What to Expect Behind the Walls When Renovating a Century Home in Toronto

If you open up the walls of a century home in Toronto, you should expect to find some mix of knob and tube wiring, galvanized or lead plumbing, little to no insulation, lath and plaster, balloon framing, and possibly asbestos or hidden moisture damage. Most homes built before 1930 contain at least three of these, and the only way to know for sure is careful testing before demolition and a healthy contingency fund of 15 to 20 percent.

That is the short answer. The long answer is more interesting, and honestly, more useful if you are about to sign a renovation contract. Toronto has one of the largest stocks of pre-1930 housing in Canada. The Victorian rowhouses of Cabbagetown, the Edwardian semis of Riverdale and the Junction, and the 1920s detached homes across the west end were all built with materials and methods that stopped being used decades ago. When your contractor takes down that first sheet of plaster, you are looking at a time capsule. Some of what is inside is a problem. Some of it is actually good news.

Here is a room-by-room, layer-by-layer look at what renovators in Toronto actually find, what each discovery tends to cost, and how to plan so none of it derails your project.

1. Knob and Tube Wiring

This is the most common find, full stop. Knob and tube was the standard wiring method in Toronto homes from the 1880s through the 1930s, and plenty of it is still live inside older walls. You will recognize it by the ceramic knobs that hold wires away from the framing and the ceramic tubes that guide wires through joists.

The wiring itself was well made for its time. The problems are everything around it. There is no ground wire, the rubber insulation gets brittle with age, and decades of amateur splices and overloaded circuits create real fire risk. It also cannot legally be buried in insulation, which matters a lot when you want to insulate those cold exterior walls.

Two practical points that catch homeowners off guard. First, most insurance companies in Ontario will either refuse coverage or charge a steep premium on a home with active knob and tube. Second, even if your inspector saw modern wiring at the panel, splices to old knob and tube are often hiding in walls and ceilings. Budget roughly $10,000 to $25,000 to fully rewire a typical Toronto semi, more for a large detached home.

2. Old Plumbing: Galvanized Steel, Lead, and Cast Iron

Behind bathroom and kitchen walls, expect galvanized steel supply pipes that have been corroding from the inside for 60 to 100 years. The telltale sign is weak water pressure on the upper floors. Corroded galvanized pipe does not just slow your shower, it eventually pinholes and leaks inside the wall cavity.

Some pre-1950s Toronto homes also still have a lead water service line running from the city main into the basement, and occasionally short runs of lead pipe inside. The City of Toronto has replacement programs for the public portion, but the private side is on you. Drain stacks are usually cast iron, which can be perfectly fine or completely rotted at the base, and you will not know until it is exposed.

If the walls are already open for your renovation, replacing supply lines with PEX or copper is far cheaper than doing it later as a standalone job. Most whole-home repipes in Toronto land between $8,000 and $20,000 depending on size and access.

3. Little or No Insulation

Open an exterior wall in a century home and there is a good chance you will find nothing but a dusty air gap between the brick and the plaster. Insulation simply was not part of the building standard when these homes went up. Where you do find something, it is often sawdust, newspaper, seaweed, or crumbling early batts that have slumped to the bottom of the cavity.

One material deserves special caution: vermiculite. This pebble-like loose fill, common in attics and sometimes in wall cavities, may contain asbestos if it came from the Libby mine that supplied much of Canada. Never disturb it without lab testing first.

The upside is that a renovation with open walls is the single best opportunity you will ever get to bring the home up to modern comfort standards. Spray foam or dense-packed cellulose in the cavities transforms how a century home feels in a Toronto January and cuts heating bills noticeably. We covered why this matters so much in our guide to the importance of thermal insulation in home renovations.

4. Asbestos and Lead Paint

Homes built or renovated before the 1980s frequently contain asbestos, and in century homes it shows up in specific places: pipe wrap on old heating lines, duct tape and boiler insulation, some plaster and drywall compounds, vinyl floor tiles and their adhesive, and vermiculite insulation. Lead paint is nearly guaranteed on original trim, doors, and window frames in any pre-1960 Toronto home.

Neither material is dangerous while sealed and undisturbed. Demolition is exactly what disturbs it. Ontario regulations require proper assessment and, where asbestos is found, remediation by trained workers following Type 1, 2, or 3 procedures depending on the material. A designated substances survey before demolition typically costs a few hundred to a couple thousand dollars, and abatement can run from $2,000 for a small pipe-wrap job to $15,000 or more for widespread material. Skipping the survey is not a shortcut, since contractors are legally required to stop work when suspect material turns up, which costs you far more in delays.

5. Lath and Plaster Walls

The walls themselves are part of the surprise. Instead of drywall, century homes have thousands of thin wood strips nailed across the studs, coated with two or three layers of plaster, often the original horsehair variety. Plaster is heavy, brittle, and extremely messy to remove. Demolition of lath and plaster costs more and takes longer than drywall demo, and the debris weight adds disposal fees.

