What thickness of drywall is required by Ontario Building Code for residential interior walls in Toronto?

The Ontario Building Code does not mandate a specific drywall thickness for standard residential interior partition walls — 1/2-inch (12.7mm) drywall is the industry standard used in virtually every Toronto home. However, the code does specify minimum drywall thickness and type for fire-rated assemblies, and understanding these requirements is essential for any renovation project in the GTA.

For most interior walls — bedroom partitions, hallway walls, closet walls, and general living areas — 1/2-inch regular drywall is the standard choice. It provides adequate rigidity when installed on studs spaced 16 inches on centre, which is the standard framing layout in residential construction across the GTA. This is what you will find in the vast majority of Toronto homes built from the 1950s onward, from Scarborough bungalows to modern Vaughan subdivisions.

Where the Ontario Building Code does get specific is with fire-rated assemblies. The wall and ceiling between an attached garage and the living space must achieve a minimum 45-minute fire resistance rating, which requires 5/8-inch Type X drywall on the garage side with all joints properly taped and finished. This is non-negotiable — it is a life-safety requirement. Similarly, condo party walls (the walls between separate dwelling units) must achieve a minimum 1-hour fire resistance rating and meet STC 50 for sound transmission, typically requiring double layers of 5/8-inch Type X drywall on resilient channel or a staggered-stud assembly. Furnace rooms and utility rooms containing gas-fired appliances also require fire-rated enclosures.

For ceilings, while 1/2-inch drywall is technically acceptable, most professional drywall crews in the GTA strongly recommend 5/8-inch drywall on ceilings. The reason is practical — 1/2-inch drywall is more prone to sagging between joists over time, especially in humid conditions or when installed on 24-inch joist spacing, which is common in newer Toronto homes. The extra thickness of 5/8-inch board provides significantly more rigidity and resistance to sag. If you are finishing a basement ceiling in a GTA home, 5/8-inch is the professional standard.

Moisture-prone areas like bathrooms, kitchens, and laundry rooms should use moisture-resistant (green board) or mould-resistant (purple board) drywall, though the code does not mandate this for all wet areas. In Toronto's humid summers, mould growth behind standard paper-faced drywall in poorly ventilated bathrooms is a common and expensive problem. Purple board with its fibreglass facing eliminates the paper food source that mould feeds on, making it a smart investment at $24-$32 per 4x8 sheet versus $14-$20 for regular board. Note that no type of drywall — including green board and purple board — is acceptable as a tile substrate in shower or tub surrounds. Cement board is required in those locations.

From a cost perspective in the current GTA market, the price difference between 1/2-inch and 5/8-inch drywall is modest — roughly $4-$6 per sheet more. For a typical basement finishing project of 800-1,200 square feet, upgrading to 5/8-inch throughout might add $200-$400 in material costs, which is negligible compared to the $5,000-$12,000 total drywall scope. Upgrading to Type X where fire ratings are required adds a similar modest premium.

The bottom line for Toronto homeowners: use 1/2-inch regular drywall on standard interior walls, 5/8-inch on ceilings, 5/8-inch Type X wherever fire ratings are required by code, and moisture-resistant board in wet areas. If you are unsure about fire separation requirements for your specific project — particularly garage walls, basement finishing, or condo renovations — consult with your contractor and your local building inspector. Getting the drywall type wrong on a fire-rated assembly means tearing it all out and starting over, which is far more expensive than using the correct board from the start.