Primer ยท Odours
Sewer Smells and Drain Flies
A sewer smell in the house, or tiny moth-like flies around the sink, almost always points to a specific drain problem. Most of them you can find yourself.
- Plain steps you can do yourself, safely
- Clear signs for when to stop and call
- Written for Newmarket houses and water
- A plumber on the phone if it goes sideways
What goes wrong in the pipes
Every drain in your home has a trap, a bend that holds water to block sewer gas from rising into the room. When a trap dries out, a seal fails or a vent clogs, the gas comes through. Drain flies are a separate clue: they breed in the slimy film inside drains and overflow openings. Both problems are annoying, and sewer gas in quantity is unhealthy, so tracking down the source is worth an afternoon.
Have these on hand
- Flange plunger
- Cup plunger
- Bucket
- Rubber gloves
Hands-on steps
- Pour water into unused drains. Basement floor drains, guest showers and laundry tubs lose their trap water over time. A litre of water refills the seal.
- Add a spoonful of mineral oil to a rarely used floor drain after filling it. The oil slows evaporation.
- Clean overflow holes. Bathroom sink overflows collect gunk; flush them with hot water and a bottle brush.
- Scrub the drain film. A stiff brush and enzyme cleaner reach the slime where drain flies lay eggs.
- Check the toilet base. A smell around the base can mean a failed wax ring. Look for movement or staining.
- Use tape to test for flies. Lay clear tape over a drain overnight. Flies stuck to it show you which drain is the nursery.
Mistakes that turn a small repair big
- Spraying insecticide at drain flies. It kills adults but not the eggs in the film.
- Pouring bleach repeatedly. It does little to the film and can harm septic systems.
- Sealing a floor drain with tape or a plug permanently; it may be needed in a flood.
Where smells hide in Newmarket homes
Finished basements often have a floor drain tucked under a laundry sink or hidden behind a furnace. Those drains are easy to forget, and they dry out first. Basement bathrooms that are only used by visitors are another common source. Mark them on your calendar for a monthly splash of water.
Kitchen smells are slightly different. A dishwasher drain hose without a high loop can pull sink water back into the machine, leaving a sour odour that people blame on the sink. Lifting the hose to the underside of the counter often fixes it.
When to put the tools down and call
If the smell persists after every trap is filled, or it gets stronger when fixtures drain, a vent stack may be blocked or a pipe cracked inside a wall. Call (365) 517-0623. A wet pit with stale water can also smell, so if your sump is involved see sump pump repair. A noisy tankless heater venting poorly is a different source of odour; tankless installation covers venting basics. Weak flow from fixtures can be a related sign; see low water pressure.
Asked on site
Are drain flies harmful?
They are mostly a nuisance, but they indicate organic buildup in your drains that should be cleaned.
Why does the basement smell after a dry summer?
Trap water evaporates. Refilling the floor drain usually solves it within minutes.