we’ll do as little harm as possible and keep a watchful eye on it, always. That’s as close to symbiotic as we can get. Keep that in mind as you start designing your treehouse. Have a ton of fun and stay very safe (both good topics for their own conversation), but remember – do as little harm as possible. Avoid heavy pruning. Keep intrusions into the tree as few as possible; which means, to the point of this topic, place your GLs carefully. Remember that they’re incredibly strong – rated anywhere from 3,000 to 6,000 pounds or more, depending upon the type of GL and how they’re attached. Small and medium-sized treehouses don’t need many to stay aloft.

Sample Deck plan

A straight board tacked temporarily in the tree is a great way to get a sense of platform height. Mark two points and use them to help map the exact location of the trunks. That will let you draw an accurate plan for your treehouse platform.

Assuming you’re comfortable with basic design and building (and if you’re not, please stop now) place the GLs for stability of structure, careful distribution of load, and maximum flexibility for the tree. First, imagine where you’d put a post if your treehouse was actually an earth-bound shed. Then play the reduction game; can some of the “post points” be replaced by beams carrying the load to other points? GLs and the brackets that are used with them are designed to allow tree and treehouse to move, so stress forces don’t get trapped and take their wrath out on your good work. Make sure that movement is possible when you sketch your foundation plan. Of course, make sure you have a very good sense of the tree’s layout, not just at the deck height, but also at 6 to12 inches or more below the deck where the GLs will go. And be warned — never put a GL closer than 2 feet above or below another GL or any trunk penetration. Doing that can cause the wood between the GLs to rot, in turn causing both anchors to fail. Definitely not a good thing.

If you have any doubts, it’s never a bad idea to run your plan by an engineer familiar with treehouses or a veteran treehouse builder. It might be a quick telephone conversation. It might be a more careful examination. Many will give your plans a solid review for just the cost of a few hours time – likely less than the cost of the GL you’re about to buy. Even if it takes a site visit, a few bucks here could mean a lot of money and frustration saved down the road.

Drilling into the tree

So, the tree has spoken and the deal’s been made. You know what that means?

Yup.

It’s time to rip a wound into a living thing that will never heal.

All right, that’s a little macabre, I know. But it’s true, and it’s the essential mindset to have before you start spinning a drill anywhere near that tree. Trees don’t heal, they compartmentalize. Make a clean, correct hole that’s a perfect match for the GL, and the tree will shut down and seal off the wood next to the anchor (making it dry and strong), and spend the next few years closing the wound in and adding strength-increasing “reaction wood” to the trunk underneath. Because of the interplay between GL and tree, your treehouse foundation will get stronger over time and the tree will learn to forget you’re there.

Botch the job, though, and no one is safe. Incorrectly installed GLs can fail, undermining your efforts and putting people at risk; and poorly done work can introduce stress and infection agents into a tree that can kill it in surprisingly short order. Some of the best architects and designers in the treehouse industry have stories of treehouses crashing to the ground or good trees dying soon after work was completed. These pioneers learned the hard way. We don’t have to. All we need to do is be careful.

Now that you’re paying attention, let’s install a GL.

First, make sure you have the right gear.

Two common types of GL include the original (gray above) and a larger version for supporting heavier loads. The classic GL will work for most people, but make sure you have the custom drill bit (top left) and a good pipe wrench to set that GL right.

There are different types of GLs in use. The classic GL still used by Michael Garnier at his Out-n-About resort has a 1 1/4-inch thick steel shaft about 12 inches long, with a 1-inch thick by 3-inch wide collar pressed onto the shaft. These GLs offer 3,000 to 4,000 pounds of support, or more if they have a cable backup attached (we’ll get to that in a little bit). For heftier challenges, different builders use longer GLs with thicker collars in the middle. The at-home builder should rarely need more than the basic GL, but you might want to use a thicker-collared version if you’re expecting a heavy load or using a tree that grows very rapidly.

In addition to the right GL, you’ll need the right hardware to go with it. That means strong welded -metal brackets that attach the GL to the beams they carry.

The proper bracket connects the GL to the beam supporting the treehouse, while allowing for enough movement to keep the tree happy.

