System Failures of Zip Line Hardware in Trees Expert Article

Trees are commonly utilized as support systems for zip lines, tree houses, and other structural applications. These arrangements can be more complex than traditional construction methods because they introduce a living organism into the system that is susceptible to disease, death, and decay.

In this article, forensic arborist Christopher Larson discusses the use of trees as structural support members and highlights several issues relevant to their failure.

Zip Line Hardware in Trees

Any installation of hardware into a tree should be carefully considered in order to reduce the likelihood of future failure. Considerations should include the selection of appropriate tree species, installation location on the tree, tree size, hardware selection, and installation techniques.

Drilling any hole into a tree creates a wound that opens the tree to infection and decay. The long-term integrity of the system is supported by proper planning and craftsmanship which limits wounding and promotes compartmentalization of decay. 

Trees, as living systems, are unique amongst construction materials, because they have the ability to grow, compartmentalize damaged tissues, and die. Proper care, planning, and inspection will provide the best possible chances for long-term structural safety.

An arborist should be consulted early in the construction planning process to evaluate the viability of proposed trees, and periodically after construction to perform follow-up inspections of the system.

Wood Decay

Trees have the ability to compartmentalize damage and limit the spread of decay when they are wounded. Research shows that the discoloration and decay that frequently results from the installation of dead-end and through-hardware is limited if the tree is in good health.

Several factors influence the degree of discoloration and decay, and the tree’s ability to compartmentalize the wound:

• Tree species and genetics
• Tree vitality
• Condition of wood penetrated by drill hole
• Installation time of year
• Size, depth, position, and number of holes drilled

The image above is a European beech showing discoloration associated with drilling a hole for through-hardware installation (29 years since installation). A more recent drill hole is evident at the bottom of the photograph: faint discoloration with the same shape as the dark drill hole discoloration.

Species Selection

Some trees have a more robust wound response than others; they can compartmentalize damaged/exposed tissues more effectively, thus limiting the spread of decay and damage. Trees that are good compartmentalizers are better candidates to have hardware installed versus poor compartmentalizers. 

Wood discoloration and decay associated with hardware installation varies with species, and the ability to compartmentalize can be different in individual trees depending on the condition of the tree.

Tree Size Matters

In most applications, the trunk is the most likely position for zip line anchors. Trunk diameter and wood strength should both be considered when selecting trees for anchor installation.

Methods and Installation of Hardware

When proper hardware is installed, the hole is typically drilled through the entire trunk, and a bolt is inserted with a washer and nut on the opposite end of the bolt secure it in place. For through-hardware, installation holes should be drilled no more than 1/8 inches larger than the diameter of the hardware to be installed. Holes should not be drilled closer together than the diameter of the trunk being drilled or 12 inches, whichever is less.

The installation site must be inspected to assure that hardware is installed into sufficient thresholds of sound wood. Furthermore, the standards of arboriculture state that when installing through-hardware, heavy-duty or heat-treated round steel washers should be between the bark of the tree and the nut. The washer should not be countersunk into the wood.

The image above shows a longitudinal view of discoloration and decay extending from drill hole for hardware installation (note countersinking of washers into wood)

Wood Poles

Wood poles, whether treated or not, can decay as readily as living trees. Wood poles used as part of any zip line system should be inspected for any type of incipient decays.

Inspections

It is critical that a detailed inspection be conducted on the tree before and after any hardware is installed. If the tree and its associated parts are in poor health or structure, the hardware installation in most cases should not occur, or alternate options should be considered.

The adverse physiological consequences of installing hardware by drilling holes in trees cannot be overlooked because any wound requires the tree to reallocate resources to compartmentalize the wound.

Trees with hardware installed require detailed inspections and assessments by a qualified arborist. Those inspections should involve a full tree risk assessment of the tree’s root flare, trunk, and canopy. The arborist should recognize and remove tree branches with preexisting defects that may break or fail onto zip lines or may sway into the pathway of the zip line corridor.

More so, the inspecting arborist should perform an aerial inspection of the areas of the tree where the hardware has or will be installed.

Forensic Tree Investigations

The experts at Robson Forensic are frequently retained to investigate injuries involving the design, maintenance, and operations of zip line courses. Within the context of a zip line investigation, Robson Forensic is positioned to provide a thorough and comprehensive investigation by addressing every aspect of the case, from structural engineering and material failures, to recreational programming and tree forensics.

For more information submit an inquiry or call us at 800.813.6736.

Featured Expert

Christopher Larson, BCMA, TRAQ

Board Certified Master Arborist & Tree Expert

Chris Larson is a Board Certified Master Arborist with over 20 years of experience in urban forestry, arboriculture, and landscape design/maintenance. He applies his expertise to a broad range of… read more.