Tree Protection Zones in North Bay Construction Projects

What is a Tree Protection Zone?

A Tree Protection Zone (TPZ) is the designated area around a tree — or group of trees — that must be protected from construction activity to prevent root damage, soil compaction, and physical injury to the canopy or trunk. It is not a suggestion. In most North Bay jurisdictions, it is a condition of permit approval.

The TPZ is typically measured as a radius from the trunk center. The default is a radius equal to 1 foot per inch of trunk diameter at breast height (DBH), with a minimum of 8 feet and a maximum of 20 feet unless otherwise specified by a project arborist. A tree with a 24-inch DBH, for example, would have a minimum TPZ radius of 24 feet.

Standard TPZ Calculation

TPZ radius (ft) = DBH (inches) × 1 ft
Minimum 8 ft · Maximum 20 ft (unless arborist specifies otherwise)

TPZs apply to all trees meeting the jurisdiction's protected size threshold — regardless of species, condition, or perceived value. A stressed or young tree still inside the TPZ is still protected.

Within the TPZ, the following activities are generally prohibited unless explicitly authorized by a project arborist:

Parking or storing vehicles, equipment, or materials
Dumping concrete washout, paint, solvents, or chemicals
Grading, trenching, or any excavation within the critical root zone (CRZ)
Reworking or compacting soil
Pruning or removing any part of the tree without a qualified arborist assessment
Removing or compacting mulch or leaf litter layer

North Bay Jurisdiction TPZ Requirements

The table below summarizes current TPZ and tree protection requirements across major North Bay jurisdictions. This is a practical summary — always confirm with the issuing jurisdiction before relying on these figures for permit planning.

Jurisdiction

Permit Triggers

TPZ Standard

Arborist Report

Marin County
Marin County Code, Chapter 22.17

Removal, pruning >25% crown, or any work within 100 ft of protected tree on unincorporated parcels

1 ft per inch DBH; minimum 15 ft radius from trunk center

Required prior to permit issuance; specify TPZ boundaries and mitigation

Sonoma County
Sonoma County Tree Ordinance, Ord. 6091

Tree removal, grading, or construction within 100 ft of protected tree

1 ft per inch DBH; minimum 10 ft radius; project arborist may require larger

ISA TRAQ report required; include TPZ plan, root protection measures, and monitoring schedule

Napa County
Napa County Code, Tree Preservation Ord.

Any grading or construction within 50 ft of heritage tree

Minimum 15 ft radius; project arborist may specify CRZ-based calculation

Heritage tree report required; must identify all protected trees and TPZ boundaries

City of Santa Rosa
Santa Rosa Municipal Code 17.08

Development, subdivision, or any grading within 100 ft of protected tree

1 ft per inch DBH radius; minimum 15 ft

Tree inventory and TPZ plan submitted with entitlement application

City of Petaluma
Petaluma Municipal Code 18.70

Any work within critical root zone of protected tree

CRZ defined as 1 ft per inch DBH; arborist may specify additional buffer

Tree protection plan required; arborist inspection at permit final

City of San Rafael
San Rafael Municipal Code 12.04

Tree removal permit or any grading within 50 ft of heritage tree

1 ft per inch DBH radius; project arborist required on all heritage tree projects

Pre-construction survey and TPZ installation verification required

City of Novato (MCOSD)
Marin County Stormwater WCOSS guidelines

Any land disturbance within 50 ft of creek setbacks or mapped heritage oak

1 ft per inch DBH; minimum 15 ft radius; creek setback may supersede

Oak woodland assessment required for projects affecting heritage oaks

City of Sonoma
Sonoma Municipal Code 13.20

Grading, development, or work within 50 ft of protected heritage tree

1 ft per inch DBH minimum; heritage trees default to 15 ft minimum

Heritage tree report with TPZ boundaries required at permit submittal

City of Healdsburg
Healdsburg Municipal Code 17.16

Any construction, demolition, or grading within 50 ft of protected tree

Project arborist to define TPZ based on species, age, and site conditions

ISA arborist report required; TPZ plan must be approved prior to grading permit

City of St. Helena
St. Helena Municipal Code 17.42

Any removal, grading, or construction within TPZ of protected tree

1 ft per inch DBH; minimum 10 ft; city may require larger for heritage species

Tree protection plan required; post-construction inspection by city arborist

Note: Protected tree thresholds (size in DBH required before permit is triggered) vary by jurisdiction and species. Refer to the North Bay Tree Ordinance Reference for protected species lists per jurisdiction.

Common TPZ Compliance Failures

Violations are discovered during city inspections, at final inspection, or — more expensively — when a tree dies two to three years after construction and the liability lands on the property owner. The most common failures we see on North Bay projects:

Improper installation — or no installation at all

TPZ fencing is placed at the drip line rather than at the calculated radius. Or it's installed late — after the concrete crew has already driven through the root zone. A TPZ is not valid if it's not in place before any equipment enters the site.

