What is the minimum STC rating required by Ontario Building Code for walls between dwelling units?

The Ontario Building Code requires a minimum Sound Transmission Class (STC) rating of 50 for walls, floors, and ceilings separating dwelling units. This applies to all party walls in semi-detached homes, townhouses, duplexes, condominiums, and any building containing two or more separate dwelling units, including secondary suites and accessory dwelling units.

An STC 50 rating means that loud speech on the other side of the wall is barely audible, and normal conversation is essentially inaudible. To put this in perspective, a standard interior wall with single-layer 1/2-inch drywall on wood studs rates around STC 33-35 — you can hear normal conversation clearly through it. At STC 40, loud speech is audible but unintelligible. At STC 50 (the code minimum), only very loud sounds like amplified music or a loud TV are faintly perceptible. The jump from STC 33 to STC 50 represents a dramatic improvement in day-to-day livability, which is exactly why the code sets this threshold for shared walls.

The specific reference is Ontario Building Code Section 9.11, which addresses sound transmission in residential construction. This section requires that the apparent sound transmission class (ASTC) between dwelling units be at least 47, or alternatively that the wall assembly achieve a laboratory STC rating of at least 50. The distinction matters — ASTC is measured in the actual building and accounts for flanking paths (sound travelling around the wall through floors, ceilings, and connections), while the laboratory STC rating is measured under ideal conditions. Because real-world performance is always somewhat lower than lab ratings due to flanking, the code accepts either metric.

For Toronto-area construction, achieving STC 50 typically requires one of several proven wall assemblies. The most common approach in new residential construction across the GTA is a single wood or steel stud wall with insulation in the cavity and a double layer of 5/8-inch Type X drywall on at least one side, achieving STC 50-55. For condo and multi-residential buildings, the standard is typically a steel stud assembly with mineral wool insulation, resilient channel on one side, and 5/8-inch Type X drywall — this achieves STC 55-60. Staggered stud walls (2x4 studs staggered on a 2x6 plate so no stud touches both sides) with insulation achieve STC 50-56. Double stud walls with an air gap between the two stud rows and insulation in both cavities can reach STC 60-65.

It's worth noting that the STC 50 requirement is a minimum, and many GTA homeowners and condo developers choose to exceed it. In practice, STC 50 still allows some sound transmission — a loud party or amplified music next door will be faintly audible. For comfortable living in Toronto's densely packed semi-detached homes and condos, many acoustics professionals recommend targeting STC 55-60 for party walls, which provides a much more satisfying level of privacy.

For homeowners creating a secondary suite or accessory dwelling unit in Toronto — increasingly common given the city's housing policies — the STC 50 requirement applies to the separation between the primary dwelling and the suite. This catches many homeowners off guard during basement apartment conversions, where the floor/ceiling assembly between the main floor and the basement suite must meet both the fire separation and sound transmission requirements. A typical floor assembly needs mineral wool insulation between joists, resilient channel on the ceiling below, and double drywall to meet STC 50.

If you're renovating a shared wall in a Toronto semi-detached or adding a secondary suite, the STC 50 requirement will be checked during building inspection. This is not optional or advisory — it's a code requirement that must be met for the permit to be closed. Your drywall contractor should be specifying assemblies that meet or exceed STC 50 and should be familiar with the tested assemblies listed in the Ontario Building Code appendix.

For proper assessment of your existing wall assembly and a soundproofing plan that meets code, get matched with an experienced drywall professional through the Toronto Construction Network.