What's That Smell? Understanding Sewer Gas in Your Home

A sulphur smell — like rotten eggs — coming from somewhere inside your home is unpleasant, but more importantly it's a signal that something in your plumbing system isn't working as it should. In most cases it's a sewer gas issue: gases from the wastewater system are making their way back into your living spaces. Here's what causes it, what the risks are, and how to track down the source.

What Is Sewer Gas?

Sewer gas is a mixture of gases produced by the decomposition of organic waste in the drainage and sewer system. The smell typically comes from hydrogen sulphide — the compound responsible for that distinctive rotten egg odour. In high concentrations, sewer gas can be harmful, and methane (also present in sewer gas) is flammable. While the concentrations in a domestic setting are rarely dangerous, persistent exposure is not something to be relaxed about, and the underlying cause needs to be found.

How Does Sewer Gas Get Into the House?

Your home's drainage system is designed with a series of barriers to prevent sewer gas from entering your living spaces. The most important of these is the P-trap — the curved section of pipe beneath every sink, shower, floor waste, and bathroom fixture. The P-trap holds a small amount of water at all times, and that water acts as a seal, physically blocking sewer gas from passing through. Your vent pipes (the pipes that exit through your roof) are the other part of the system — they allow air into the drain lines to keep water flowing properly, and they vent gases safely outside.

When either of these systems fails, gas can get through.

Common Causes

1. A Dry P-Trap

This is the most common and easiest-to-fix cause of sewer gas smell in Australian homes. If a drain hasn't been used for a while — a guest bathroom, a laundry tub that's rarely used, a floor waste in the garage — the water in the P-trap can evaporate, breaking the seal. Sewer gas then has a clear path into the room.

Fix: Run the tap or pour a litre of water down the drain to refill the trap. If the smell disappears within an hour, a dry trap was the culprit. For drains that are rarely used, running them briefly every few weeks will prevent this from happening again.

2. A Cracked or Broken P-Trap

Plastic P-traps can crack over time — particularly on older fittings or where pipes have been knocked or stressed. A cracked trap may still hold water most of the time, but a hairline crack can allow gas to pass through even when the water seal appears intact. If you've topped up a trap and the smell persists, a cracked fitting is worth investigating.

3. A Blocked or Damaged Vent Pipe

Vent pipes exit through the roof and allow the sewer system to breathe. If a vent pipe becomes blocked — by a bird's nest, leaf debris, or a small animal — negative pressure builds up in the drain lines. This can literally suck the water out of P-traps throughout the house, removing the seal. It can also cause gurgling sounds from drains and toilets.

A blocked vent is a job for a plumber — clearing it involves accessing the roof and the pipe from above.

4. A Cracked Drain Pipe

A crack in a drain pipe within a wall cavity or under the floor can allow sewer gas to escape directly into the structure of the house. This is harder to diagnose without camera equipment, but persistent smells that don't resolve after checking traps and vents often point to a pipe issue. A CCTV drain inspection can locate the problem without any demolition.

5. A Loose or Poorly Sealed Toilet Base

The wax seal (or rubber seal on more modern installations) between the toilet base and the floor waste prevents gases from escaping around the toilet. If the toilet rocks, has been moved and reinstalled, or if the seal has dried out and cracked, sewer gas can escape around the base. A toilet that wobbles slightly, or one where you notice the smell specifically around the toilet area, may have a failing base seal.

When to Be More Concerned

In most household situations, the concentration of sewer gas is an unpleasant nuisance rather than an immediate danger. However, you should take it more seriously if:

• The smell is very strong or getting noticeably worse over a short period
• Multiple people in the household are experiencing headaches or nausea
• The smell is coming from a room with an enclosed or poorly ventilated space
• You can hear gurgling from multiple drains at once — this suggests a systemic blockage rather than a single dry trap

In these cases, call a plumber rather than continuing to investigate on your own.

What You Can Do First

1. Run water down every drain in the affected area — sinks, showers, floor wastes, laundry tubs
2. Check for any drains that are clearly not being used regularly and flush them
3. Check around the toilet base for any rocking or movement
4. Open windows to ventilate the space while you investigate
5. If the smell persists after topping up all traps, call a plumber