What drywall requirements must be met for a legal secondary suite under Toronto's multiplex zoning rules?

A legal secondary suite in Toronto requires fire-rated drywall assemblies achieving a minimum 1-hour fire resistance rating between the suite and the primary dwelling, along with sound isolation meeting STC 50 requirements under the Ontario Building Code. Toronto's multiplex zoning rules (adopted in 2023–2024) allow up to four units on most residential lots, and each unit must meet the full Ontario Building Code requirements for a separate dwelling unit — which has significant implications for drywall specifications, materials, and installation quality.

The fire separation between dwelling units is the single most important drywall requirement. The Ontario Building Code requires a minimum 1-hour fire resistance rating for walls and floor/ceiling assemblies separating dwelling units within the same building. This typically requires one of the following drywall assemblies on the separating wall:

• Two layers of 5/8-inch Type X drywall on each side of the wall framing (four layers total) on standard wood stud framing
• One layer of 5/8-inch Type X drywall on each side of a staggered-stud or double-stud wall assembly with insulation in the cavity
• A tested and listed assembly from the Gypsum Association's Fire Resistance Design Manual that achieves the required 1-hour rating

The fire separation must be continuous from floor to ceiling and wall to wall, with no gaps, unsealed penetrations, or interruptions. Every electrical box, plumbing penetration, HVAC duct, and structural connection through the fire-rated wall must be fire-stopped with approved materials. In practical terms, this means the drywall contractor must coordinate closely with electrical and plumbing trades to ensure that all penetrations are properly sealed before the final layer of drywall is installed.

Sound Transmission Requirements

STC 50 (Sound Transmission Class 50) is the minimum requirement for walls and floor/ceiling assemblies between dwelling units under the Ontario Building Code. Standard single-stud walls with one layer of drywall on each side achieve roughly STC 33–38 — well below the requirement. Meeting STC 50 requires deliberate acoustic design in the wall and floor assemblies.

Common approaches to achieve STC 50 in GTA secondary suite projects include resilient channel on one or both sides of the separating wall, with two layers of 5/8-inch Type X drywall mounted on the resilient channel and fibreglass or mineral wool insulation in the cavity. This assembly can achieve STC 50–55 when properly installed. The critical detail with resilient channel is that no screw can penetrate through the resilient channel into the stud behind it — a single screw that short-circuits the channel dramatically reduces the sound isolation. This is a common installation error that defeats the entire purpose of the resilient channel, and it's why sound isolation work should always be done by experienced drywall professionals.

For floor/ceiling assemblies between stacked units (common in Toronto's new multiplex conversions), achieving STC 50 typically requires two layers of 5/8-inch Type X drywall on resilient channel on the ceiling below, mineral wool insulation in the joist cavity, and a sound-dampening underlayment beneath the flooring above. QuietRock sound-dampening drywall ($55–$90 per 4x8 sheet) can simplify these assemblies by providing higher STC performance in a single layer compared to standard drywall.

Additional Drywall Requirements for Secondary Suites

Each dwelling unit must have its own enclosed entry with fire-rated drywall separating shared corridors or vestibules from the living spaces. Ceiling heights must meet the Ontario Building Code minimum of 2.1 metres in habitable rooms and 1.95 metres in basements, which affects bulkhead planning around mechanical systems and ductwork.

Moisture-resistant drywall (green board or mould-resistant purple board) should be used in all bathrooms, kitchens, and laundry areas within the secondary suite. In basement secondary suites — the most common type in Toronto — the exterior walls require R-20 insulation with a 6-mil polyethylene vapour barrier before drywall installation.

The drywall scope for a typical basement secondary suite in Toronto runs $8,000–$18,000 depending on size, number of rooms, and the complexity of the fire-rated and sound-rated assemblies. For a full secondary suite conversion including all trades (electrical, plumbing, HVAC, framing, drywall, flooring, kitchen), budget $50,000–$120,000 in the current GTA market.

A building permit is mandatory, and the City of Toronto inspector will specifically check the fire separation assemblies, sound isolation details, and vapour barrier installation before and after drywall is hung. Get matched with a drywall contractor experienced in secondary suite fire-rated assemblies through Toronto Drywall Installers — it's free and ensures you're working with someone who understands these code requirements.