The foundational 12-point summation system for evaluating tree failure risk in urban environments.

The Matheny & Clark Hazard Rating is calculated by summing three scores: Failure Potential + Size of Part + Target Rating = Hazard Rating (3–12). This additive model was a deliberate design choice by the original authors — by treating each component as an independent 1-to-4 integer, the system produces a linear hierarchy where every additional point of risk increases priority by the same increment. A tree with a Hazard Rating of 10 is unambiguously higher priority than one rated 7, regardless of which components contributed the score. This simplicity is the method's greatest strength for municipal tree management: it allows a city forester to sort an inventory of 10,000 trees by a single numeric column and allocate pruning and removal budgets from the top down. Unlike qualitative matrix systems that produce categorical outputs (Low, Moderate, High), the 12-point scale provides granular differentiation — a tree scoring 9 can be distinguished from one scoring 8, enabling precise budget allocation when resources are limited.

The total score range of 3 to 12 divides naturally into four action thresholds that guide management decisions. Trees scoring 3–4 represent low hazard — structurally sound trees in low-use areas that require only routine monitoring on a standard maintenance cycle. Scores of 5–7 indicate moderate hazard where defects are present but failure is not imminent; these trees benefit from increased inspection frequency, scheduled pruning, or installation of support systems. The high hazard range of 8–9 signals that significant structural defects coincide with frequently or constantly occupied target zones, demanding priority mitigation within a defined timeframe. Extreme hazard scores of 10–12 require immediate abatement — typically removal, structural cabling and bracing, or target restriction — because the combination of imminent failure potential, large part size, and constant target occupancy creates an unacceptable risk. In the digital form, the inspector selects each component from a dropdown and the Total Hazard Score field calculates the sum automatically, enforcing the valid 3–12 range and eliminating the arithmetic errors that plagued paper-based field workflows.

The Matheny & Clark method deliberately separates tree risk into three orthogonal dimensions, ensuring that each factor contributes independently to the final score. Failure Potential evaluates the tree's structural condition: an arborist examines roots, trunk, and crown for defects such as decay, cavities, cracks, codominant stems, included bark, and root plate instability, then assigns a rating from 1 (no significant defects, failure unlikely) to 4 (failure imminent or already in progress). This assessment is informed by the Visual Defect Inventory completed earlier in the inspection but is ultimately a professional judgment — two trees with identical defect checklists may receive different Failure Potential ratings based on species-specific wood properties, defect interaction patterns, and site wind exposure. In the form, the Failure Potential field presents four options that correspond directly to the rating scale, and the inspector selects the one that best characterizes the most critical failure scenario observed.

Size of Part rates the physical magnitude of the tree component most likely to fail, measured by diameter in inches. A small branch under 6 inches (rating 1) poses minimal energy even if it falls on a target, while a whole-tree failure exceeding 30 inches diameter (rating 4) represents catastrophic impact capable of destroying structures and causing fatalities. This component ensures that a large, structurally sound tree near a target is rated differently from a small, severely defective one — mass and impact energy matter independently of structural condition. Target Rating assesses how often people or high-value property occupy the potential impact zone beneath or adjacent to the tree. An identical defect on an identical tree scores differently depending on whether it overhangs a wilderness trail visited once a week (rating 1) or a school playground occupied eight hours a day (rating 4). This component connects the biological assessment to the human consequence, making the final Hazard Rating a true risk metric — combining probability, magnitude, and exposure — rather than merely a condition score.

Failure Potential (1–4)

Size of Part (1–4)

Target Rating (1–4)

The Matheny & Clark method requires a Level 2 Basic Visual Assessment — a systematic 360-degree walk-around inspection conducted from ground level without invasive testing tools. The inspector examines the tree from the root collar upward, progressing through three anatomical zones, and documents every observable structural defect using the form's multi-select checklists. This defect inventory serves as the evidentiary basis for the Failure Potential rating: the type, severity, location, and combination of defects observed determine whether the tree receives a 1, 2, 3, or 4 for that component. The checklist approach ensures that inspectors follow a consistent observation sequence regardless of experience level, reducing the risk of overlooking a critical zone. Each defect zone in the digital form presents a predefined list of indicators drawn directly from the original 1994 ISA evaluation form, with an option for photo documentation of each finding.

For a comprehensive overview of all supported inspection methods, visit the standards library.

The Matheny & Clark 12-point system was the dominant tree risk assessment method in the United States from its publication in 1994 until approximately 2011, when the International Society of Arboriculture introduced the Tree Risk Assessment Qualification (TRAQ) as a qualitative replacement. TRAQ uses a likelihood-by-consequences matrix that produces categorical risk levels (Low, Moderate, High, Extreme), aligning with broader risk management frameworks like ISO 31000 and ANSI A300 Part 9. Despite this transition, Matheny & Clark remains in active, widespread use for several compelling practical reasons that keep the 12-point system relevant in modern arboriculture.

First, many municipal tree ordinances and utility vegetation management contracts were written around the 12-point numeric score and have not been updated. A city that references "Hazard Rating 8 or above requires priority abatement within 30 days" in its municipal code cannot switch to TRAQ without a legislative or contractual revision — a process that can take years. Second, the numeric scoring system provides a linear ranking that is inherently sortable — a property that qualitative categories do not share without additional weighting rules. For large-scale urban tree inventory programs where thousands of trees must be prioritized against a fixed annual budget, the ability to sort by a single integer column and draw a budget cutoff line remains operationally superior to categorical groupings. Third, the method's simplicity makes it well-suited to training programs and community-based volunteer tree monitoring efforts where participants may not hold ISA Certified Arborist credentials. The complete inspection form captures species identification, DBH (Diameter at Breast Height in inches), age class, vigor class, site characteristics, and a structured defect checklist alongside the three-component hazard evaluation, producing a comprehensive tree record that serves both risk management and long-term canopy inventory purposes.

The form also captures recommended abatement actions — Prune, Remove, Cable/Brace, Monitor, or None — linking the hazard score directly to a management response. This closed-loop design ensures that every inspection produces an actionable output, not just a score. The abatement recommendation, combined with the numeric hazard priority, feeds directly into work order systems and capital planning models.

The standard is published through the International Society of Arboriculture (ISA).