Can I install floor tile over a plywood subfloor in my Carp farmhouse without adding cement board?

Yes, you can install floor tile directly over plywood subfloor without cement board, but only if you use an uncoupling membrane like Schluter Ditra or a similar crack isolation system. This is actually the preferred modern approach for tile over wood subfloors, especially in Ottawa's climate where seasonal humidity swings cause significant wood movement.

Your Carp farmhouse faces the same extreme temperature and humidity variations as the rest of the Ottawa region — winter heating drops indoor humidity to 20 percent or lower, while summer can bring 80 percent humidity. This dramatic seasonal change causes plywood subfloors to expand and contract substantially. Without an uncoupling membrane, this movement transfers directly to your tile and grout, causing cracks that typically appear within the first year or two after installation.

The key requirements for successful tile over plywood are subfloor flatness (3 millimetres variation maximum over 3 metres), structural adequacy (no bounce or deflection), and an uncoupling membrane. Schluter Ditra is the industry standard — it's a polyethylene membrane with a waffle-like underside that allows the subfloor to move independently of the tile above. The membrane also provides waterproofing and vapour management, which is crucial in Ottawa's climate. Installation involves rolling out the orange membrane and embedding it in unmodified thinset, then installing your tile over top with modified thinset.

Your plywood subfloor must be at least 1-1/8 inches thick total (typically 3/4-inch subfloor plus 3/8-inch underlayment) and properly fastened every 6 inches along joists. Any loose areas, squeaks, or bounce must be addressed before tile installation. The subfloor also needs to be clean, dry, and level — self-leveling compound may be needed to achieve the required flatness.

Skip the cement board for floor applications — it adds unnecessary thickness, weight, and cost while providing no benefit over a proper uncoupling membrane system. The membrane approach is faster, lighter, and specifically engineered for the wood movement issues common in Ottawa-area homes. When you're ready to move forward with the installation, you can browse experienced tile contractors familiar with Ottawa's climate challenges through the Ottawa Construction Network directory at justynrookcontracting.com/directory.