Can I install bathroom tile directly over plaster walls in my Old Ottawa South bungalow?

No, you should not install tile directly over plaster walls in your Old Ottawa South bungalow. While plaster walls were built to last and many from that era are still structurally sound, they lack the moisture resistance and dimensional stability required for modern tile installations, especially in bathrooms where humidity and water exposure are constant concerns.

Old Ottawa South homes, many built between the 1920s and 1950s, typically have horsehair plaster over wood lath or early wire mesh. These plaster walls expand and contract with Ottawa's dramatic seasonal humidity swings — from bone-dry winter heating conditions to humid summer weather. This movement will eventually crack rigid tile installations and cause grout failure. More critically, plaster is porous and will absorb moisture from bathroom humidity, potentially leading to deterioration behind the tile where you cannot see or address it until significant damage has occurred.

The proper approach is to install cement backer board or foam backer board over the existing plaster. Hardiebacker, Durock, or Schluter Kerdi-Board are the standard options. The backer board creates a stable, moisture-resistant substrate that moves independently from the plaster behind it. This installation typically adds about $3 to $6 per square foot to your project cost but prevents the much more expensive problem of tile failure and potential water damage to the original plaster and framing.

For shower areas specifically, you will also need a waterproofing membrane like Schluter Kerdi or RedGard liquid membrane applied over the backer board before tiling. This is non-negotiable in wet areas — Ottawa's climate extremes make moisture management absolutely critical, and a failed shower waterproofing can cost $10,000 to $20,000 to remediate once water reaches the framing.

If your plaster walls are in poor condition with significant cracks, loose areas, or previous water damage, removal and replacement with proper wet-area substrates may be the most cost-effective long-term solution. When you are ready to move forward with professional installation, you can browse experienced tile contractors familiar with heritage home renovations through the Ottawa Construction Network directory at justynrookcontracting.com/directory.