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With careful planning, some math, and a little help, you’ll forge memories your kid will never forget.

A while back, my son and I started talking about building a zip line in the backyard. He was pretty into the idea, so I started researching how to best go about it. While I wanted to create an epic experience that he’ll remember forever, my biggest concern was that we create something safe. Initially, I started looking at zip line kits—some as low as $100—but I wasn’t prepared to put my son’s life in the hands of a low-budget system. They seemed awfully inexpensive, and I realized I needed to know more about what I needed, as well as what I wanted.

Eventually, I purchased this kit from Zipline Gear, with some extra components to round out our backyard setup. But before I got to that point, there was a lot more careful planning than I anticipated. One of the advantages of going with a kit from a company like Zipline Gear is that you’ll have a resource to help answer questions that come up—both during planning and once you get started putting it up. I can’t stress how helpful their support was during this process. You may be able to shop each component of the system and save some money, but then knowing that each part is designed well enough for your planned zip line is all on you. Plus, the kits come with an informative instruction booklet.

Here’s how to build your own zip line—and make sure it’s safe.

Before getting started, it’s important to learn some of the terms you’ll need to know, in order to plan your zip line.

In the planning stage, you’ll just need a couple of tools to get started.

Look for trees that are healthy and at least 12 inches in diameter at the attachment point, which needs to be on the main trunk. Avoid trees with dead branches, rot, or hollow sections. The tree should be rooted in firm soil—steer clear of loose, sandy, or wet, swampy soil.

Poles or posts, likewise, need to be 12 inches in diameter and sunk into the ground 4 feet or 10 percent of the pole’s length plus 2 feet—whichever is greater. Poles should also be set in concrete, at least 6 inches thick around the pole. If you plan on using poles, talk to an engineer to be sure your pole can handle the forces applied to it.

You can set up a zip line on a flat ground, with a higher starting point, and ending on the ground. But this gets more difficult as the length of the run gets longer. Ideally, you’ll want a gentle slope with the starting anchor/tree at a higher elevation that the ending one. Once you’ve selected your trees, measure the distance between them—you’ll need this to calculate the cable slope or drop, the cable sag, and the heights of your start and end points.

We’ll use a zip line length of 100 feet as an example. You’ll want to multiply the length by 3-6 percent, 3 being the maximum slope you can run ending on the ground by putting your feet down or using a simple passive brake that mounts on the cable. This is what is typically used on playground zip lines—basically a spring on the cable that prevents a sudden stop. Anything above 3 percent, up to 6, will require a bungee braking system or a trolley with a brake the rider activates.

So, your slope in this case will be 6 feet.

Again, we’ll use a length of 100 feet as an example. Sag is important to get right for a couple of reasons. Having the right amount of sag helps prevent too much tension being applied to the trees, and it helps create a gentle up slope as the rider approaches the end of the run, so you don’t have to rely solely on braking to stop.

So, your sag in this case will be 2 feet.

If you’re setting up a zip line on flat ground, your elevation change is easy—it’s 0. If not, you’ll need to use a sight level to calculate the elevation change between the start and end points. Standing at the starting point, mark your tree with painter’s tape, paint, or something easily visible. Have a friend or helper next to the tree at the ending point—they may need a ladder. Look through the sight level and tilt it until the bubble inside lines up with the center horizontal line. Have your friend mark the tree at that point—you’ll need to direct them higher or lower, until their mark lines up in the sight level.

Measure from the mark on the starting anchor to the ground, and from the mark on the ending anchor to the ground. Subtract the start from the end. As an example, we’ll use 6 feet for the start and 12 for the end.

So, your elevation change in this case will be 6 feet.

Now you have all the numbers you’ll need to calculate the starting and ending anchor heights. Your ending anchor height will be your sag, from above, plus 7 feet.

So, your ending anchor in this case is 9 feet.

Your starting anchor will be your line slope from above, plus your ending anchor height, minus your elevation change, which will look like this:

So, your starting anchor height in this case is 9 feet. Note that the anchor heights are measured from the ground at each tree, and in our case, the starting tree is already 6 feet higher than the ending tree.

