Tree protection zones explained: what builders and homeowners need to know

A tree protection zone (TPZ) is the fenced area around a tree that stays off limits during construction. It protects the roots that keep the tree alive and standing. Your arborist and your municipality set its size, usually from the trunk diameter. When retained trees get damaged, it's mostly because someone ignored this zone.

If you're building near trees in Greater Victoria, the TPZ is the line on your site plan that matters most to the trees. Below is how it's sized, what's banned inside it, and how to work with it.

What is a tree protection zone?

A TPZ is a defined area around a tree's trunk where construction activity is restricted or banned. It's marked on the plans and fenced on site, so machines, materials and digging stay off the roots the tree depends on.

You'll also hear the term critical root zone (CRZ). The CRZ is the part of the root system a tree needs to survive. The TPZ is the protected area you fence to keep the CRZ safe. Guides and bylaws don't all use the same names. Saanich's bylaw, for example, says "protected root zone."

Why so much space? Roots spread far wider than most people expect, and most of them sit close to the surface. Oregon State University Extension notes that most of a tree's roots are within the top 18 inches (about 45 cm) of soil. A single pass with an excavator or a pile of fill can damage them.

How is a TPZ sized?

Most methods start with the trunk diameter, measured about 1.4 m (4.5 ft) above the ground. Arborists call this DBH, or diameter at breast height. Multiply it by a set factor and you get the radius of the protected circle.

Oregon State also suggests comparing the result with the dripline (the edge of the canopy) and using whichever is larger.

Treat these as starting points. Species, age, health and site conditions can all push the right size up or down. Each municipality has its own rules, and your arborist sets the actual TPZ for your site in the tree protection plan. Check the rules for your area in the bylaw table on our consulting and permitting page or in our municipal guides, such as Saanich and Victoria.

What's not allowed inside a TPZ?

Anything that packs down soil, cuts roots or changes the grade is off the table unless your plan specifically allows it. The extension guides we reviewed agree on what does the damage.

Purdue's guide warns that as little as 2 inches (about 5 cm) of fill over the root zone can kill a tree. So "just a little topsoil" isn't harmless.

What does tree protection fencing look like?

Tree protection fencing is a sturdy, easy-to-see barrier set up at the edge of the TPZ before any work starts, and it stays up until the job is done. Each municipality has its own rules on height, materials and signs.

As one local example, Saanich's bylaw requires a barrier at least 1.2 m high with solid wood or metal posts for permitted work near protected trees. Its standard drawing (Schedule C) shows a 2x4 wood frame with orange snow-fence mesh and signs reading "DO NOT ENTER – Tree Protection Zone" on every face or at least every 10 m. In Victoria, the City says a tree permit won't be issued until barriers are installed and approved, and it may ask for a letter from your arborist confirming the installation.

A lot of TPZ breaches start with the fence: it leans over, gets moved for a delivery, or loses its signs.

Can you work inside a TPZ?

Sometimes. If a driveway, footing or service line has to cross the zone, crews can often do the work with low-impact methods while an arborist watches.

• Hand digging lets crews find roots before they're damaged.
• Air or hydro excavation uses compressed air or water to remove soil while leaving roots intact. UC IPM recommends it over regular digging near roots.
• Tunnelling or boring under the roots avoids open trenches. Oregon State suggests going at least 18 inches (about 45 cm) below the root zone.
• Root pruning, when unavoidable, should be clean cuts made under supervision.

Saanich's guidance expects a certified consulting arborist to supervise work inside the protected root zone and to document root sizes affected, methods used and photos in a memo afterward.

What happens if the TPZ is breached?

The tree may decline slowly, often over several years, or become less stable. You won't always see root damage right away.

It can also cost you, and cause trouble with your permit. Saanich states it may take enforcement action if a protected tree is damaged in a way not consistent with the permit. Victoria collects securities for retained trees and returns them once work is complete and permit conditions are met. If a breach happens, call your project arborist right away so the damage can be assessed and documented. Fines can be steep. For example, Oak Bay's bylaw sets a $3,000 minimum and Langford's a $5,000 minimum. Check your municipality's bylaw for its penalties.

How does a TPZ fit into a tree protection plan?

The TPZ is one part of a tree protection plan. The plan identifies the trees to keep, maps each TPZ, specifies the fencing, and sets out the methods for any work near roots. Many municipalities ask for it with a building or tree permit.

The project arborist is the person who makes the plan work on site. That usually means confirming the fencing before work starts, supervising digging near roots, visiting at planned points during the build, and writing the letters the municipality needs, such as a letter of assurance or a post-construction letter for security release.

At Oakstead, tree protection plans start at $1,450 for a single lot, and project arborist site visits are $325 each (plus GST). On larger projects, the tree protection plan usually sits inside a broader tree management plan. That plan adds the full tree inventory, the reasons each tree is kept, removed or replaced, and the replacement and post-construction care plans. Oakstead is consulting only. If trees need pruning or removal, we refer you to vetted local partners and take no referral fees.

Planning a build near trees? Request a consultation and we'll help you set a TPZ that works for the tree and the project.