The type and style vary based on how you’re using the GLs, but the most common one features a steel plate about 2 inches wide and 12 inches long, with a looped matching steel piece on the bottom side. The top part bolts to the beam, usually with 1/2-inch galvanized lag bolts, and the lower loop goes around the smooth shaft of the GL. Add a nut to the end of the GL, and the system allows a limited range of motion in all directions except up and down – two directions you don’t want your treehouse to head.

You can buy GLs (about $125 and up) and basic brackets (typically $35 and up) from several treehouse shops online, notably Michael Garnier himself (www.treehouses.com). Renowned treehouse engineer Charley Greenwood has a detailed design schematic for the standard GL available as a PDF file on his Web site, www.treehouseengineering.com, and if you know a good metal shop they might be able to make some for you. Be warned, however, the job’s not as easy as it might seem. Real GLs, the ones that can carry you and your kids in safety, use high-strength steel and very demanding production techniques. Don’t let anyone tell you softer steel is just as good, or that heat treatment isn’t necessary. Larger GLs can be harder to find, and a bit more expensive (usually upwards of $200). If you’re in need of anything you can’t find, drop me a note (john@peacemakertreehouses.com). Chances are I have it or I’ve looked for it too, and I’d be happy to help you get what you need.

One last note about gear: Make absolutely certain you have the right drill bit to go with your GL. If you’re using the standard GL, Michael Garnier rents the right custom-made bit for about $50. Before you laugh at me and grab the old drill bit set from the garage, just consider this. To install the GL, you need to drill three different diameter holes. The first is 1 inch wide, just right to allow the special threads on the inside of the GL to get a firm grip on the tree. The second is a 1 1/8-hole to allow the slightly larger smooth shaft of the GL to sink snugly into the tree without tearing out the threads you just tapped in. The third is 3-inches wide and creates the all-important seat for the special pressed-on collar. All three holes have to share the exact same center, and all have to be set to the precise depth in relation to the tree and each other. Get any of them wrong and the GL might not seat, or big gaps inside the tree will create a sloppy fit and pose a risk of fungal infection. Trust me, you need the right bit. Rent it. Buy it. Have an engineer and a metalworking buddy examine a GL and make one. Whichever you prefer. Just don’t get into this without it.

The last bits of gear you need for the job are more common — a high-power corded drill, a fairly beefy pipe wrench and something to let you set an exact level.

A water level can be a great tool for setting GL heights on multiple trunks. Remember to always work from the same starting point, and mix a little vodka in the water if the temperature dips below freezing.

The first step in the field is to set your level. If you’ve done the design, whether on a napkin or with your CAD program, you’ve already decided how high the treehouse should be. My advice is to set that height based on comfort levels, namely those of the intended users and the tree. Try to keep in the bottom third of the tree, where trunks are thicker, winds are gentler and the rules of leverage are on your side. If the treehouse is for younger kids, there’s no reason to get too lofty. If it’s over their heads, it’s high to them, and a fall from a few feet is a lot less dangerous than a fall from a few dozen.

With your height known, mark that level on the trunk with a Sharpie or a soft marker. Make it as precise to the height you want as possible. Now, do a little math. How thick is the decking material you plan to use? (Remember not to get fooled by the “nominal thickness” and use the actual thickness. A “five-quarter” board is actually four-quarters, or 1 inch, thick). How about the joists that will support the deck? Are there beams below that? And how far from the center of the GL to the top of the bracket you’re using? All of these dimensions lower the height at which you’ll be installing the GL in relation to the deck.

A galvanized nail is used to mark the exact spot where the GL will be attached to the tree. Note the ladder is secured to the trunk. Always make sure you put safety first.

Once you figure out that height, make a mark directly under the first spot. This is the height for your GLs. Now, use whatever level system you prefer to transfer this height to any other trunks that may be involved in the design. Always work from the original point to the other points directly, don’t use a new point as a starting place for a third. Do the latter and you risk compounding any slight margin or error you might have in your level.

(A quick side note: If you’re using a single trunk and will be installing “knee braces,” setting that GL height is a bit more complicated. My advice — take your time. Use the method above to get your top height, then install the upper beam with some temporary bracing or ropes to keep it level. With that beam up, work down from the location on the beam that will receive the knee brace [make sure it’s far enough out to offer meaningful support] to find out where a 45-degree angle intersects the trunk. Make sure you account for the size of all the hardware you will use, and that will give you the lower GL spot. Of course, remember the 2-foot rule. And this final caution: If you’re doing a lot of knee bracing, it’s likely a good time to call a pro and get some help. Proper design and construction for knee braces is critical to the safety of the platform.)

