Whether you're a DIY enthusiast looking to install your own backyard zip line or a professional in the recreational industry, understanding zip line slope and sag requirements is crucial for safety and effective operation. These two components determine the profile your zip line needs to follow to ensure an exciting, yet safe, ride from start to finish.
In this guide, we’ll outline the basic concepts of zip line slope and sag, which serve as the foundation for calculating your ideal setup.
Disclaimer: The content provided in this article is designed for informational purposes only and is intended for use in backyard zip line installations under 1,000 feet in length. While we have made an effort to provide accurate information, some of the concepts have been approximated for the sake of simplicity and understanding. This guide does not replace the expertise of an engineer or an industry professional. If you are planning to install a zip line, it is strongly recommended to consult with such a professional to verify your plans and ensure the safety and integrity of your installation. Always remember, safety should always be your primary consideration in any recreational activity.
The slope of a zip line refers to the vertical drop from the upper anchor to the lower anchor. Ideally, the slope should be between 3% and 6%, meaning that for every 100 feet of horizontal distance, the cable should drop 3 to 6 feet in elevation.
Use this simple formula to calculate your zip line slope:
Slope (%) = (Vertical Drop ÷ Cable Length) × 100
|
Cable Length (ft) |
Recommended Drop (ft) |
Approx. Slope (%) |
|
100 |
3–6 |
3–6% |
|
150 |
4.5–9 |
3–6% |
|
200 |
6–12 |
3–6% |
For an in-depth walkthrough of how to calculate your anchor heights and see how slope affects ride speed, watch this short instructional video.
When the slope is at 3% or less, riders are likely to lose momentum before reaching the end of the zip line, an effect sometimes referred to as a “gravity brake”. If the slope is closer to 6%, riders tend to maintain higher speeds all the way to the end, requiring some form of braking system such as a bungee brake or spring stop to manage the final slowdown.
The sag of a zip line, which reflects how tight the cable is, refers to how much the line dips when supporting its maximum intended load. A reliable rule of thumb is to keep sag at 2% or more of the total zip line length. For example, a 100-foot zip line should sag at least 2 feet at its lowest point when loaded with the maximum weight.
Use this quick formula to calculate zip line sag:
Sag (ft) = Cable Length × 0.02
|
Cable Length (ft) |
Minimum Sag (ft) |
|
100 |
2 |
|
150 |
3 |
|
200 |
4 |
To check your zip line sag, measure the cable height at its lowest point both without any load and with the maximum intended load. If the difference between these two heights is less than 2% of the total line length, the cable is too tight, which can overload your hardware or anchors.
Always make sure your backyard zip line setup allows enough sag to balance performance and hardware durability.
Remember to maintain at least 7 feet clearance over the entire zip line avenue when the line is supporting the heaviest riders. This ensures safety for riders and protects the integrity of your zip line.
Moreover, consider any elevation change from the starting point to the ending point. Use tools such as a site level, laser level, or GPS to measure this difference, then add or subtract that value when you calculate anchor heights for your backyard zip line setup. Confirm the clearance at the lowest point under load, and recheck after any tension adjustments.
For more information on industry-recommended zip line design and performance standards, refer to the ASTM F2959 Standard Practice for Aerial Adventure Courses on the ASTM International website.
When choosing a location for your zip line, finding a landscape with a natural slope of 3% to 6% will make installation much easier. Also, if you plan to build launch or landing platforms, construct them approximately 5 to 6 feet lower than where the cable is anchored. Ideally, install the zip line and test it before building the platforms. It’s much easier to make adjustments to the zip line cable height or tension than to your platform.
Before letting anyone ride, always perform a Weight and Speed Test. There are many factors that impact a rider’s speed, and these guidelines should be thought of as a starting point for planning purposes, not as a guarantee for safe performance. The maximum allowable slope without a professionally engineered braking system is 3%. Any slope over this and up to 6% should never be attempted without appropriate braking measures. A high-speed collision with the endpoint can cause serious injury or even be fatal.
Before using your zip line, always perform a Weight and Speed test to confirm that your setup performs as expected. Factors such as slope, line length, rider weight, and braking system all affect speed and stopping distance, so it’s best to treat these guidelines as a starting point for your own adjustments.
For a zip line without a professionally engineered braking system, the maximum slope should not exceed 3%. Slopes between 3% and 6% are possible only when an appropriate braking system—such as a bungee brake or spring stop—is installed and tested.
Completing a few trial runs at different weights helps verify that your backyard zip line setup maintains a comfortable and controlled ride from start to finish.