It is not all bad. Intact plaster deadens sound better than drywall and has real heritage character. The practical rule most Toronto renovators follow: if a wall is staying put and does not need new wiring, plumbing, or insulation behind it, patch and keep the plaster. If you are opening the wall anyway, take it back to the studs and re-board, because piecemeal plaster repair around new services rarely ends well.

6. Balloon Framing and Structural Surprises

Many Toronto homes from this era were balloon framed, meaning the wall studs run continuously from the foundation right up to the roof, with floors hung off them. Those long open stud cavities act like chimneys in a fire, so building code requires fire blocking to be added whenever the walls are opened. Your contractor should price this in, not discover it mid-project.

Behind the walls you may also find undersized or notched joists, beams cut by past plumbers, and load-bearing walls exactly where you hoped to open up the floor plan. Century home layouts were chopped into small rooms, and nearly every wall between them is doing structural work. Removing one usually means a steel or engineered beam, an engineer’s drawings, and a permit. If an open-concept main floor is the goal, read our breakdown of what it costs to reconfigure a floor plan before you fall in love with the sledgehammer.

Also Read: Can My Family Live in the House During a Major Renovation in Toronto?

7. Moisture Damage, Rot, and Pests

Toronto’s freeze-thaw cycles and humid summers have had a hundred years to work on these houses. Common finds once walls open up include rotted sill plates and rim joists where the wood meets the foundation, water staining below old window openings, deteriorated mortar on the inner face of brick walls, and evidence of past or present pests, from carpenter ants to mice highways running through the stud bays.

Small areas of rot are routine and cheap to sister or replace while the framing is exposed. The expensive version is hidden long-term leaking, often from decades-old roof or flashing failures, that has compromised a structural member. This is the category that most often triggers change orders, which is exactly why the contingency fund exists.

8. Abandoned Chimneys, Dead Ducts, and Odd Voids

Century homes almost always contain abandoned infrastructure: bricked-over fireplaces, chimney flues that no longer vent anything, old gravity-furnace ducts the size of a hallway closet, capped gas lines that once fed wall sconces, and mysterious framed voids from renovations done in 1955. None of this is dangerous by default, but unused chimneys eat up valuable interior space and their unsupported masonry sometimes rests on nothing more than a wood beam. Removing a redundant chimney during a major renovation frees up square footage on every floor it passes through.

The Good News Behind the Walls

It is not all hazards and invoices. Toronto century homes hide genuine assets:

• Old-growth lumber. The studs and joists are denser and stronger than modern equivalents, which is part of why these homes have lasted a century already.
• Double-brick construction. Two full wythes of brick give solid structure, excellent sound blocking, and thermal mass that modern wood-frame homes cannot match.
• Salvageable character. Behind bulkheads and dropped ceilings, renovators regularly uncover original mouldings, brick feature walls, and even stained glass that previous owners covered rather than removed.

How to Prepare Before Anyone Swings a Hammer

• Get a designated substances survey. Test for asbestos and lead before demolition, not after a surprise stops the job.
• Scope the wiring and plumbing. An electrician and plumber can inspect accessible areas and give you a realistic worst-case number up front.
• Carry a 15 to 20 percent contingency. On century homes this is not padding, it is the plan. Our Toronto home renovation cost guide breaks down where the money actually goes in 2026.
• Confirm permits and heritage status early. Structural changes, new wiring, and plumbing all need City of Toronto permits, and homes in Heritage Conservation Districts carry extra approvals.
• Build slack into your schedule. Hidden conditions are the number one reason old-home projects run long. Our guide to realistic renovation timelines in Toronto shows what to expect room by room.

Frequently Asked Questions

Is it worth renovating a century home in Toronto?

Usually, yes. The structure, lot location, and character of pre-1930 Toronto homes are hard to replicate, and once the wiring, plumbing, and insulation are modernized, you get a home that performs like new but feels like nothing built today. The key is going in with accurate expectations and a proper contingency.

How much extra should I budget for hidden problems?

Set aside 15 to 20 percent of the renovation budget as contingency. On a major century home project, discoveries like knob and tube, asbestos, or hidden rot are the norm rather than the exception.

Can I leave knob and tube wiring in place if it still works?

Legally it can remain in some situations, but practically it is a dead end. Insurers increasingly refuse it, you cannot insulate around it, and any renovation that opens the walls is the cheapest moment you will ever have to replace it.

Do I need to test for asbestos before renovating?

Yes. Ontario requires suspect materials to be assessed before they are disturbed, and contractors must stop work if suspect material is found mid-demolition. Testing first protects your health, your timeline, and your budget.

Plan for the Surprises, Enjoy the Result

A century home renovation in Toronto rewards the prepared. When you already know that the walls may hold old wiring, tired plumbing, empty cavities, and a few genuine treasures, none of it feels like a crisis. It is just the work. The homeowners who end up happiest are the ones who test early, budget honestly, and work with a team that has opened up enough old Toronto walls to know what is coming.

If you are getting ready to renovate a century home and want that kind of experience on your side, the design-build team at Senso Design has spent over 20 years helping GTA homeowners bring heritage houses into the modern era, permits, surprises, and all.