Now you have your marks. Before you grab the drill, make sure you understand at exactly what point along the circumference of the tree your GL should enter. It should be as perpendicular to the beam above it as possible, and always point directly into the center of the trunk.
This also is a good time to double-check the trees. Does they look and sound solid at the spots you’ve marked? If you have any doubt, set a new spot. There’s no point sinking a GL into a rotted trunk.

OK, now you’re at the right height and you know exactly the right spot. It’s time to drill.

Use a corded high-torque drill to turn the custom GL bit into the tree. Remember to hold on tight, watch your level and make sure you have a good place to stand.

Here, again, a note on safety. If installing the GL takes you off the ground, make sure you and any heavy tools you take with you are secure. If you’re on a ladder, strap the top tightly to the tree so it won’t push away when you lean in with that drill (more of Newton’s handiwork). While you’re at it, strap yourself to the tree with a secure lanyard so you stay put. A high-power drill and that big custom bit can kick you around a lot. However you’re up there, make sure you and your tools are protected from falling and there’s no one and nothing breakable underneath the work site.

So up you go, drill in hand with the bit loaded and locked in. Find your spot. Put the small threaded tip of the bit firmly against the trunk. And wait.

Just one more time -– is this the right spot? Are you and the tools safely tethered in? Remember, once you pull that trigger, there’s no going back.

All that being double-checked, it’s time to break a sweat.

With the bit against the tree, make sure you are pointed toward the heart of the trunk and make very sure you’re level. The GL must be set right on level. If the end droops down, the beam will want to slide off and the GL will likely fail. If the end is high, the beam will slide into the trunk, damage the tree, and the GL will likely fail. This is so important that Michael Garnier’s crews use a custom guide they strap to the trunk to make sure they get it right. If you have a steady hand and you’re not 40 feet up a Douglas fir, you probably don’t need the jig, but just be very — you guessed it — careful.

Crews at Michael Garnier’s Out-n-About resort in Oregon install GLs while suspended more than 40 feet up in a douglas fir. To make sure the GLs are level, Michael developed a special jig that straps to the tree and guides the drill.

Game time. Pull the trigger.

For a few seconds, it may seem like the bit won’t go in. That’s especially true on trees with very thick and soft bark. Lean on it a bit, and the threaded tip will bite, pull the big auger bit into the tree, and that bit will try to pull you in fast. Hold tight, keep control and watch your level. This is just the beginning. At the base of the auger bit (the part that looks like a cork screw) is that 3-inch wide Forstner bit. As that approaches the tree, make sure you have a good grasp on that drill and you’re firmly planted on whatever it is you’re standing on. Before the Forstner section hits, it’s a good idea to pull the bit out to clear the hole. Don’t back the drill bit out by going in reverse. That leaves most of the wood still in the hole. If you can’t pull the bit back while it’s turning forward (and don’t feel bad if you can’t; that’s no easy task), then hold the drill firmly, let it come to a stop, and run it in reverse slowly for a turn or two –- just enough to get that threaded tip to let go of the tree. Once you do that, you should be able to pull the drill bit the rest of the way out while its spins forward. You’ll know you’ve got it right when all of the wood chunks start falling to the ground.

Now, stage No. 2. If you’re using the standard GL, you want that Forstner section of the custom drill bit to sink about 1 inch below the inside of the bark – just enough to seat the wide collar on your GL firmly inside the cambium layer of the tree. If you’re using a GL with a thicker collar, set your depth so you get about two-thirds of the collar inside the bark. That will give you the added strength, and still leave room for the tree to grow.

Here’s something else to keep in mind. As you start to chew with that wide section, some wetter or more fibrous tree species can clog the bit, leaving you spinning and pushing and doing a whole lot of nothing. You may have to pull the bit out a few times to clean it. Be careful. The bit is sharp, and might be hot. Unplug the drill before you mess with the bit, then use a sharp tool to carefully remove the wood fibers that build up on the leading edge of the Forstner bit’s teeth. Take your time and be patient. You’ll get there.

