What Ontario Building Code requirements apply to drywall installation in a legal basement apartment in the GTA?

A legal basement apartment (secondary suite) in the GTA must meet extensive Ontario Building Code requirements that go far beyond standard basement finishing — the drywall scope alone involves fire separations, sound ratings, specific board types, and assembly configurations that are significantly more demanding than a typical rec room or home office. Getting these requirements right is essential because the municipality will inspect every element before issuing occupancy, and failure to meet code means tearing out finished work and redoing it.

The most critical drywall-related requirement is the fire separation between the basement apartment and the main dwelling above. The Ontario Building Code requires a minimum 1-hour fire resistance rating for the floor-ceiling assembly separating the two dwelling units. This typically means 5/8-inch Type X fire-rated drywall on the basement ceiling, installed as part of a fire-rated assembly that includes the floor structure above. Depending on the assembly design, this may require a single layer or double layer of Type X — a common configuration is two layers of 5/8-inch Type X on resilient channel, which achieves both the fire rating and the sound transmission requirements simultaneously.

Sound transmission between the basement apartment and the main dwelling must meet a minimum STC 50 (Sound Transmission Class 50) rating under the Ontario Building Code. Standard drywall on regular framing achieves roughly STC 33–38, which is well below the requirement. To reach STC 50, the ceiling assembly typically needs resilient channel (metal hat channels screwed to the joists with drywall screwed to the channels, creating a decoupled connection), combined with batt insulation in the joist cavities and the fire-rated drywall. It is absolutely critical that no drywall screw penetrates through the resilient channel into the joist above — a single short-circuiting screw transmits vibration directly through the assembly and can reduce the effective STC rating by 10–15 points, defeating the entire purpose of the resilient channel installation.

For even higher sound isolation — which many GTA landlords pursue to minimize tenant complaints — options include double-layer drywall with Green Glue compound between layers ($15–$25 per tube, one tube per 4x8 sheet), QuietRock soundproof drywall ($55–$90 per sheet), or staggered-stud wall assemblies at the perimeter that mechanically decouple the basement walls from the main structure.

Fire Separation Details

The 1-hour fire separation extends beyond just the ceiling. Every wall, floor, and ceiling element that separates the basement apartment from the main dwelling, from common areas (shared hallways, stairwells), and from service rooms (furnace room, electrical panel room) must achieve the required fire resistance rating. This means:

The furnace/mechanical room must be enclosed with fire-rated drywall if it serves both units or is accessible from the basement apartment. All penetrations through fire separations — electrical wires, plumbing pipes, HVAC ducts, recessed lights — must be sealed with fire-rated caulking or intumescent fire stop to maintain the fire separation integrity. Recessed pot lights in a fire-rated ceiling assembly must be IC-rated (insulation contact) and fire-rated or enclosed in fire-rated boxes.

The door between the basement apartment and any shared area must be a solid-core or fire-rated door with a self-closing device. The drywall framing around this doorway must maintain the fire-rated assembly continuously — no gaps, no unsealed headers.

Additional Code Requirements Affecting Drywall

Vapour barrier: A continuous 6-mil polyethylene vapour barrier on the warm side of all insulated exterior walls and any wall assembly against the foundation, sealed at all seams and penetrations with acoustic sealant. This goes up before drywall and must be inspected before boarding begins.

Ceiling height: The Ontario Building Code requires a minimum clear ceiling height of 1.95 metres (6 feet 5 inches) in basements and 2.1 metres (6 feet 11 inches) in habitable rooms. This measurement is taken from the finished floor to the finished ceiling (bottom of the drywall, including any resilient channel depth). In many older GTA homes with 7-foot basement ceilings, the combination of resilient channel (approximately 1/2 inch), drywall (5/8 inch), and any dropped framing for ductwork or plumbing can eat into the available headroom significantly. Careful planning of the ceiling assembly is essential to maintain minimum height requirements.

Egress: Every bedroom in the basement apartment must have an egress window meeting minimum size requirements (0.35 square metres, minimum 380mm wide and 558mm high, maximum 1,500mm sill height). The drywall framing around egress windows must not reduce the clear opening size below these minimums.

Moisture-resistant drywall: The bathroom in a basement apartment should use moisture-resistant (green board) or mould-resistant (purple board) drywall on walls and ceilings. Shower and tub surrounds require cement board — never drywall of any type — as the tile substrate.

Cost Implications

The drywall scope for a legal basement apartment is substantially more expensive than a standard basement finish due to the fire-rated board, resilient channel, sound insulation, fire stopping, and the additional labour involved in meeting code requirements. Expect the drywall scope alone to run $8,000–$18,000 for a typical 600–900 square foot basement apartment, compared to $5,000–$12,000 for a standard basement finish of similar size. The total cost for a complete legal basement apartment conversion in the GTA — including electrical, plumbing, HVAC, egress windows, separate entrance, and all finishes — typically ranges from $50,000–$120,000.

This is emphatically a project for experienced professional contractors. The interplay between fire ratings, sound transmission, vapour management, and code compliance requires expertise across multiple trades, and the consequences of errors are expensive to fix and potentially dangerous. Find experienced basement apartment conversion contractors through the Toronto Construction Network at torontoconstructionnetwork.com.