Double Sink Plumbing Convert: A Canadian Homeowner Guide

Adding a second sink to a shared bathroom is one of the most requested upgrades in Canadian homes: two people getting ready at once stop negotiating over one bowl. The encouraging part of a double sink plumbing convert is that if you already have one working sink, most of the rough-in is already in the wall. The honest part is that "just add a sink" still touches drainage, venting and water supply, and each has rules. Here is how it works, where the decisions are, and when to call a pro.

How the plumbing behind a double vanity works

Every sink needs three connections: hot and cold supply, a drain, and a vent. The drain carries waste water out through a P-trap, the curved fitting that holds a plug of water to block sewer gas. The vent is the part people forget: a pipe that lets air into the drainage system so water flows out smoothly without siphoning the trap dry.

A single vanity has one set of each. A second bowl can be served two ways:

• Shared drain (most common): both bowls connect to a single trap through a continuous waste assembly, the tee-shaped tailpiece kit under most double vanities. The existing drain stub stays where it is, ideally centred between the bowls.
• Separate drains: each bowl gets its own trap and wall connection. This is the route when the bowls sit far apart or a drawer bank occupies the middle of the cabinet.

Supply is the easy half: a plumber, or a confident DIYer where local rules allow, tees off the existing hot and cold lines to feed the second faucet. Drainage and venting are where the real decisions live.

The decision path: one drain or two

1. Measure the run. A comfortable double vanity generally starts around 152 cm (60 inches). Compact doubles exist near 122 cm (48 inches), but elbow room gets tight.
2. Find your existing rough-in. Pull or look behind the old vanity and note where the drain stub and supply valves sit relative to where the new bowls will land.
3. Check the spread between bowls. If both sit reasonably close to the existing drain, a shared trap with a continuous waste is the simplest, least invasive option. Codes limit how far a trap arm can run from its vent, so confirm the allowance with your local authority or plumber.
4. Plan for two drains if the bowls are far apart. That usually means opening the wall, adding a second sanitary tee, and confirming the venting still protects both traps. It is a bigger job, and typically where permits come into play.
5. Confirm the cabinet works with the pipes. Drawer banks, centre doors and open shelving all collide with traps differently. Choose the vanity before anyone cuts a pipe.

A realistic conversion sequence

1. Select the vanity and top first, since bowl positions dictate everything downstream. Our bathroom vanities collection includes double-sink models in the common Canadian widths, with Canada-wide shipping from Vancouver to Halifax.
2. Shut off the water at the valves, disconnect, and remove the old vanity and top.
3. If the drain moves or a second one is added, open the wall and rough in the new drainage and venting to local code, inspected and leak-tested before the wall closes.
4. Patch the wall, then set the cabinet level and anchor it.
5. Install the top, faucets, pop-up drains, traps and supply connections.
6. Fill and drain both bowls, check every joint dry-handed, and re-check after a day of normal use.

Common mistakes in a double sink conversion

• Buying the vanity last. Roughing in plumbing before the cabinet is chosen is how bowls end up misaligned with traps and drawer boxes.
• Ignoring the vent. A second bowl added far from the existing vent brings gurgling, slow draining and traps that siphon dry, letting sewer smell into the room.
• Building an accidental S-trap. Flexible accordion drain kits and creative re-piping under the cabinet often create trap configurations that codes across Canada do not permit.
• Assuming the old drain line can take anything. Older homes, particularly with galvanized drainage, may have narrowed internal diameter from decades of buildup, and two bowls will show it quickly.
• Running supply lines through an exterior wall. In a Canadian winter, from the Prairies to the Maritimes, that is a freeze risk. Keep water lines in interior walls wherever possible.
• Forgetting what sits above the counter. Two faucets mean two sets of tap holes, plus mirrors and wall sconces for both stations; plan them alongside the plumbing so you are not patching drywall twice.

Permits, codes and the Canadian context

The National Plumbing Code of Canada is the model document, but what governs your project is the version your province or territory has adopted, plus municipal bylaws. Quebec, British Columbia and others maintain their own editions. As a general pattern, replacing a vanity and faucet in the same location is routine maintenance, while relocating or adding drainage and venting usually needs a permit and inspection. Confirm with your local building department before opening a wall; condos often require licensed trades for any plumbing work.

When you buy fixtures and fittings, look for CSA or cUPC certification marks. Established brands sold in Canada, including Kohler, Riobel, Kalia and Canadian drain specialist OS&B, carry the appropriate listings.

When to call a licensed plumber

Call a pro for anything behind the wall: moving a drain, adding a second one, new venting, or tying into supply lines you cannot fully see. Also call one if the home has older galvanized or mixed-material drainage, if the bathroom sits above finished living space where a leak gets expensive, or if your municipality or condo requires licensed work.

What a handy homeowner can often do, where local rules allow: remove the old vanity, install the new cabinet and top, set the faucets and sink drains, and connect traps and supplies to an existing, unmoved rough-in.

Rounding out the project

Beyond the vanity, budget for two matching bathroom faucets, drain assemblies, sound shut-off valves, and the mirror and lighting plan above the counter. If you are weighing wall-hung against floor-standing cabinets, or one long trough bowl against two separate sinks, our buying guides hub walks through those trade-offs in more depth. And if you renovate professionally, the Vatero trade program is built for this kind of repeat project work.

Frequently asked questions

Can two sinks share one P-trap?

Usually yes, when the bowls sit close together and connect through an approved continuous waste fitting into a single trap, the standard arrangement under most factory double vanities. Codes limit the distances involved, so if your bowls sit far apart, plan on a second trap and confirm the layout locally.

Do I need a permit to convert to a double sink?

If the existing drain, vent and supply stay put and you are only connecting new fixtures to them, many municipalities treat it as replacement work. The moment drainage or venting moves or is added, a permit and inspection are typically required. Rules vary by province and municipality, so a call to your local building department settles it.

Will two sinks drain slower than one?

Not when the drainage and venting are sized correctly. Both bowls rarely empty at the same instant, and a properly built branch handles the load. Slow draining or gurgling after a conversion almost always points to a venting problem or a partially blocked older line, not to the second bowl itself.

What size vanity do I need for two sinks?

Around 152 cm (60 inches) is the comfortable starting point. Compact doubles near 122 cm (48 inches) fit smaller rooms but feel tight in daily use. Measure the wall, subtract clearances for the door swing and toilet, and if the number comes up short, one generous single bowl often serves better than two cramped ones.