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Sewer Smell in House in Thornton, CO: Causes and Fixes
Sewer gas should stay in the pipes. When you can smell it, something is letting it out, and most causes are simple. A few are not, so it helps to work from the easy checks to the serious ones.
By the Thornton Plumbing Co. team ·
Dry traps
Every drain has a U-shaped trap that holds water and blocks gas. If a drain is rarely used, such as a basement floor drain, a guest bath or a laundry sink, the water can evaporate and the smell comes up.
Pour water down any unused drain, then add a spoonful of cooking oil so the water evaporates more slowly. In a dry Colorado climate, traps dry out faster than many people expect.
Vents and the roof
The vent pipes admit air so drains can run freely, and they carry gas up and out. A blocked vent, a vent too close to a window or a cracked vent pipe can cause odor and slow drains.
A bird nest, leaves or a frost cap in winter can plug a vent. A plumber can examine the vent from the roof or send a camera through it.
Toilets and wax rings
A toilet with a failed wax ring or a loose bolt can leak gas from the base. You may also see a stain or feel a wobble. Replacing the ring and checking the flange fixes it. See toilet repair.
Broken or cracked drain pipes
A crack in a drain inside a wall or under a floor lets gas out and sometimes water. The smell may be strongest in one room. A plumber can use smoke or a camera to find it.
In basements, check the floor drain and the sump pit. A sump cover that is not sealed can let gas up.
Smell clues
Where the smell is strongest points to the cause.
| Where it is strongest | Likely cause | Try this |
|---|---|---|
| Basement floor drain | Dry trap | Pour water in and add a little oil |
| Near a toilet | Wax ring or flange | Check for a wobble |
| Through the house | Vent or cracked drain | Call for a smoke or camera test |
| Yard or near the cleanout | Damaged lateral | Book a camera inspection |
The sewer lateral
Sewer gas in the yard or a smell that appears with slow drains can mean a problem in the lateral. Cracks, roots and sags let gas escape and block flow. A camera inspection reveals the real condition of the pipe.
Thornton's wastewater is collected through about 511 miles of city sewer and treated at Metro Water Recovery. The pipe from your house to the city main is generally the owner's.
When to act at once
If the smell is strong, you feel dizzy or have a headache, or you see sewage, leave the area and call. Gas from sewage contains hydrogen sulfide, which is hazardous at high levels. Natural gas has its own smell and is a separate emergency: leave and call 911 or the gas company.
If several drains are slow and the smell comes with gurgling, a main line blockage is likely. Stop using water and phone the 24/7 line. The drain cleaning page explains how blockages are cleared.
Gas, drain or something else
A rotten egg smell is not always sewage. Natural gas has an odorant added so leaks can be noticed, and it smells similar. If gas seems likely, get out of the house, avoid light switches and anything with a flame, and ring the gas utility or 911 from the yard. Plumbing comes afterward.
Sewer gas smells more like a mix of sulfur and decay. It tends to show up near a particular drain, a floor drain or a basement corner.
Dry traps and vents
Every drain has a curved trap that holds water and blocks gas. A floor drain or a seldom used bathroom can dry out, and then the smell comes in. Pour a quart of water down it, and add a little mineral oil to slow evaporation.
A blocked vent on the roof can make fixtures gurgle and let gas into the house. A cracked vent pipe in a wall can do the same. A plumber can smoke test the system to find leaks.
Signs of a bigger problem
If the smell comes with slow drains, backups or wet spots in the yard, the sewer line may be damaged. A broken pipe leaks into the soil and releases odor. See sewer camera inspection and sewer line repair.
A wax ring that has failed under a toilet can let gas up around the base. Rock the toilet gently to see whether it is loose.
Quick cleanup tips
Run water in every fixture, including the shower and the laundry sink. Clean the P-trap under sinks. Check the water heater's drain pan. Open windows while you investigate. If the smell remains, call a plumber and describe where it is strongest.
When to call a plumber
Phone if the odor persists after you refill traps, if more than one drain gurgles, or if you see wet spots near the line. A smoke or camera test finds the source without tearing out walls. Odors that come with illness symptoms, headaches or a rotten egg smell throughout the house deserve fast attention, and anyone who feels unwell should step outside for fresh air.
Basements and floor drains
Basement floor drains are the most common source in houses that sit empty for stretches. A drain that only sees water a few times a year loses its seal as the trap evaporates. Refill it every month, and ask about a trap primer if the drain sits in a laundry room that rarely gets used. A cracked floor drain body can also leak gas, and that repair needs a plumber.
Quick answers
Asked about sewer smell
A floor drain trap that has dried out is the likeliest reason. Pour in some water, and if the odor lingers, have the vent and the lateral checked.
A faint whiff is mostly unpleasant, while heavy concentrations are hazardous. Open windows and call a plumber.
Yes. A damaged lateral or a broken cleanout cap can release it.
Usually not unless there is sudden damage. Ask your insurer.
Connected services
Drain cleaning & unclogging
Unblock a sink, tub, washer line or main drain, then learn what caused the stoppage. Open page
Sewer Line Repair
Correct a broken, root-invaded or bellied sewer lateral, whether the repair is one spot or a longer run. Open page
Related reading
Tree roots in the sewer line
Roots find tiny cracks and grow into a dense mat that catches everything. Here are the signs, the fixes and what keeps them from returning. Open page
Toilet will not flush
A toilet that will not flush usually has a handful of causes, from a lifted chain to a blocked main line. Here is how to sort them. Open page
Tracking down a hidden leak
Reading the meter, dyeing the toilet tank and walking each room will turn up most leaks. Here is the order to do them in. Open page
Sewer odor that keeps coming back?
Call and say where the odor is strongest so the right contractor is lined up.