Decay Testing NSW — IML Resistograph + ISA TRAQ Workflow for Defensible Decay Findings

The Resistograph measures the mechanical resistance offered by wood as a 3mm drill is advanced through it at a constant feed rate. The output is a continuous graph of resistance against drill depth (every 0.01mm). Sound wood reads as steady high resistance. Cavities read as zero resistance. Decayed zones read as reduced resistance, often with characteristic patterns that indicate the type and extent of decay. Interpretation requires arboricultural training — the graph is data, not a conclusion. The conclusion comes from an AQF5 reading the graph against the species, the visual indicators, and the structural context.

Minimally. The Resistograph uses a 3mm drill (about the size of a small nail). The drill hole closes within weeks through normal cambial response and is sealed by the tree’s compartmentalisation processes. For trees in good general health the test is effectively non-destructive. For trees already in poor condition with compromised compartmentalisation, the test is one of the few diagnostic tools that doesn’t make the condition worse — especially compared with the alternative of removing the tree on visual evidence alone and discovering it was salvageable.

No. The Picus 3 sonic tomograph is a different decay-detection tool that uses sound-wave travel time across the stem to map internal density — it produces a colour tomogram of the cross-section rather than the depth-graph an IML Resistograph produces. Both tools have their merits and their limitations; some firms use both, some use one. We use the IML Resistograph plus visual + probing + ISA TRAQ workflow. For applications where sonic tomography is specifically required (e.g. some heritage tree reviews where the conservation plan calls for it), we can refer to a specialist with Picus equipment and integrate the findings into our TRAQ-aligned report.

Varies. A single suspect zone on a single stem might need 2–3 tests at different angles to characterise the extent of compromise. A larger heritage tree with multiple potential defect zones (main stem, codominant unions, major branch attachments) might warrant 6–10+ tests. Test locations are selected by the AQF5 based on the visual + probing findings — we don’t drill randomly, and we don’t drill more than the assessment justifies. The number of tests is agreed at scoping based on the visual pre-assessment.

Yes. Aerial Resistograph testing on upper stems and major branch attachments is delivered as part of our in-house aerial inspection capability. Most consulting arborists outsource aerial work or don’t offer it; we deliver it directly. Particularly relevant for heritage trees where upper-stem cavities, codominant union compromise or major branch attachment defects need to be diagnosed without dismembering the tree.

That’s what it’s structured to do. Reports include the raw Resistograph graphs as data appendices, photo-stamped test locations, GPS reference, ISA TRAQ-aligned risk classification, and the integrated remedial recommendation — signed by an AQF Level 5 Consulting Arborist. Our principal also delivers expert witness work on tree-related disputes, so the reporting format is shaped by experience defending findings on cross-examination rather than just satisfying a council planner.

ISA TRAQ (Tree Risk Assessment Qualification — the framework decay findings are interpreted within), Arboriculture Australia Minimum Industry Standards (work practice baseline), and AS 4373 (Pruning of Amenity Trees) for any remedial pruning specifications that flow from the testing. AS 4970-2025 applies where the testing intersects with retained-tree assessment on development sites.