Plaster and Drywall Repair in Older Thornhill Homes

Published 2026 — Rhodex

Check what is behind the paint before you patch. Plaster and drywall need different repairs, and in older Thornhill homes sanding unknown paint or joint compound can release lead or asbestos. Small drywall patches are routine; plaster usually calls for a plasterer.

Thornhill has houses from many eras, and the oldest ones, especially around the Yonge Street village core, were finished with plaster rather than drywall. A crack that a quick tub of filler would fix in a newer house can come straight back in a plaster wall, and the same filler can make a loose plaster ceiling worse. Here is how to tell what you have, what it takes to repair, and what to settle before anyone picks up a sander.

First, find out what the wall is made of

Drywall is a gypsum board faced with paper, usually about half an inch thick. Traditional plaster is applied wet in coats over wood lath strips or, from about the late 1930s, over rock lath, a gypsum board with a paper face that was widely used under residential plaster. Plaster walls feel harder and colder to knock on, and a thumbtack will barely go in.

The reliable check is a look at an edge. Remove an outlet or switch cover plate and look at the cut edge of the wall, or look through the opening for a ceiling light. Wood strips with gaps behind a thick coat mean lath and plaster. A uniform board with paper on both faces is drywall. Be aware that some renovators screwed drywall over old plaster, so a wall can be both.

Why the repair is different

Hairline cracks: Both materials crack as a house settles through the seasons. In plaster, a fine crack can be raked open slightly, filled and painted. If the crack keeps returning, the plaster may be losing its grip on the lath.
Bulging or sagging plaster: This means the plaster keys, the lumps that grip the lath, have broken. Pressing filler over it hides the problem for a season. Loose sections are usually re-secured with screws and washers or cut out, which is plasterer's work, especially on a ceiling.
Holes: A small hole in drywall takes a patch and a few thin coats of compound. A hole in plaster needs a firm backing so the fill does not sink, and the finished surface has to be blended into a wall that is usually not perfectly flat.
Water stains: Fix the leak first, in either material. Wet plaster that has gone soft generally has to come down, and wet drywall is replaced rather than dried and painted.

Many tradespeople also price plaster differently because it is slower: wet coats need to dry between passes. Expect a plasterer's quote to be built around the job, not a flat patch fee.

Lead paint and asbestos come before the sanding

Health Canada says a home built before 1960 probably contains lead-based paint, and for homes built from 1960 to 1990 the exterior may contain it and interior paint may hold smaller amounts that can still be harmful, especially to young children. Its advice is not to use sanders, heat guns or blowlamps on it, because they create toxic dust and fumes, and to consider hiring an expert. Paint chips can be sent to a lab for testing. Leaving intact paint alone, or covering it, can be safer than removing it.

Asbestos is the other question. Drywall joint-filling compound and plaster are both on the lists that Ontario's safety association publishes of building materials that have contained it. Ontario's asbestos regulation exempts the owner of a private home that the owner or the owner's family occupies, but the same guide says the rules still apply to the constructors, employers and workers on a private project. In practice that means a contractor working in your house has duties around asbestos even where you do not, and a rental unit changes the owner's position too. If your walls and ceilings are older, ask a contractor whether they sample first. Do not scrape a textured ceiling yourself to find out.

Permits and heritage rules

Patching, skimming or replacing plaster and drywall inside your home does not need a building permit in Vaughan or Markham. If the repair opens walls to change wiring or plumbing, those jobs carry their own rules, so see our guide on how your side of Yonge Street changes the permit rules for small jobs.

Parts of Thornhill sit inside a Heritage Conservation District. Vaughan's Thornhill district plan lists interior work among the minor alterations that need no heritage permit, and the Ontario Heritage Act excludes the interior of buildings from the permit requirement in a district. Markham requires a heritage permit for exterior alterations in its district. We did not find a Markham list that speaks to plaster specifically, so if your home is designated individually, ask the city's heritage staff before you start.

What it typically costs

These are general estimates from published contractor guides, not quotes. For drywall, Toronto-area and Ontario sources put a small hole or patch at roughly $75 to $300, and many contractors set a service-call minimum, commonly in the $150 to $200 range, so repairing several spots in one visit costs less per spot. Larger patches, ceilings and sections that must be replaced run higher. We could not find a reliable Toronto-area per-square-foot figure for plaster repair. Sources differ widely, so treat any plaster number you see online with suspicion and ask for a written quote for your specific damage. Prices before HST are common, so ask which one you are being given.

Quick answers

How do I tell if my walls are plaster or drywall?

Cut a small inspection hole or remove an outlet cover and look at the edge. Drywall is a paper-faced gypsum board; plaster sits on wood strips or a perforated board behind it, and it is noticeably thicker and harder.

Can I patch plaster with drywall compound?

Only for hairline cracks and shallow dings. Larger holes in plaster need a backing or a plaster-based fill, because plain compound shrinks and cracks again around the edge.

Do I need a permit to repair a plaster wall in Thornhill?

No, patching walls and ceilings does not need a building permit in either Vaughan or Markham. Heritage rules in the Thornhill districts cover exterior changes; the Ontario Heritage Act leaves building interiors out.

Is it safe to sand old plaster or joint compound?

Not until you know what is in it. Health Canada says homes built before 1960 probably contain lead paint, and joint compound and plaster in older buildings can contain asbestos, so test before you sand.

What does a small drywall patch cost in the Toronto area?

Published Toronto-area contractor guides put a small hole patch at roughly $75 to $300, often with a service-call minimum. These are general estimates, and plaster work is usually quoted by the job.

Before you pick up the putty knife

A cracked wall in an older house is often an easy repair, but only once you know what the wall is and what is in its paint. Look at the edge, test before you sand, and match the repair to the material. If it turns out to be plaster over lath, or the ceiling is coming down in sections, that is a good point to get a written quote from someone who works with it regularly. Describe the repair on Rhodex and a vetted craftsman will quote it.

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Plaster or Drywall? Repairing Cracked Walls and Ceilings in an Older Thornhill Home