Smelly Drains and Sewer Flies: What's Causing Them and How to Fix Them

Why does your drain smell? The trap, the slime or the pipe

Your drain smells because air from the sewer is getting past the trap, or because something in the trap itself is rotting. Drains such as floor wastes hold water in seals and traps. That water is the seal between your room and the sewer. Sydney Water says pouring water into a floor waste replenishes water sitting in seals and traps, which is why a refill is the first thing to try.

The three causes look different. A dry trap happens in a shower nobody uses, a floor waste that never gets water, or a home left empty for a few weeks. Build-up happens in drains you use every day: Unitywater says hair, food scraps and other material can build up in the S-bend of the kitchen or bathroom drain. The third cause is a fault further down the line, such as a blocked pipe. Unitywater also says a smell inside the house may mean a blocked toilet or wastewater pipe.

You can deal with the first two at home. The third is regulated drainage work for a licensed plumber.

Smell or flies? Match what you notice to the likely cause

What you smell, and where, narrows down the cause before you touch anything. Use the table below as a starting point, then follow the matching section of this guide. A smell can have more than one cause, so if the first step doesn't fix it within a day or two, move to the next likely cause.

Rule out gas first: a rotten-egg smell can be a gas leak

A rotten-egg smell can come from the sewer or from leaking gas, and gas is the one to rule out first. The Queensland Government says fuel gas has a chemical called mercaptan added to give it a distinctive odour, similar to rotten eggs. Unitywater describes sewer odour the same way. The same page warns that LPG is heavier than air and can pool in low areas and drains, so a gas smell near a drain is not proof it is the drain.

Think gas if the smell is strongest near a cooktop, gas hot water system, heater, the gas meter or LPG bottles. The Queensland Government steps if you smell gas are:

• Don't light any fires, and don't turn electrical devices or light switches on or off.
• Turn off the gas appliance.
• Open doors and windows to ventilate the area.
• If you can still smell gas, turn off your gas supply and contact your gas supplier or a gasfitter for advice.
• If you can still smell gas after turning off the supply, call 000. Your gas supplier's emergency number should be on your gas bill.

Fixes to try at home, in this order

The home fixes for a smelly drain go from cheapest and safest to most effort: refill, clean, flush. Queensland lets you do some drain care without a licence. The QBCC lists cleaning or maintaining a ground-level grate for a trap on a sanitary drain as unregulated work. Work through these steps one at a time, so you know which one fixed it.

1. Refill the trap. Run the tap or shower for a minute, or pour a bucket of water into a floor waste. Flush any toilet that hasn't been used for a while.
2. Clean the grate and the removable waste. Lift the shower or floor waste grate, pull out hair and soap, and scrub the grate and the sides of the waste you can reach.
3. Flush with hot tap water to rinse loosened slime through the trap.
4. If you use a drain product, pick one made for the job and follow the label exactly. Use one product at a time.

Never mix drain or cleaning products, including bleach and vinegar. Better Health says chemicals may create a poisonous gas or liquid when mixed. If the smell comes back within days of a refill and a clean, the cause may be in the pipe, and the next step is a plumber, not a stronger chemical.

Drain flies (sewer flies): how to identify them and get rid of them

Drain flies are a sign of slime in a drain, and getting rid of them means cleaning out that slime. The University of Kentucky's entomology department describes drain flies, also called moth flies, as small, dark-winged, non-biting gnats. Their wings are covered in scales, so they turn into a smudge of fine dust when swatted. They rest on walls or ceilings and make short hopping flights when disturbed.

The larvae live in the fine slime layer in sink or floor drains and in toilets that are rarely used. The same source says drain flies are often seen after a holiday and usually disappear once water starts moving through the traps again. Many flies over several weeks usually means a breeding site that has to be found.

• Find the drain: lightly coat the inside of a clear plastic cup with vegetable oil or petroleum jelly, turn it upside down over each drain and leave it for a few days. Flies coming out of that drain stick to the cup.
• Clean it: scrub the grate and the waste to remove the slime. Cleaning pipes and traps is the most effective control.
• Hot water from the tap may give short-term control, but drain fly larvae are hard to drown because they trap air bubbles.
• Don't pour insecticides down drains to kill drain flies.

