Managing tree decay and risk through Resistance Drilling

Both trees and palms are susceptible to internal decay which can lead to an increased risk of failure. This may occur due to a disease, fungal rot or even wildlife which results in damage or loss of woody tissue which can reduce the strength of a tree's cross section. Just because a tree has decay doesn’t necessarily mean that it will fail, the mode of failure is often a product of a number of factors. It is therefore crucial to have trees assessed by experienced and qualified arborists to ensure the risk to human life, infrastructure and the surrounding landscape is acceptable.

There are many ways that an arborist can assess a tree. One common practice is to complete a tree survey or conduct a risk assessment on each individual tree to identify potential risks or diagnose any existing issues or diseases that may result in an increased likelihood of failure. The International Society of Arboriculture classes this as a ‘Level 2: Basic’ risk assessment.

However, in some instances, visual assessments have limitations. Technical instruments are therefore used to detect, quantify and map internal decay. A common practice to detect decay and cavities in trees and timber is through Resistance Drilling. This is known as ‘Level 3: Advanced Assessment.’

Resistance Drilling Origins

Since its development in the 1980s, thin needle resistance recording has been the most standard practice for gathering data globally on urban trees and wood to assess hazard and stability levels.

Originally, resistance drilling was developed to measure latewood  density in oak tree rings to reconstruct previous climates. With this, experts observed that this method was able to detect defects in timber and trees.  

Since it’s development, researchers have conducted various experiments on spring-driven recording mechanisms to prove their effectiveness. However, many did not meet the requirements needed for it to be considered a reliable measurement tool.

In the early 1990s, resistance drilling devices were modified and were incorporated with electronic regulation and recording when it was officially labeled as a Resistograph® - an electronic resistance drilling device that provides a linear scaled profile, revealing the density of wood and its changes accurately. For clarity, Resistograph® is now a trademarked name for a brand of resistance drill made by Rinntech. At Canopy Consulting, we use an IML-RESI PowerDrill ® PD-400 as we believe it has increased technological advantages.

Many specialised arborists and wood inspectors utilise this approach to uncover hidden flaws or cracks or quantify visible decay. It is important to remember that experience by the assessor is of the utmost importance as the information provided needs to be given context in relation to the tree and its environment. 

How Resistance Drilling  Works

The IML-RESI PowerDrill ® PD-400 is an electronic resistance drill which drives a small diameter (3mm) spade bit to a maximum depth of 400mm into a tree. As the bit penetrates the drill’s resistance is simultaneously plotted as a graphic profile which is used to determine the internal strength of the wood.  

Decay in trees is a natural biological process in trees and commonly occurs without causing structural weakness. However, depending on the volume and position of decay, this can lead to an increased likelihood of failure and potential risk to persons and property. 

The Ri/R, or One-third rule, (Mattheck, et al., 2015) has been applied as a general threshold evaluation to assess the structural integrity of a stem cross-section at the nominated test height(s). The rule suggests that as soon as the outer intact shell wall (Ri) of a hollow or decayed tree stem is less than one-third of the local radius (R), the stem is significantly more predisposed to failure under wind loads. 

The rule has historically been applied as a clear demarcation of when a tree may become ‘unsafe’ (Rinn, 2018). The rule has also been contested as inaccurate (Gruber, 2008) though has been confirmed by a number of other studies to show a correlation between a 30% residual shell wall and increased probability of failure (Dahle, et al., 2017). (Mattheck, et al., 2015) conclude that the one-third rule should ‘never be applied dogmatically but always in light of prevailing conditions. (Dahle, et al., 2017) suggest the judicious application of these values to trees in urban areas.

Material properties of wood, including elasticity and density, vary by species and growing conditions, and though many studies reference these wood properties, most of the work comes from samples taken from forest or plantation trees, and defect free sections of lumber (Dahle, et al., 2017). Correspondingly, (Rinn, 2018) illustrates the limitations of the Ri/R rule in the context of the ‘urban tree’ which is typically non-circular in cross section with internal decay or defects not always located concentrically. 

Limitations of Resistance Drilling

Despite its usefulness, there are still questions about its accuracy. In an article published by Western Arborist by Frank Rinn, he explained that most inaccurate evaluations of trees and timber came from misreading of the measured profiles. The measured values must have a clear relationship to the actual (physical) characteristics of the material examined.

Density or strength are well defined material qualities that may be utilized to characterize key "wood conditions" features. The features are quite obvious in the instance of detection of rot in wood: Wood breakdown due to degradation of the fungus and is often indicated by material weight loss or density change. 

Furthermore, in order to retrieve the correct data, it is highly important to have the profile interpreted immediately on the spot for several reasons. This is to avoid additional drilling in cases wherein surrounding parameters influence the profile or data retrieved.  

Therefore, by analysing data from Resistance drill testing and in combination with a thorough site and tree assessment; and the application of arboricultural knowledge including species characteristics, an educated assessment of the likelihood of failure, and therefore risk, can be formulated.

If you are concerned about possible decay in your trees, reach us at Canopy Consulting through 0432-633-402 or visit


Dahle, G. A., James, K. R., Kane, B., Grabosky, J. C., & Detter, A. (2017). A Review of Factors That Affect the Static Load-Bearing Capacity of Urban Trees. Arboriculture & Urban Forestry, 43(3), 89-106.

Gruber, F. (2008). Untenable failure criteria for trees: I. The residual wall thickness rule. Arboricultural Journal(31), 5-18.

International Society of Arboriculture. (2017). Tree Risk Assessment Manual (Second Edition ed.). Champaign, Illinois: International Society of Arboriculture.

Mattheck, C., Bethge, K., & Weber, K. (2015). The Body Language of Trees - Encyclopedia of Visual Tree Assessment (1st Edition ed.). Karlsruhe, Germany: Karlsruhe Institute of Technology - Campus North.

Rinn, F. (2018). The Visual Tree Assessment One-Third Rule: Frequently Applied, but Mostly Irrelevant. Georgia: Society of Municipal Arborists.

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Canopy Consulting
Macquarie Park NSW 2113
1300 122 667
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