Sewer smell in the house: the seven usual causes

Sewer gas smells like rotten eggs with something sharper behind it. It arrives in one room and drifts, comes and goes with the weather, and it is the one plumbing complaint where the fix ranges from a glass of water to opening the slab. The good news is that the causes are few and they line up from most to least likely, so a homeowner can usually narrow it down in an afternoon without tools.

First, rule out the smell that is not sewer gas

Natural gas and propane are odorized to smell like sulfur or rotten eggs too, and the two get confused. If the smell is near the water heater, the range, the dryer, or the gas meter, or if it is strong and sudden anywhere, treat it as gas. Do not flip switches, do not light anything, leave the house, and call the gas utility from outside. Everything below assumes that has been ruled out.

Narrow it down before you fix anything

Sewer gas has to come from an opening in the drain system. Every drain and toilet has a trap, a bend that holds water as a seal, and every trap connects to a vent through the roof. The smell means a seal is missing, a vent is not doing its job, or the pipe itself is open somewhere. Walk the house with three questions:

• Which room? Stand in each bathroom, the laundry, the kitchen, and the garage with the doors closed for a minute. The room where it is strongest is where the opening is.
• Which fixture? Get close to each drain. Cup a hand over a shower drain and sniff. Sniff at the base of each toilet. Open the cabinet under each sink.
• When? Worse after a flush points at a toilet seal. Worse when the washer drains points at the standpipe. Worse on a windy day or when the air conditioning runs points at a vent problem, since pressure changes pull air through whatever opening exists.

The seven usual causes

1. A dry trap in a fixture nobody uses. The guest bath, the utility sink, the shower in the pool bath, a floor drain in the garage. Water in a trap evaporates in a few weeks in our heat, faster when the house is closed for the summer, and the empty trap is a direct opening to the sewer. Run every unused fixture for a minute and wait an hour. This resolves most cases.
2. A failed wax ring under a toilet. The ring seals the toilet to the drain flange. When a toilet rocks, or the ring has aged out, gas comes up around the base, and sometimes water too after a flush. Signs: the toilet moves when you push it, the floor around it is discolored, or the smell is strongest at floor level behind the bowl.
3. A blocked, disconnected, or missing vent. Vents let air in behind draining water so traps do not get siphoned dry, and they carry gas out above the roof. A vent blocked by a bird nest or leaves, or one cut short in an attic during a past remodel, lets gas back into the house. The tell is gurgling at one fixture when another drains, and traps that keep emptying on their own.
4. The washing machine standpipe. The washer hose drops into an open pipe with a trap below it. If the hose is pushed too far down, or the standpipe trap is missing on an older installation, the machine’s discharge blows the seal out or gas rides straight up. The laundry is a common source and easy to overlook.
5. A garbage disposal. Food that never fully ground out sits under the splash guard and in the chamber and decays. It smells like sewage without being sewage. Clean the underside of the rubber guard, run ice and cold water, and see if the smell goes.
6. A cracked or separated drain under the slab. This is the one to worry about. In a pre-1975 home the cast iron under the floor rots from the inside, and a channel in the bottom of the pipe lets gas and water into the fill under the slab. It surfaces at cracks in the tile, at baseboards, and near the lowest bathroom. Often the smell comes and goes with rain as the water table moves.
7. A septic problem. On septic in Lehigh Acres, Alva, or eastern Estero, a full tank, a failing drain field, or a missing tank lid puts the smell in the yard first and the house second. Soft ground, greener grass over the field, or drains that all run slow at once point here.

What is safe to do yourself

Fill every trap. Pour a bucket of water into any floor drain. Tighten the two closet bolts on a rocking toilet a quarter turn each, no more, and see if the rocking stops. Pull the washer hose back so only a few inches sit in the standpipe. Clean the disposal. From the ground, look at the roof for a vent pipe with a nest or a screen packed with debris. Those are the limits. Pulling a toilet to replace a wax ring is possible for a careful homeowner with a helper, but a flange that turns out to be cracked or corroded, common on older cast iron, is a plumber’s fix.

Where this stops being DIY

If every trap is full and the smell is still there after a day, stop guessing. A smell that lives at floor level, comes with rain, or sits in a house with original cast iron drains needs a camera down the line and sometimes a smoke test, where the plumber pushes harmless smoke into the drain system and watches where it comes out. A vent that is open in the attic, a broken flange, and a cracked drain under the slab are all found that way, and none of them is fixed with a jug of anything from the hardware store.

Send us a note with where the smell is strongest and when it comes, and whether running the unused fixtures changed anything. That much narrows the visit before it starts, and the fix is usually smaller than the smell suggests.