Decay detection and structural assessment of a retained tree

A tree decay detection assessment is what you commission when a visual inspection has found something that changes the risk picture but cannot say how much sound wood is left. It sits outside the BS 5837 development survey entirely. A BS 5837 survey categorises trees for their suitability for retention alongside development. A decay investigation asks a narrower question, which is whether a particular stem or limb can be expected to hold up under foreseeable loading, and what that means for the consent application or the retention strategy.

The order matters. Visual assessment first, then targeted investigation only where the visual findings raise a question the eye cannot settle. Running a decay detection device on every mature tree on a site is a waste of the client's money and produces data nobody will interpret properly.

When a visual inspection is not enough

Most trees are resolved on visual evidence. The trigger for going further is a visible defect whose extent is hidden: a basal cavity, a shear crack, a bulge or rib indicating adaptive growth over an internal void, a historic branch failure with associated dysfunction, or a fruiting body of a decay fungus at a structurally important position.

A second trigger is proximity. The same defect on a tree over a lane used twice a week and a tree over a school entrance produce different decisions, and the investigation is justified where a more precise answer would change what you recommend. If removal is the answer either way, the instrument adds nothing.

A third is an application. Where a council has refused, or is likely to refuse, consent for works to a protected tree, a quantified condition report carries more weight than a description. The tree survey documentation supporting a planning application serves a different purpose, so keep the two reports separate rather than folding one into the other.

Fruiting bodies and what they tell you

Identification of the fungus is the single most useful piece of field evidence, because different species behave differently in the wood they colonise. Some cause a selective delignification that leaves a pale, fibrous residue and reduces stiffness gradually. Others cause a brittle fracture failure in which strength is lost faster than the outward form suggests. White rots, brown rots and soft rots do not fail in the same way, and a report that records "bracket fungus present" without naming the organism has not done the work.

Position matters as much as species. A bracket at 4m on a lateral limb is a limb problem. The same species at the root collar of a leaning stem is a stability problem, and stability problems are not resolved by crown reduction.

Absence of fruiting bodies means very little. Many decay fungi fruit seasonally or intermittently, and an inspection in February may find nothing on a tree that produced brackets the previous October. Record the season of inspection and say what that limits.

Decay detection equipment and its limits

Three families of tool are in common use, and each answers a different question.

None of these measures strength. They measure a proxy for the condition of wood, and the step from proxy to structural conclusion is a judgement made by the person holding the report. Sonic tomography in particular produces colour images that clients read as photographs of decay, which they are not. Say so in the report.

Drilling is invasive. A drill path admits air and can carry material inward, so the number of paths should be the minimum needed and the positions should be recorded. Where a stem is already compromised, the argument for not drilling it is a serious one.

Interpreting results against target occupancy

A residual wall thickness figure on its own decides nothing. The interpretation depends on the ratio of sound wood to radius, the geometry of the section, the height and weight of the crown above the defect, the exposure of the site, and what is beneath the tree and for how long each day.

Target occupancy is the part clients resist and reviewers should insist on. A quantified estimate of hours of occupancy beneath the crown converts an abstract probability of failure into a probability of harm, and it is the only way to justify retention of a defective tree over an infrequently used area. Record how the figure was derived, not just the figure.

Where a tree is being retained through construction, the assessment interacts with the protection strategy, because compaction and root severance change the loading picture within the design life you have just assessed. The practical sequencing of that sits alongside retaining a tree through a construction programme.

Risk assessment frameworks in use

Several structured frameworks are used in the UK, quantified and matrix-based, and the choice is less important than consistency and transparency. What a reviewer looks for is a stated framework, stated inputs, and a conclusion that follows from them. A report that names a framework and then reaches a conclusion the framework's own scoring does not support is worse than a report with no framework at all.

Two habits improve every report of this kind. State the inspection interval you are recommending and why, and state what would change your advice, because the second of those tells the client what to watch for.

Reporting to support a consent application

Where the tree is protected, a decay investigation is often the difference between refusal and consent. The report should stand alone for a tree officer who will not visit with you.

1. Identify the tree unambiguously by reference to a plan, a tag number and the Order schedule where one exists.
2. Describe the defect and the field evidence, with dated photographs.
3. State the investigation method, the number and position of measurements, and the equipment used.
4. Present the raw output as well as the interpretation, so the officer can form their own view.
5. Set out the options considered, including doing nothing and monitoring, with the consequence of each.
6. Specify the recommended works in the terms a contractor can price, cross-referenced to a proportionate pruning specification.

Officers refuse applications supported by conclusions they cannot check. Showing the working is not a courtesy, it is what makes the recommendation usable, and it is the same discipline that makes pruning specifications enforceable.

Monitoring instead of removal

The strongest outcome of a good investigation is often a retention plan rather than a felling licence. Reduction of the crown to lower the lever arm, removal of the target rather than the tree by re-routing a path, installation of a bracing system with a defined inspection regime, or simply a shorter re-inspection interval with agreed triggers.

Where retention forms part of a wider scheme, the tree's long-term future usually needs to appear in the Landscape Strategy as well, with the space around it protected in the layout rather than promised in a paragraph. A veteran tree retained with a 6m clear zone and a 15-year inspection schedule is worth more, in planning terms and in fact, than a replacement planted to discharge a condition.

One honest uncertainty: none of these tools tells you when a tree will fail, and practitioners who present them as though they do eventually meet a tree that fails anyway. The report's job is to make a defensible decision at a point in time, and to say when that decision should be revisited.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. Arboricultural reports of this kind should be prepared by a suitably qualified and experienced arboriculturist, and councils will normally expect professional authorship. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.