If the flies keep coming after a thorough clean, have a plumber check the drain.

Toilet smells but isn't blocked: what to check and when to call

A toilet that flushes normally but still smells has either a simple cause you can check or a pipe fault that needs a plumber. Start with the simple one: a toilet in a spare bathroom can lose its water seal or grow slime if it isn't used. Flush it a few times and give the bowl a clean, then see if the smell goes.

If it doesn't, look around the base of the pan after a flush. Water on the floor or a damp patch around the pan is something to show a plumber. Unitywater says a smell inside the house may mean a blocked toilet or wastewater pipe. Take photos of what you see, so the plumber knows where to look.

Gurgling is a separate clue. Redland City Council says that in severe wet weather, a gurgling toilet usually means the overflow relief gully outside is doing its job. Redland City Council says calling a plumber isn't necessary in wet weather unless there is an actual blockage within your property's wastewater line. Our blocked toilet guide covers what to do if the toilet starts to back up.

Smell outside the house: check the overflow relief gully

For a sewer smell outside the house, the overflow relief gully is the first thing to check. Redland City Council describes the overflow relief gully (ORG) as a drain-like feature with a grated lid in the ground outside your home. The ORG sends wastewater outside, instead of into the house, if there is a blockage within your property.

Here is what you can safely check, without opening anything:

• The grate should sit loosely and should not be covered by pot plants, tiling, paving or landscaping.
• Logan City Council says an ORG should sit 75 mm above the surrounding finished ground level, except in a path or paved area.
• Downpipes and water tank overflows should not drain into the ORG.
• A home with more than one bathroom may have more than one ORG, so check them all.
• A home connected to the sewer before the 1970s may not have an ORG at all. Redland City Council suggests talking to a plumber in that case.

Unitywater says long periods of rain can cause a temporary wastewater smell to come up through floor drains. A smell that only appears in wet weather and goes once it dries out may be the network, not your pipes. A smell or wet grate in dry weather needs the checks in the next section.

Whose pipe is it? Your drain versus the council or utility sewer

The pipes on your property are yours to look after, and the sewer main belongs to your council or water utility. Redland City Council says the internal wastewater pipes on your property are your responsibility to maintain. Logan City Council says it repairs blocked and damaged sewerage mains outside your property boundary, and that for a problem with the sewerage pipes on your property you should call a licensed plumber in the first instance. Logan also says it is the property owner's responsibility to keep the ORG properly fitted and maintained.

The ORG helps you tell the two apart. Redland City Council says an ORG that keeps overflowing when nobody is using water may mean a blockage in the council network, so contact council. An ORG that overflows when you flush or run a tap may mean a blockage in your own line, so call a plumber.

Urban Utilities says it will maintain and operate the sewerage service from the property connection point and respond to sewage overflows in its network. For local help on your side of the line, see blocked drains in Capalaba or Victoria Point drain clearing.

When a smelly drain is a job for a licensed plumber

A smelly drain becomes a job for a licensed plumber once refilling and cleaning haven't fixed it, because anything past the grate is regulated work. The QBCC says all regulated plumbing and drainage work must be done by a licensed plumber or drainer, and lists unblocking or repairing pipes as regulated work. Even unblocking sanitary drainage is minor work that must be performed by a QBCC licensee. The unregulated list owners may do is short, and covers tasks such as cleaning a ground-level grate or replacing the cap on a ground-level inspection opening.

Call a plumber if you notice any of these:

• The smell comes back within days of refilling and cleaning the trap.
• Several drains smell at once, or drains are slow to empty.
• The ORG overflows when you use water in the house.
• Drain flies keep appearing for weeks after a thorough clean.
• Damp ground, sewage or a smell in the yard along the line of the drain.

If a drain keeps smelling or blocking and there are trees near the line, our guide to tree roots and pipe relining explains what to look for. Jim Monger Plumbing and Gas works under QBCC licence 15504687 and gas licence L90908. For help with a drain, see our blocked drain service or call 0418 871 269.