Basement and Foundation Crack Sealing in Thornhill

Published October 2026 — Rhodex

A thin, dry crack that is not growing is usually a seal-and-watch job in a Thornhill basement. A horizontal, widening or leaking one needs its cause found first. Vaughan and Markham both list interior basement damp-proofing as generally not needing a permit.

Much of Thornhill was built in the decades after the war, and the houses on both sides of Yonge Street have had fifty years or more of wet springs and freeze-thaw cycles on their foundations. A crack in a basement wall is one of the most common things homeowners find when they finally clear the storage shelves, or when a buyer's inspector does it for them. Most are not an emergency. This guide covers how to sort them, what the two cities say about permits, and what sealing typically costs.

Sort the crack before you seal it

Hairline, dry, vertical or diagonal: usually shrinkage in the concrete as it cured. Seal it if it bothers you, and otherwise note it and watch it.
Up to roughly a quarter inch and damp at times: often minor settling. Worth sealing, because it lets in moisture and soil gas.
Horizontal, stair-stepping through block joints, wider than about a quarter inch, or growing: treat it as a question for a foundation specialist or engineer, not a caulking gun. These widths are rules of thumb, not an engineering standard.

Pencil-mark each end of the crack, write the date, and photograph it next to a ruler. A wide crack that has not changed in years is less worrying than a hairline that appeared last month and keeps lengthening. If doors or windows upstairs have started sticking or a wall bows inward, skip the monitoring and call someone.

Epoxy, polyurethane, and what neither one fixes

Contractors commonly inject from inside with one of two materials. Epoxy is rigid and is generally used on dry cracks to bond the concrete back together. Polyurethane is a flexible foam that expands into a crack and is the usual choice when it leaks. Neither fixes a wall that is still being pushed inward. If the water keeps coming back after sealing, the cause is usually outside: grading that slopes toward the house, a downspout emptying beside the wall, or a failed exterior membrane. Sealing from inside treats the symptom, so ask any contractor what they think is causing the crack and how they know it has stopped.

Which side of Yonge you're on, and what each city says

Thornhill sits across the Vaughan and Markham border, and the permit rules are written by each city. For the sealing job itself the two lists agree:

Markham lists “interior damp-proofing basements” and “maintenance and repairs” among the work that does not need a building permit. Its list does not mention exterior damp-proofing, so ask before anyone digs around the outside of the foundation. Markham's Building Standards Department is at 905-475-4858.
Vaughan lists damp-proofing basements among the work that generally does not need a building permit. Treat that as a general rule rather than a guarantee, and confirm your scope with Vaughan Building Standards at 905-832-8510.

Underpinning, new openings and other structural changes are a different category and are normally handled as permit items on either side of the border. If a quote includes any of that, ask who pulls the permit.

When the TRCA is worth a look

Thornhill has ravines and creek valleys running through it, and the Toronto and Region Conservation Authority regulates development in and near river valleys, watercourses, wetlands and hazardous land such as steep slopes and floodplains. Its permit page lists excavating or placing fill, additions, decks and landscaping, and does not address interior repairs. If the repair means digging outside on a lot that backs onto a ravine, run your address through TRCA's Regulated Area Search first (416-661-6600).

What sealing typically costs

These are general estimates from public Canadian price guides, not quotes, and the guides do not agree with each other:

HomeStars' main foundation-crack guide gives epoxy injection at about $500 to $1,200 and polyurethane at about $500 to $1,500.
Its Toronto guide gives roughly $450 to $1,100 per crack, and two other HomeStars guides give $300 to $800.
Larger, structural or multi-crack jobs, and anything involving excavation or drainage, are priced very differently and should be quoted after an assessment.

Get a written quote that names the material, the number of cracks and the warranty, and get more than one.

Sealing before you sell

In a competitive resale market a basement crack tends to come up at the buyer's inspection whether you mention it or not. A small crack that is clean, sealed and documented is far easier to explain than one a buyer's inspector finds first. Anything wider or growing is worth having assessed before you list, so you know how big the job really is. For the rest of the pre-listing list, see Small Repairs Before Listing a Thornhill Home.

Quick answers

Can I seal a small basement crack myself in Thornhill?

Yes, a thin, dry crack that is not growing can be sealed from inside. Wider, leaking, horizontal or growing cracks should be looked at by a foundation specialist or engineer first.

Do I need a permit to seal a basement crack in Thornhill?

Usually not for interior sealing. Vaughan and Markham both list basement damp-proofing as generally not needing a permit, but confirm your exact scope with your city.

How much does foundation crack injection cost in Thornhill?

Public Canadian price guides put one crack at roughly $300 to $1,500, depending on the guide and the material. These are general estimates, not quotes.

Should I fix a basement crack before listing my Thornhill house?

A small sealed crack is easy to explain to a buyer. A hidden one is harder, so seal what is small and have anything larger assessed before you list.

Does the TRCA matter for a crack repair?

Only if the work is outside, near a valley or watercourse. Interior sealing is not what TRCA's permit page describes, but check its address search before digging.

Seal what is small, ask about what is not

Most basement cracks in Thornhill houses are the small, dry kind, and sealing them is a quick, inexpensive job. The skill is in telling them apart from the ones that are telling you something about the wall. If you want a person to look at yours, describe it here.

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Sealing a Basement or Foundation Crack in Thornhill: Which Cracks Are Small, Which Side of Yonge You're On, and What It Costs