Now repeat that process wherever you need to install a GL. Remember every time to do that double check –- is this the right spot? Is your angle right? Are you and your tools and the work area safely secured? Is the drill level? A good carpenter can work around almost any error, but this is one of those things that puts the “almost” in that sentence. It has to be done right the first time. So take your time, and be, yup, careful.

Setting the GL

If all went well, you have a good, clean hole right where you want it. Outside of sap running time in spring (always a good time to avoid setting GLs), there should be a minimal amount of liquid present. Use a light to make sure the hole is clear of any debris, and that the trunk looks solid all the way in.

When turning the GL into the hole, make sure you’re aligned correctly and the GL is level.

All right. You’re halfway home. It’s time to install the GL. Remember, GLs are pretty heavy, so make sure the work area remains safe.

One other thing is very important to remember: Don’t leave that hole in your tree open for very long. The air is full of things that love to eat wood, and you’ve just punched a hole through the tree’s natural defense. Steel is easy for a tree to handle. Hungry fungus is another thing. Treehouse engineer Charley Greenwood even recommends adding 3M DP190 epoxy to the GL, to lock in the bolt and to seal and protect the wood.

You can usually get the GL started in the hole by hand. Remember the last inch or so of the hole is slightly wider to accommodate the unthreaded part of the shaft, so that gives you a head start. With your level close by, start the GL by hand and, again, make sure you’re pointed toward the center of the tree and level. Turn in that GL as far as you can. Don’t be afraid to grunt a few times. The more you can do by hand; they less you’ll have to wrestle with that pipe wrench.

Individual strengths vary, of course, but few treehouse builders I know use anything smaller than a 24-inch pipe wrench. If you’re off the ground, make sure the wrench has its own tether. You don’t want that monster falling on anything. And it’s a good idea here to wear a hard hat. One of the worst jobsite injuries I ever got came from pulling a steel tool down on my own head. Not a proud moment, I can tell you.

It can be a workout, but a strong pipe wrench at least 24 inches long is the best way to turn in a GL.

With the jaws of the wrench set to fit just over the outer (that’s very important) shaft of the GL, as close to the middle collar as possible, check to make sure you have it facing the right way (the teeth on the wrench’s jaws should be biting into the GL as you turn in). You can probably get one-third of a turn done before you’re bumping the ladder or yourself, so just spin the wrench back up and keep turning. It’s always best to pull the wrench down, rather than push it up, to set the GL. This let’s your weight work for you and saves your back and shoulders a whole lot of pain. Don’t get discouraged. Turning in a GL is hard work and you will get tired. Just keep turning, you’ll get there.

“There” is, of course, that moment when the inner face of the collar spins firmly into the clean-shaven surface of the tree your custom drill bit left in its wake. It’s extremely important to make sure that collar is seated all the way and snug. That collar is one of the key design features of the GL. It creates a larger face to rest on the end grain of the wood below the GL, and it helps the GL resist failure by using the wood’s natural resistance to crushing across the grain. Keep turning the GL. Unless you’re using a 6-foot pipe and a few guys on the ground pulling the end with a rope (don’t laugh, I’ve seen it tried), it will be very hard to turn too far. Just keep turning until you feel like you’ve hit a wall. Sometimes it will even give you a slow squeak to let you know the collar has found its home. Give it a few more tugs. If they wall’s still there, you’ve got it.

When you feel the inner face of the wide collar turn flush and tight against the newly cut hole, you’ve hit your mark — the GL is installed. A soft grinding wheel and a little rustproofing paint will clean the GL’s outer shaft just fine.

Turning the GL in with the pipe wrench has one flaw – it can chew up the shaft of the GL a bit. In the early days of GLs, some builders used a special cap nut installed on the machine threads on the outside of the GL shaft. That would let them use a big socket and a long breaker bar. It’s a good system for getting the GL in, but the problem starts when you have to turn the cap nut backwards to get it off. Sometimes, doing that turns the whole GL backwards, pulling that collar off the tree. That’s a big risk to take; so most builders now just accept that the GL’s outer shaft may need a quick touch with a light grinding wheel and a little rustproofing paint. Unlike an un-set collar, there’s essentially no risk in scuffing up the shaft. So just smooth out the rough edges, give it a blast of paint, and you’re done.

