What plumbing is needed for a basement bathroom rough-in in a Toronto home?

A basement bathroom rough-in in a Toronto home requires three drain lines (toilet, sink, and shower or tub), corresponding water supply lines (hot and cold), a vent stack connection, and in most cases, breaking and excavating the concrete slab to install the underground drain piping — all of which must be done by a licensed plumber and inspected by the City of Toronto Building Division before the concrete is repoured. This is the most invasive and expensive component of adding a bathroom to a finished basement.

The drain system is the most complex part of the rough-in because basement drains must connect to the home's main sanitary sewer line, which typically runs under or through the basement slab. For a three-piece bathroom (toilet, sink, shower), the plumber will need to install a 3-inch drain line for the toilet (connected to a toilet flange set at the correct height for the finished floor), a 2-inch drain line for the shower, and a 1.5-inch drain line for the sink vanity. These drain lines must slope at a minimum of 1/4 inch per foot toward the main drain connection to ensure proper gravity flow. The plumber marks the layout on the concrete floor, then cuts the slab with a concrete saw and excavates the trench — typically 12 to 18 inches deep — to install the new ABS drain piping. After the piping is installed and inspected, the trench is backfilled with gravel and the concrete is repoured.

If the bathroom drain connections are below the level of the main sewer line — which happens in some GTA homes, particularly those with shallow sewers or in areas like the Beaches, Lakeshore, or Mimico where homes sit close to Lake Ontario — a sewage ejector pump is required. The ejector pump sits in a sealed pit below the bathroom floor and pumps waste up to the main sewer line. Ejector pump systems cost $2,000 to $5,000 installed in the GTA and must be properly sized for the fixtures they serve.

Water supply lines are simpler. The plumber runs new hot and cold water supply lines (typically 1/2-inch copper or PEX) from the existing water distribution system to each fixture location. PEX (cross-linked polyethylene) has become the standard for new residential plumbing in the GTA because it is flexible, resistant to freezing and bursting, and faster to install than copper. Supply lines for the shower include a mixing valve installed in the wall cavity, accessible for future maintenance.

Venting is essential for proper drain function and is the component most often misunderstood by homeowners. Every fixture drain needs to be connected to a vent stack that allows air into the drain system, preventing siphoning of the water traps that block sewer gas from entering the home. The bathroom vent can typically tie into the home's existing vent stack, but the connection must comply with Ontario plumbing code requirements for distance and sizing. If the existing vent stack is too far from the new bathroom location, an air admittance valve (AAV, sometimes called a Studor vent) may be permitted as an alternative in some situations, though the City of Toronto has specific restrictions on AAV use.

A backwater valve on the main sewer line is strongly recommended — and in many GTA municipalities, required — when adding basement plumbing. This valve prevents sewage from backing up into your basement bathroom during heavy rainfall when the municipal sewer system is overwhelmed. Many GTA municipalities, including Toronto, Mississauga, and Brampton, offer rebates of $1,000 to $3,250 for backwater valve installation.

Budget $8,000 to $15,000 for the rough-in plumbing alone in a GTA basement bathroom, depending on the distance from the main drain, whether the slab needs to be cut, and whether an ejector pump is required. If your home was built with a pre-installed rough-in (capped drain and supply lines already under the slab), the cost drops to $2,000 to $5,000 for connecting to the existing rough-in — a significant saving that makes these homes particularly attractive for basement finishing.