Can I install new drywall directly over existing plaster walls in a Toronto century home or must I strip first?

Yes, you can install drywall directly over existing plaster walls in many cases, and this is actually the preferred approach for many Toronto century home renovations — but only if the plaster is structurally sound and the walls are reasonably flat. Stripping plaster to the studs is the alternative when the existing plaster is too damaged, but it comes with significant cost, mess, and potential asbestos concerns.

Installing 1/4-inch or 3/8-inch drywall over intact plaster — a process called overlay or re-covering — is common practice in established Toronto neighbourhoods like Cabbagetown, the Annex, Riverdale, High Park, Rosedale, and Leslieville where century homes (pre-1945) with original plaster-and-lath walls are being renovated. The key requirement is that the existing plaster must still be firmly bonded to the lath behind it. Test this by pressing on the walls in multiple locations — if the plaster feels spongy, moves, or has large areas that sound hollow when tapped, those sections have lost their bond (called delamination) and will not support new drywall. A few small hollow spots can be addressed by re-securing the plaster with plaster washers and screws before overlaying, but if more than 20-30% of a wall is delaminated, stripping to the studs becomes the better option.

The overlay process involves screwing 1/4-inch or 3/8-inch drywall directly through the plaster and lath into the wood studs behind them. Finding the studs is the first challenge — century home studs are often irregularly spaced (not the modern standard of 16 inches on centre) and a stud finder may struggle to read through thick plaster and lath. Professional crews often locate one stud and then probe with a finish nail at intervals to find the pattern. Screws must be long enough to pass through the drywall, the plaster (typically 1/2 to 3/4 inch thick), the wood lath, and penetrate at least 5/8 inch into the stud — so 2-inch to 2-1/2-inch drywall screws are standard.

There are several practical considerations specific to Toronto century homes. Trim and millwork will need to be removed and reinstalled or replaced, since the new drywall surface sits 1/4 to 3/8 inch forward of the original plaster. This affects door casings, window trim, baseboards, crown moulding, and electrical cover plates — all of which will need extension rings or adjustments. In homes with original heritage millwork that the homeowner wants to preserve, this is a significant consideration. Electrical boxes will also be recessed by the thickness of the overlay and will need box extenders to bring them flush with the new surface — this is a code requirement.

The cost for overlaying runs approximately $2.50-$4.50 per square foot including materials and labour, compared to $5.00-$9.00 per square foot for full plaster stripping and new drywall installation. The savings come primarily from eliminating the extremely labour-intensive and messy demolition of old plaster and lath. Plaster demolition generates enormous amounts of heavy debris — a single room can produce 1,000-2,000 pounds of plaster rubble — and the dust is pervasive. Disposal costs in the GTA run $300-$600 per room for plaster debris removal.

Asbestos is a critical consideration for any plaster disturbance in Toronto homes built before 1990. Plaster, joint compound, and textured coatings in older homes may contain asbestos fibres. If you choose to strip plaster rather than overlay, Ontario Regulation 278/05 requires testing before disturbance, and if asbestos is found, certified abatement professionals must handle the removal — adding $3,000-$8,000 or more to the project cost. Overlaying avoids disturbing the existing plaster entirely, which eliminates this risk and cost.

The decision comes down to wall condition: if your century home's plaster is cracked and stained but still firmly attached to the lath, overlay is the faster, cleaner, and less expensive approach. If the plaster is falling off the lath in large sections, or if you need to access the wall cavity for new insulation, electrical, or plumbing work, stripping to the studs and installing new drywall is the right call. Either way, this is professional territory — the irregular framing, unknown wall cavity conditions, and trim work in century homes require an experienced drywall contractor.