Fencing removed early

After framing or rough grading, the site superintendent pulls the TPZ fencing to make room for materials storage. The tree is now unprotected for the most damaging phase of construction — structural work and heavy equipment movement.

Grade changes within the TPZ

Adding as little as 4 inches of fill over the root zone reduces oxygen exchange in the soil and kills fine roots. Even "temporary" material storage on the TPZ soil causes compaction that takes years to remediate. We see this most often when the original arborist was not present during site logistics planning.

Trenching within the critical root zone

Utility installation — irrigation, electrical, drainage — routinely cuts roots within the TPZ because it happens after the TPZ plan was written, by a contractor who was never given a copy. Root severance within the CRZ can take 18–24 months to manifest as canopy dieback, by which time the permit is closed and the liability is uncertain.

Wrong species assessment

A native oak identified by the contractor's "arborist" as a valley oak (higher threshold) turns out to be an interior live oak (lower threshold) — meaning the tree was protected at a smaller size than the permit required, and no permit was pulled. The city issues a stop-work and a remediation requirement. All of this was preventable with a qualified pre-construction survey.

What Makes a TPZ Plan Defensible

A TPZ plan is defensible when a city inspector, planning director, or opposing counsel can look at it and clearly understand what trees were assessed, how the TPZ was calculated, what protections were installed, and who was responsible for monitoring. Vague language and hand-wavy specifications do not hold up.

A complete, defensible TPZ plan includes the following components:

01

Tree inventory with DBH and TPZ radius

Every tree meeting the jurisdiction's protected threshold, listed with species (confirmed by a qualified arborist — not a landscaper), DBH, canopy spread, health condition, and calculated TPZ radius. Photographs of each tree with a site plan overlay are standard practice on defensible plans.

02

Site plan showing TPZ boundaries

A scaled drawing (even a professional-quality PDF from SketchUp or AutoCAD is fine) that overlays each TPZ radius on the existing topography. Include the location of proposed structures, utilities, grading limits, and the construction staging area.

03

Fencing specification and installation requirements

Specify the fence type (chain link is standard; orange plastic mesh is generally insufficient), the exact location (setback from the TPZ edge, not the trunk), the height, and the gate specification. State clearly: the TPZ is established before any equipment enters the site.

04

Root pruning protocol

Any root cutting within the TPZ must be performed by a qualified arborist — not a backhoe operator. Specify the maximum root cut diameter by species and the follow-up treatment (wound paint, mycorrhizal treatment, irrigation protocol). Document what was actually cut vs. what was planned.

05

Monitoring schedule

A TPZ plan that ends at installation is incomplete. Specify inspection points: pre-construction, after rough grading, during utility installation, and at project completion. Each inspection should be documented in writing with photographs — this is your record that the plan was executed.

06

Remediation triggers

If a protected tree shows signs of stress during construction (crown thinning, leaf drop, epicormic shoots), what happens? The plan should specify a monitoring protocol, a threshold for requiring a hazard assessment, and a contact for the project arborist. This protects the property owner from post-construction liability claims.

Consulting Arborist vs. Tree Service

Many TPZ plan failures start with a well-meaning but misapplied assumption: that the tree company doing the work can also serve as the project arborist. For TPZ documentation that will be submitted to a permitting authority, this is a meaningful distinction.

Tree Service / Tree Worker

Performs pruning, removal, and cabling work
May not hold ISA certification (not required in CA to operate)
Limited or no expertise in tree risk assessment or soil science
Has a financial interest in recommending work (removal, pruning, treatment sales)
Cannot submit TPZ plans or arborist reports to North Bay permitting agencies
Appropriate for executing arborist-directed work, not for independent assessment

ISA-Certified Consulting Arborist

Provides independent, fee-for-service assessment with no work to sell
Holds ISA Certified Arborist or Tree Risk Assessment Qualification (TRAQ)
Trained in tree biology, root systems, soil conditions, and risk assessment
Can submit TPZ plans and arborist reports to all North Bay jurisdictions
Can serve as expert witness in tree-related disputes or litigation
Required for permit-level tree impact documentation in Marin, Sonoma, and Napa

The distinction is not about quality of work — it's about role. A tree service performs the work. A consulting arborist assesses, plans, and monitors. For any project where a city inspector, planning department, or neighbor might challenge your TPZ documentation, you need a qualified consulting arborist as the author — not as the crew.

CanopyMetrics provides ISA TRAQ-qualified consulting arborist services for development projects, permitting, and TPZ plan preparation across all North Bay jurisdictions. We do not perform tree removal or maintenance work — eliminating the conflict of interest that makes vendor-sourced reports unreliable.