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OBC DIVISION B · PART 9 · SECTION 9.29 — INTERIOR WALL AND CEILING FINISHES

What water-resistant backing and finish is required around tubs and showers in Ontario?

Walls in showers and above tubs with showerheads need a waterproof finish extending at least 1.8 m (6 ft) above the floor, over moisture-resistant backing. Around tubs without showers, the protected zone extends above the tub rim.

Code reference
Interior walls exposed to shower spray must have a waterproof finish, such as tile over suitable backing or integral tub-shower surrounds, extending not less than 1.8 m above the floor in showers and above bathtubs equipped with showers, with a lesser height above the rim of tubs without showers. Backing behind the finish must resist moisture degradation.
OBC Division B, Subsection 9.29.2.

Paraphrased for clarity — open this provision in the BUILDAiD Library for the exact wording.

What this means in practice

The finish and the backing are both regulated. Tile itself is not waterproof; grout lines leak, so what is behind the tile matters. Standard paper-faced gypsum board is a poor tile base in wet zones, and current practice, reflected in referenced installation standards, is cement board or coated glass-mat board with a waterproofing membrane in showers. Measure the 1.8 m zone from the floor, not the tub rim, and carry protection around exposed edges. Failed tub surrounds are among the most expensive hidden-rot repairs in renovations.

Plain language

The wall around your shower must be genuinely waterproof to about 1.8 metres, six feet, above the floor, and the board behind the tile has to tolerate moisture. Ordinary drywall behind shower tile eventually turns to mush.

Related questions

What thickness of drywall is required and how must it be fastened in Ontario?

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This page is provided for general information. Building code requirements depend on your specific project, occupancy and municipality, and code editions change. Always verify requirements against the current Ontario Building Code and confirm with your local building department.