But should we get cable?

Or maybe not done after all.

There’s one more thing to consider. Sometimes a GL alone is not enough. If a specific GL is likely to carry an unusually heavy load, or if that load will ride far out on the GL’s shaft (or even need to go beyond the threaded end), it may be time to attach some form of cable “back-up” suspender system to the GL.

These back-up systems generally come in two types.

Sometimes, a GL needs a little help from a friend. Special high-strength extensions can be slipped over the GL, or a “teardrop” nut can be threaded on its end, to allow a cable to be attached.

In one — used when total load is the issue, not how far out it rides –- a special “teardrop” nut is attached to the threaded end of the GL. A high-strength shackle, or clevis, locks into the teardrop nut and attaches to a steel cable. At the other end of the cable, typically 6 feet or so higher in the tree, is a turnbuckle and another shackle linked to a galvanized lag bolt. Adding the upper connection creates a triangulated system with the tree, the cable and the GL forming the three sides. The normal load carrying ability of the GL can as much as double in this setup.

In the other system –- used when the load is carried further out on the GL or beyond — a special section of high-strength pipe gets slipped over the shaft of the GL. A steel loop or other connection is welded to the end of the pipe, and the cable attaches there. The longer the pipe, the longer the cable and the higher the upper lag bolt connection is set into the tree. In the most extreme cases, used on many of the treehouses at Michael Garnier’s Out-n-About resort, pipes as long as 6-feet or more extend the GL out beyond the foundation of a single-tree structure. The cables on these systems connect to another set of GLs higher in the tree, creating an incredibly strong and dynamic support system.

All of that, as you can guess, is likely more than you’ll ever need. Most do-it-yourselfers putting up smaller treehouses find themselves on the lighter side of the load equation.

But how light is light?

Long pipe extensions over multiple GLs reach out to cables beyond the rim of this treehouse platform. These cable “backups” support this treehouse high in a douglas fir at the Out-n-About resort in Oregon.

Calculating load can get complex, especially when you factor in the motion and wind forces a treehouse can face. If you even suspect the minimum GL load strength of 3,000 pounds might not be high enough, contact an engineer or a professional treehouse builder. Never play around with safety. If they suggest the teardrop nut backup system, then you should follow that advice and install it in the manner I describe below.

What might be easier for the DIY builder to spot is the need for a small extension on the GL. Usually, they’re needed when the trunk has a slight lean, and the beam that will ride above the GL needs to sit out toward the end of the shaft. Remember, you need to leave a few inches clearance between the beam and the tree to allow for future growth. Trees grow taller from the top, but they get stronger by adding width at all heights, so make sure you leave some room. Don’t be tempted to squeeze a fat beam on a GL by jamming it against the trunk.

What you need here is a custom extension, typically what’s knows as a high-strength “schedule 80” pipe with a heavy-duty welded loop on one end. You can usually buy these for about $30 from Michael Garnier. If he doesn’t have them, drop me a note and I’ll help you find a few. Unlike the GL itself, this is a fairly easy piece to make, so you might want to see if a local metal shop can get it done for you. The key things to remember here: The inner dimension of the pipe must exactly match the outer dimension of the GL’s shaft; The outer dimension must exactly fit into whatever bracket you’re using; The pipe must be the heavy schedule 80 type; and the welded loop on the end must be strong enough to handle the full load. Don’t make the pipe any longer than you need, typically 1 foot is enough. And remember to give it a good coat of rustproofing paint.

The width of this beam and the load on the GL required a custom pipe extension and a cable backup. Note how the same bracket is used to allow tree and beam to move. The angle of the cable headed back toward the trunk keeps the extension in place.

Installation on the GL is simple –- just slip the pipe over the GL’s outer shaft, make sure it’s all the way against the outside of the collar, and then install the bracket as you would if you were using only the basic GL. As you mount the beams and begin building the platform, make sure the pipe stays put and doesn’t slide off the GL.

