Resistograph Tree Decay Detection — Microdrill Internal Inspection
Invasive Tree Decay Detection
The IML Resistograph is a precision instrument used for invasive tree decay detection, providing direct, measurable data on the internal condition of wood that cannot be obtained through visual inspection or non-invasive techniques alone. Urban Tree Management uses the Resistograph to assess the structural integrity of trees where decay is suspected, to confirm and refine the findings of sonic tomography investigations, and to provide objective evidence of internal condition for risk management, insurance, planning, and legal purposes across Greater Manchester, Lancashire, Cheshire, Merseyside, and nationwide.
How the Resistograph Works
The Resistograph drills a very fine needle into the wood at a precisely controlled speed, simultaneously measuring the resistance encountered as the needle advances through the stem or branch. Dense, sound wood offers high resistance. Decayed, degraded, or hollow wood offers significantly lower resistance — and the instrument plots these readings in real time as a continuous resistance profile across the full diameter of the tree at the point of drilling.
The instrument measures both the rotational resistance required to keep the needle drilling and the axial pushing force required to advance it. Different types of decay affect these two measurements in different ways, and analysing both together allows a more detailed and differentiated picture of the internal condition to be built up than either measurement alone would provide. The resistance profile is displayed on a digital screen in the field and can be exported as a graphical image for inclusion in the inspection report.
What the Resistograph Measures
The primary application of Resistograph testing in arboriculture is measuring the thickness of the residual wall in trees with suspected internal cavities or decay — that is, determining how much sound, structurally competent wood remains between the outer surface of the stem and the decayed or hollow interior. This measurement is critical to assessing whether the remaining wood is sufficient to support the tree's own weight and withstand wind loading without structural failure.
Resistograph data also allows the exact location and depth of decay to be pinpointed within the stem, the extent of decay to be measured objectively rather than estimated, the type of decay to be differentiated based on the resistance profile, and changes in internal condition to be tracked over time through repeat testing at the same drilling points. Where sonic tomography has already been carried out, Resistograph drilling at the sensor locations provides a direct physical confirmation of the tomographic image, significantly increasing confidence in the findings and the robustness of any management recommendation that follows.
Reporting and Analysis
All Resistograph investigations carried out by Urban Tree Management are documented in a professionally produced report that includes the resistance profile graphs, the surveyor's interpretation of the findings, and clear management recommendations based on the data.
Reports are produced in a format suitable for submission to local planning authorities in support of tree work applications, for use in insurance claims and legal proceedings, for inclusion in duty of care records maintained by landowners and facilities managers, and for use by tree surgeons commissioned to carry out works based on the findings.
When is Resistograph Testing Required?
Resistograph testing is typically commissioned in one of three situations.
- The first is as a follow-up to a visual tree inspection where signs of decay are present but the external evidence is insufficient to determine the structural significance of the decay.
- The second is as a confirmatory test following sonic tomography, where drilling at the sensor locations provides physical data to verify and refine the tomographic image.
- The third is for periodic monitoring of trees where decay has previously been identified and a baseline has been established, allowing the rate of progression to be tracked over time and management decisions to be reviewed as the tree's condition changes.
In all cases Urban Tree Management will advise on the most appropriate level of investigation for the specific tree and situation before any testing is carried out.
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