Why Do My Drains Smell Like Rotten Eggs, and How Do You Fix It?

Work out which smell you have first

Every remedy online gets applied to every smell, which is why so many of them fail. The smells below have different causes and different fixes, and telling them apart takes a minute of attention rather than any equipment.

The dry trap: the most common cause, and it is free to fix

Every outlet in the house is sealed against the sewer by a small plug of water held in a trap. There is no valve, no membrane and no mechanism. The water is the seal. Lose the water and there is nothing between the sewer and the room.

Traps lose their water 2 ways. They evaporate, which happens to any fixture that goes unused, and they get siphoned, which happens when a large flow elsewhere pulls air through them. Gurgling from a nearby fixture is the audible version of the second, and it usually turns up in the same house as the smell.

• Guest bathrooms, second showers and rarely used basins are the usual offenders.
• Laundry floor wastes are next, because nothing routinely discharges through them once a washing machine outlet hose sits in the standpipe instead.
• The overflow relief gully outside holds a water seal too, and it is fully exposed to sun and wind.
• A holiday house, a rental between tenants or a home whose owners have been away a month will usually have several dry traps at once.

The remedy is to run every tap, shower and floor waste for 30 seconds, flush every toilet, and pour a couple of litres down anything with no tap over it. Then wait a day. If the smell has gone, that was the whole problem. For a fixture that will keep going unused, a small amount of cooking oil poured in after refilling floats on the seal and slows evaporation considerably.

When only the hot taps smell, it is the anode

This is the cause most drain articles never mention, and it accounts for a large share of rotten egg complaints. A storage hot water cylinder contains a sacrificial anode, usually magnesium, whose job is to corrode instead of the steel tank. Where the water supply carries sulphate and the tank has been sitting warm and undisturbed, sulphate-reducing bacteria use the hydrogen generated at that anode and produce hydrogen sulphide. Hydrogen sulphide is the rotten egg smell, and it arrives only with hot water because that is where it is being made.

• The giveaway is that cold water is odourless from every tap, and hot water smells from every tap.
• It is usually worst on the first draw of the day and after a holiday, because the water has been sitting.
• It is not a health risk at the concentrations involved, and it does not mean the tank has failed.
• The permanent remedy is replacing the magnesium anode with an aluminium or zinc alloy anode, which does not feed the same reaction.
• Raising the stored temperature also suppresses it, and a storage cylinder should be held at 60 degrees anyway to control Legionella, with a tempering valve limiting what reaches the tapware.

The reason this is worth getting right rather than living with is that the anode is a wear part. If the smell has appeared and nobody has checked the anode, the more useful question is how much of it is left. An anode that has been fully consumed stops protecting the cylinder, and the next thing to corrode is the tank.

Biofilm in the waste, and what honestly shifts it

A kitchen smell that gets worse when the sink is used is coming from the pipe wall, not from the sewer. Fat, soap and food residue build a soft organic layer inside the waste, bacteria colonise it, and the smell is the by-product. The same happens in a shower waste, and in the basin overflow channel, which almost never gets cleaned.

Now the unpopular part. Bicarbonate of soda and vinegar is the most recommended remedy on the internet and it is close to useless as a drain treatment. The 2 ingredients neutralise each other on contact, and what is left in the pipe a minute later is salty water. The fizz looks impressive in a sink and has no force behind it once it is inside a 40mm waste. It will not damage anything, and it will not remove a biofilm or move a blockage.

1. Remove and clean the pop-up plug or the shower waste grate. A surprising share of kitchen and bathroom smells live on the underside of the plug itself.
2. Scrub the basin overflow with a bottle brush and warm detergent. This is the fix for a musty smell that never changes when water runs.
3. Flush the waste with very hot tap water and a good squirt of dishwashing liquid, then leave it 10 minutes and flush again. The detergent emulsifies the grease, the heat keeps it moving, and neither is hot enough to damage the pipe.
4. For a persistent kitchen smell, undo the trap over a bucket, tip it out and rinse it outside. Traps hold the worst of it, and this is the step that actually works.
5. For ongoing maintenance in a grease-prone kitchen, an enzyme or bacterial drain treatment left overnight does slowly digest organic build-up. It needs contact time and it does nothing to roots or a solid blockage.

When the smell is structural

A smell that returns within hours of every clean, or affects more than 1 room, has stopped being a housekeeping problem. The drainage system itself is letting gas escape, and there are 3 realistic ways that happens.

• A blocked or collapsed vent. The vent pipe through the roof is what lets the drain breathe. Choke it with gum leaves, a nest or a rusted-flat cowl, and the system pulls its air through trap seals instead. Gurgling and smell together is the classic signature.
• A cracked or disconnected drain under the house. In a home with a suspended timber floor and a vented subfloor, a broken sanitary drain vents sewer gas into that subfloor, and it rises through gaps in the floorboards and around skirtings. The smell seems to come from the room rather than from any fixture.
• An overflow relief gully that is buried, damaged or dry. It is meant to be the deliberate weak point that discharges a blocked sewer into the garden. If it has been paved over, raised garden beds have swallowed it, or its seal has dried out, it becomes a permanent vent for sewer gas at ground level.

None of the 3 can be confirmed from inside the house. A camera inspection with a locator establishes whether the drain is sound and, if not, where the fault sits and how deep. That decides between a patch repair, a sectional reline and an excavation, which are 3 very different jobs.

Why Adelaide houses get this more than most

Our summers are unusually hard on trap seals. A run of 35 to 40 degree days with very low humidity evaporates the water out of an unused trap far faster than a humid climate does, which is why the smell in a spare bathroom is so often a January and February problem and disappears in winter. The overflow relief gully outside is worse again, sitting in full sun with a wide grate over it.

The inner southern and eastern suburbs add a second factor. Goodwood, Unley, Hyde Park, Malvern, Norwood and their neighbours are largely sewered in vitrified clay with mortared joints, and a great many of those homes have suspended timber floors over a vented subfloor. A joint worked open by reactive clay soil, or a section cracked by a root, releases gas into exactly the space the house draws its air from. That produces a persistent whole-house smell with no single fixture responsible for it.

In the Adelaide Hills the picture changes again. On a septic system or a community wastewater scheme, odour usually means a tank overdue for pump-out or a saturated disposal trench, neither of which is a drain fault. Hills roofs also collect a heavy load of gum leaves, and a vent cowl choked with leaf litter is a common cause of smell in Blackwood, Belair, Crafers and Stirling homes.

The order to work through, and when to stop

1. Test hot against cold. If only hot smells, it is the hot water cylinder and nothing below applies.
2. Refill every trap in the house, including floor wastes and the overflow relief gully. Wait 24 hours.
3. Clean the plug, the grate and the basin overflow by hand.
4. Flush with very hot tap water and detergent, then clean out the trap over a bucket if the kitchen is the source.
5. If the smell is gone, stop. Nothing further is needed and nobody needs to be called.
6. If it has returned within a day, or it is in more than 1 room, or a fixture is gurgling as well, stop trying remedies. The cause is the vent or the drain itself and no product poured into a sink will reach it.

Marc carries a RIDGID SeeSnake camera with a locator and a Jetwave jetter, which is what makes it possible to say where the fault actually is rather than work it out by elimination. You get a text with an arrival time before he sets off, and the price is discussed before any work starts.

Questions we get asked

References

• SA Water: sewer network and property connections
• Standards Australia: AS/NZS 3500 Plumbing and Drainage
• Government of South Australia: septic and community wastewater systems