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Deck and Fence Repair on an Older Thornhill Lot: Tree Roots, Permits on Either Side of Yonge, and What It Costs

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Published 2026 — Rhodex

Tree roots often lift deck posts and push fences off line on older Thornhill lots. Vaughan and Markham set different permit lines, and both protect trees 20 cm across or wider, so check before cutting.

Older Thornhill streets have something newer subdivisions are still waiting for: mature trees. They also have decks and fences that have been sitting next to those trees for twenty or thirty years. When a deck post sinks, a patio-door landing tilts or a fence run starts to lean, the cause is often below the surface: roots that grew under a footing, or a tree trunk that has swelled into the fence. This guide covers what to check, which city's rules apply, and what the repair market looks like. Where we could not confirm a rule on the city's own page, we say so.

First, which side of Yonge Street are you on?

West of Yonge Street, Thornhill is in the City of Vaughan; east of it, the City of Markham. Your property tax bill names your city. That matters here because the two cities set different permit thresholds, different fence heights and different tree by-laws. Our guide to how your side of Yonge Street changes the permit rules for small jobs covers the split in general; this guide applies it to decks, fences and trees.

What roots actually do to a deck and a fence

The tempting fix is to chop the offending root. Slow down before you do. One university extension guide on protecting trees during construction advises against letting more than about 25 percent of the roots in a tree's protected zone be damaged, and recommends clean cuts made by someone who knows what they are doing. A tree that loses too many roots can die or become unstable, and a tree that falls on the deck you just repaired is a worse outcome than the heaved post. Ask an arborist to look at any root thicker than your wrist before it is cut.

The tree rule on both sides of Yonge

Both cities protect private trees. The City of Vaughan's page says trees with a diameter of 20 cm or more, or a base diameter of 20 cm or more, must not be injured, destroyed or removed without a permit. Its Tree Protection By-law 177-2025 (amended by 251-2025) applies on private and public property, and the City lists fines of up to $10,000 for a first offence by an individual and up to $25,000 for later ones. The City of Markham's Tree Preservation By-law 2023-164 sets a similar 20 cm line, measured at about 1.37 m above the ground, and the city says owners also need a permit to injure a tree, not only to remove it. Markham's tree application notes that a permit can take up to 45 days and that a neighbour must sign if any part of the tree is on their property.

What we could not confirm is whether cutting a root for a footing counts as "injury" under either by-law, because the by-law wording on roots was not available to us. Markham's own planning guidance says roots can extend two to three times past the canopy's outer edge, which is why digging near a big tree is treated as a risk to it. The safe rule: if a tree 20 cm or wider is near the work, call before digging or cutting. Vaughan's Forestry staff and Markham's Forestry and By-law staff (905-477-7000) can say which permit, if any, applies.

Deck repair: when the permit line is crossed

Vaughan's building permit page states that a permit is required for any deck or porch attached to a building, regardless of area or height above grade. A permit is not required for an uncovered deck that is no larger than 18.6 m² (about 200 sq ft), not more than 0.6 m (about 23.6 inches) above grade and not attached to an existing structure. Zoning setbacks apply either way. Vaughan's Building Standards department is on 905-832-8510.

Markham's published rule is tighter on size. As we read it, a detached deck under 10 m² and no more than 600 mm high does not need a permit, while a deck attached to the house, taller than 600 mm or larger than 10 m² does. We could not reopen Markham's own page to recheck the figures today, so confirm with the City before relying on them.

None of the pages we found says whether swapping some boards on an existing deck needs a permit. Boards are cosmetic. Posts, beams, footings and the connection to the house are structural. If a heaved footing means resetting posts on an attached deck, treat it as structural and ask the building department first.

Guards are the part to check on any raised deck. Ontario Building Code 9.8.8.1 requires a guard on each open side where the drop is more than 600 mm, and openings in a standard baluster guard are limited to about 100 mm. A post that flexes when you lean on it is a repair to do before the next gathering. Footing depth matters too: municipal deck guides elsewhere in Ontario, such as the City of Barrie's, put frost-protection footings at about 4 feet below grade, so a shallow tree-root-lifted pier is a sign to dig deeper, not shim it.

Fence repair: heights, the neighbour and the line

Neither city's published guidance asks for a building permit for an ordinary fence; the trigger is a pool enclosure. Heights differ. Markham's Fence By-law 277-97 allows 1.2 m in a front yard and 1.8 m in interior side and rear yards. Vaughan sets its limits by location in Fence By-law 189-2020: its page lists 1.2 m in a front yard close to the street and up to 2.0 m in many other residential cases, and a fence taller than the table allows needs a formal exemption. Replacing a fence on the same line at the same height is usually straightforward. A taller fence is when you read your own lot's table first.

A fence on the property line is shared. Ontario's Line Fences Act lets neighbours who cannot agree on the cost of building or repairing a line fence ask the municipal clerk for fence viewers, who can order how the work and cost are split. Municipalities can opt out of the process, so ask your clerk whether it is offered. Talk to the neighbour first and put what you agree in writing: it is cheaper than a dispute, and a fence leaning because of their tree is a conversation about the tree.

What it costs: general market estimates

We did not find a reliable Ontario price for repairing a single post or a few boards, and most repair figures online are American, so none are quoted here. Repairs are normally quoted by the job or by the hour, not per foot. What is published for new work gives a sense of scale. A Toronto deck builder's 2026 guide puts installed new decks at roughly $30 to $50 per square foot for pressure-treated wood, $45 to $65 for cedar and $55 to $80 for composite, with installed railing at about $55 to $65 per linear foot in pressure-treated wood and removal of an old deck at about $500 to $2,500. A national HomeStars guide gives installed pressure-treated wood fence at about $27 to $45 per linear foot and notes that Toronto sits at the higher end. A contractor cost database lists materials alone at roughly $15 to $35 for a 4x4x8 pressure-treated post and $3 to $8 per replacement board. These are general estimates, not Thornhill prices and not quotes. Working beside a protected tree, hand-digging around roots, an arborist visit or a tree permit can all move a real quote. Ask for written quotes that separate boards, framing, posts and any permit or arborist work, and add 13% HST.

Before anyone digs a post hole

Request locates from Ontario One Call before the first hole. Resetting a footing near a tree is also where utilities, roots and a property line all meet.

Quick answers

Do I need a permit to repair a deck in Thornhill?

Not always. In Vaughan, an attached deck needs a permit at any size or height. A free-standing deck up to 18.6 m² and 0.6 m high does not. Markham's line is tighter, so check your side of Yonge.

Can I cut tree roots that are damaging my deck or fence in Thornhill?

Check first. Both Vaughan and Markham protect private trees of 20 cm diameter or more, and injuring one without a permit is an offence. Ask an arborist and the city before cutting any large root.

How tall can a backyard fence be in Thornhill?

In Markham, 1.8 m in rear and interior side yards under By-law 277-97. Vaughan allows up to about 2.0 m in many residential cases. Confirm your own lot's limit before building taller.

How much does a new wood fence cost near Toronto?

A national HomeStars guide puts installed pressure-treated wood fence at about $27 to $45 per linear foot, with Toronto at the higher end. Repairs are usually quoted by the job.

Who pays to fix a fence between two Thornhill neighbours?

Ontario's Line Fences Act lets neighbours who can't agree ask the municipal clerk for fence viewers. They can order how the cost and work are split, if your city offers the process.

A short checklist

Sources

Related reading

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