Not long after the beams are in place, use your new platform, some ropes or a ladder to get 6 feet or so above the GL on the tree. Six feet is suggested because it’s way more than 2 (the danger zone, if you remember), and it usually allows for a cable to hang close to vertically, a better set up for strength. You can go a little shorter, or a few feet longer, if that’s what it takes to access a good spot on the tree.
That good spot will be as close to directly above the GL as possible, with no branches or trunk features that block the line between it and the GL below. One thing to be very careful about – make sure the upper point is behind an imaginary line that extends straight up from the loop on the pipe. If it is, the cable will hold the pipe extension on the GL as it pulls the end back toward the trunk. If it’s not, you need a longer extension. Don’t risk that sliding off once the treehouse is built.

Setting the top lag bolt is easy, especially now that you’re a veteran of GL installation. Depending upon the size of the treehouse, a 3/4- or 1-inch galvanized lag bolt is used. Galvanized is important, and most DIY folks won’t need more than a three-quarter inch lag to get the job done. Use a lag that allows for about 6 inches to be set into the tree, so an 8-inch bolt usually works. That leaves an inch for the shackle and washers you’ll install and another inch for growth.

You can drill this hole with a standard hardware store auger bit; a 5/8-inch version should do well when sinking a 3/4-inch lag. Be sure to put a piece of bright-colored tape on the bit at the depth you want to hit. This will tell you exactly when to stop drilling so you don’t do any more damage to the tree than necessary. Try to set the hole as close to level as possible, with a slight rise at the head of a degree or two still acceptable.

A lag bolt anchors the cable backup further up the trunk. Bolt, shackle, turnbuckle, clamps and cable all must be rated to handle the load, and must be intended for outdoor use.

Before turning the lag bolt into the tree, remember to put a couple of heavy-duty galvanized washers and the shackle over the shaft. Turn it in carefully with a heavy socket and a breaker bar and you’re set. Of course, always make sure the shackles and the turnbuckles you use are galvanized and rated to handle the load. Fastenal (www.fastenal.com) or a good local professional-grade supplier should have all of the hardware you need for this. Once again, if you have any question, or need some help finding stuff, feel free to drop me a note.

Between the GL extension and the upper shackle, you’ll need to install a length of high strength steel cable and a turnbuckle (for keeping the cable tight). Most builders use 3/8-inch galvanized steel “7-19” cable, meaning the cable is made of seven woven bundles, each with 19 strands of wire in them. In general, two wire clamps on each end are fine, but you should ask the vendor what the manufacturer recommends. When you install the clamps, be sure to put the big end (where the nuts go) over the tension side of the cable. The tension side is the one that’s headed to the other end, rather than the tail of cable that’s been looped around the end connection.

The turnbuckle can be at the bottom or top, based on how things lay out on your platform. You want the turnbuckle to be accessible (it will need to be tightened from time to time), but not so easy to access that a mischievous guest might decide to play with it. Open the turnbuckle about 80 percent of the way (making sure it’s even on both sides) before you put the system together.

Assembly is straightforward. Attach one end of the turnbuckle to the GL extension or the shackle at the top. Connect the cable to the other end of the turnbuckle and to the opposite end of the system (if you’re using the tear-drop nut, you’ll need a shackle on the bottom as well). Try to keep the cable ends even, and trimming it ahead of time with bolt cutters or a grinding wheel can save you some awkward work in the tree. Watch the ends -– the wire can be sharp so gloves are not a bad idea -– and carefully slide the clamps in place. Be careful not to over tighten the clamps, but make sure they’re good and tight (again, ask your vendor to what the manufacturer recommends). Make the cable as taught as you can by hand when you install it. Once it’s installed and the clamps are tight, turn the turnbuckle by hand or with a wrench and add just enough tension to the cable that all slack is gone. If you start to pull the end of the GL extension up, your getting too tight and transferring too much of the load to the top lag bolt. Once you’ve got it adjusted the way you want it, you’re done.

POST SCRIPT

The heavy-duty GL you saw installed in the pictures above now supports one of two main girders under a 240-square-foot treehouse platform in Ithaca, N.Y.

Treehouse construction, like most skilled trades, has a lot of science behind it and a lot of art in the application of that science. I tried to be thorough here, and give you a good sense of not only what to do, but also why you do it and what you need to understand before it’s done. That’s always been the way I like to approach any project.

If you’re confused by anything I’ve written, or anything in the pictures that go along with this, feel free to send me a note at johncarberry@hotmail.com. I’ll do everything I can to help complete the picture for you, and help you in your quest to find the joy and peace that only time in the trees can offer.

— John Carberry