Why does my bathroom drain smell like sewage, and is it a sign of a serious plumbing problem?

A sewage smell coming from a bathroom drain is never normal, but it's not always a sign of a serious plumbing problem — in many cases, the cause is a dried-out P-trap, which is a simple fix. That said, persistent sewer odour after addressing the obvious causes warrants a professional assessment, because it can indicate venting issues, a failed wax ring, or a cracked drain line.

The P-Trap: The Most Common Cause

Every drain in your bathroom — the sink, the shower, and the floor drain — has a P-trap, a U-shaped section of pipe that holds a small amount of water at all times. This water seal is the barrier between your living space and the sewer gas in the drain system. When the P-trap dries out, sewer gas rises directly through the drain into your bathroom.

P-traps dry out most commonly in infrequently used drains — a basement bathroom floor drain, a guest bathroom shower, or a second sink that nobody uses for weeks at a time. In Toronto's dry winter months, when indoor humidity drops and heated air accelerates evaporation, P-traps can dry out surprisingly fast. The fix is simple: pour water down the drain. Run the faucet or shower for 30 seconds, or pour a cup of water directly into a floor drain. The water refills the trap and restores the seal. For drains that are rarely used, add a tablespoon of mineral oil after the water — it floats on top of the water and slows evaporation significantly.

Other Common Causes

A failed wax ring under the toilet allows sewer gas to seep out around the base of the toilet. If the smell seems to originate from around the toilet rather than a drain, the wax ring may need replacement — a $200–$400 job for a licensed plumber, including removing and resetting the toilet with a new wax ring and supply line.

A damaged or missing P-trap can occur after amateur plumbing work, renovations where the trap was disturbed, or in very old Toronto homes where original cast iron traps have corroded through. A plumber can inspect the trap visually (under the sink) or with a camera (for shower and floor drains) and replace it if necessary.

A blocked or improperly vented drain system is a more serious cause. Every plumbing fixture needs a vent pipe that connects to the main vent stack and exits through the roof. The vent equalizes air pressure in the drain system and allows sewer gas to escape safely above the roofline. If a vent is blocked (bird nests, ice buildup in Toronto winters, or debris), clogged, or was never properly connected, sewer gas can be pulled back through the P-traps as water drains — you may notice a gurgling sound when you flush the toilet or drain the tub, accompanied by the sewer smell. This requires a licensed plumber to diagnose and repair.

A cracked or deteriorating drain pipe inside the wall or below the floor can leak sewer gas into the bathroom space. In older Toronto homes with cast iron drain stacks — common in pre-1970s construction across the city's established neighbourhoods — corrosion can create pinhole leaks or cracks that allow gas to escape. A plumber can inspect the stack with a camera and recommend repair or replacement. Cast iron stack replacement in a GTA home runs $3,000–$8,000 depending on accessibility and the extent of deterioration.

When to Call a Plumber

If refilling the P-traps doesn't eliminate the smell within 24 hours, call a licensed plumber. Persistent sewer gas in your home is not just unpleasant — it contains methane and hydrogen sulfide, which in high concentrations can cause headaches, nausea, and respiratory irritation. A professional can perform a smoke test ($200–$400) where non-toxic smoke is introduced into the drain system to identify exactly where the leak is occurring.

Don't ignore sewer odour or mask it with air fresheners. The underlying cause — whether it's a simple dry trap, a failed wax ring, or a cracked drain line — only worsens with time, and early diagnosis prevents more expensive repairs down the road.