Now, as runs get longer, the sag increases. The formulas above account for this to a degree, but if your ground doesn’t run on a constant grade, meaning there are high spots in the middle, you may need to adjust both anchors higher to account for this—if you have to do this, it will become evident when you put your zip line up and weight test it. For this reason, it’s best to leave any platform building until the zip line is fully installed.

We built our zip line with a 6 percent slope, so we definitely needed brakes. There are two options here. One is a passive bungee block, which is basically a chunk of rubber that slides on the zip line cable. This block is connected to a long bungee cord, anchored to another tree alongside the main cable. When the rider hits the block, the cable stretches, slowing them down, and then pulling them back until the bungee relaxes. These work great, but it causes you to stop farther from the tree, so landing on a platform—unless it’s free-standing—isn’t really possible.

The second braking option is having a rider-operated brake on the trolley. They recommend these for riders ages 9-10 and up. You need to pull down on the brake with your weight to activate it. We carefully tested the brake with my 7-year-old, using a safety line from the ground to be sure he wouldn’t pick up too much speed and would be able to use the brake. He did fine and was able to moderate his speed very well. I would urge you to be extra cautious when using active brakes of this type with younger children.

Assuming you still have your trees marked from the planning stage, these are the basic tools you'll need to install a zip line:

And these are the zip line components. My kit came with:

Tree savers are optional but highly recommended. I made mine from scraps of pressure-treated lumber with grooves cut in them, but you can buy plastic Tree Savers that will last longer.

That’s it, you’re ready to ride.

To use your zip line, you’ll need a trolley, a harness, lanyard, a carabiner, and a brake. That should all be included if you get a kit, except for maybe the brake. My kit came with a pretty standard trolley, and I got a BrakeHawk brake that attaches to the trolley frame.

The brake slips over the frame and is held in place by a pin. To activate the brake, you wrap you hand over the top of the frame and pull down, or pull the brake tether hanging from the back.

You’ll need to attach the tether to the harness through the loop in the front, like this, and clip a carabiner on the other end.

Now the trolley and harness are ready. Place the trolley over the cable and clip the carabiner through it. The trolley won’t stay upright unless there is a little weight on the lanyard.

Now, you’re ready to have fun on your zip line.

Note: There are trolleys that attach permanently to the cable, and these often have T-handles as well as loops for a carabiner. I chose not to go this route as I didn’t want to have what they call an attractive nuisance—something that draws people to your property to horse around with when you’re not there. When we’re done using the zip line, we take our gear inside so no one else can use it.

The longer your zip line, the more likely you’ll need to start or end high enough to require a platform. If you’re planning on building a platform, wait until you have the zip line up before you build it. Platforms for both starting and end points should be 5 feet below the height of the cable. Depending on how long your anchor sling is, you can get away with a smaller platform, about 4.5 inches square for your starting point. I found this size big enough to fit both me and my son without one of us falling off.

I didn’t put a railing on my platform as it’s only 6-7 feet high, but you should consider railings on both sides of higher platforms. Ending platforms will need to be longer, as the cable sling, turnbuckle, and stop block may be 5 feet or more away from the tree—at that distance, the platform will need to be at least 8 feet long. If you have railings on this platform, it will need to be wide enough so that a rider won’t risk banging into them as they land.

There are different ways you can build a platform that will work. I wanted to be sure the tree wouldn’t break mine up as it grows, so I looked to treehouse hardware for inspiration to build my platform. I also used salvaged deck lumber for the platform frame.

In many cases, particularly on shallow sloped zip lines, you can to engineer a landing on the ground, where the rider is close enough to simply put their feet down when they stop. Another option is a simple, free-standing platform. It can be 3-4 feet square and at a height that falls 5 feet or so below the main cable. The advantage of these platforms is that you can move them to mow the lawn or farther down the line if your riders get braver and go faster.