Sewer Smell in the House? Causes and Solutions

Every drain has a trap that holds water to block sewer gas. In a fixture or floor drain you rarely use, that water can evaporate, letting odor in. Other causes include buildup or partial blockages giving off smell, a blocked or improperly working vent, or a cracked pipe. The location and pattern of the smell helps narrow it down.

The trap is the key concept. That U-shaped bend under every sink, tub, and floor drain is designed to hold a small plug of water that seals out the sewer gas in the line beyond it. As long as the trap stays wet, you smell nothing. The trouble starts when that seal is lost — through evaporation in an unused fixture, or because a problem deeper in the system is disturbing it — and sewer gas finds its way into the room.

A dried-out trap is the most common and most benign: water evaporates from a guest bathroom or basement floor drain that hasn't been used in weeks, breaking the seal. But odor can also come from buildup itself — a grease- or sludge-coated line gives off a smell as waste sits and decomposes — or from a venting problem that lets gas escape indoors instead of out the roof. The least common but most serious cause is a cracked or broken pipe leaking gas (and sometimes sewage) into a wall or under a slab. Where and when you smell it helps tell these apart: an isolated odor near one unused fixture points to a dry trap, while a pervasive smell with slow drains points deeper.

Start simple: pour water into unused drains and floor drains to refill dry traps. If the smell persists or comes with slow drains, gurgling, or backups, the cause may be in the sewer line — buildup, a blockage, or pipe damage — and a camera inspection will pinpoint it so it can be cleaned or repaired.

The dry-trap fix costs nothing and solves a surprising share of cases: run water in every sink, tub, and shower you don't use often, and pour a bucket of water down basement and utility floor drains to refill their traps. If that clears it, you're done. If the smell lingers — especially alongside slow drains, gurgling, or a backup — the cause is in the line, and that's when a camera inspection earns its keep by showing exactly whether you're dealing with buildup, a blockage, a vent issue, or a damaged pipe.

Most sewer smells are a nuisance rather than an emergency, but a persistent one shouldn't be ignored — sewer gas is unpleasant and, at high concentrations, unhealthy, and a lingering odor can be the first sign of a cracked pipe or a developing blockage. If refilling the traps doesn't resolve it within a day or so, or the smell comes with any drainage symptoms, it's worth having the line looked at before a hidden problem grows. Call (207) 419-2600 and we'll help track down the source.