Why does my house smell like sewage?

Seven causes, from easiest to hardest

• A dry trap. Every fixture has a U-bend, the trap, that holds a plug of water to block sewer gas. Floor wastes, spare showers and laundry tubs dry out within weeks when unused, faster in hot weather, which is why new, rental and holiday homes often smell when people move in. Pour a bucket of water into each.
• A dirty shower or basin waste. Hair, soap and biofilm, the slimy layer bacteria form inside pipes, can smell like sewage. Clean the grate and the trap.
• A failed toilet seal. If the smell is strongest at the base of the toilet, or the toilet rocks, the seal between the pan and the sewer pipe may have failed.
• A blocked or damaged vent pipe. Australian plumbing rules (AS/NZS 3500) require drains to be vented, usually through a pipe above the roof, so sewer gas escapes above the house. Leaves, a nest or a broken vent can force gas back through the traps, often with gurgling. Some homes use an air admittance valve, a one-way vent under a sink or in a wall. When one fails, the smell is usually strongest near it and the nearby drains gurgle.
• The overflow gully outside. Check for a missing grate, a dry trap or sewage around it.
• A partly blocked sewer line. A smell plus slow drains or gurgling usually means the main line is partly blocked.
• A cracked or broken pipe under the house or slab, which can leak gas and sewage. A camera inspection finds it.

When it's urgent

• Sewage is visible inside or around the house.
• The overflow gully outside is overflowing.
• Several drains are slow at once.
• There's damp, smelly soil under the house.

Smell outside or in the street?

If the smell comes from a maintenance hole in the street or several neighbours notice it, the problem is likely in Sydney Water's sewer. Call Sydney Water on 13 20 90. If it's only in or around your house, it's in your private pipes or fixtures, which the owner is responsible for. Our guide to who fixes a blocked sewer explains where your pipes end and Sydney Water's begin.

Is sewer gas dangerous?

In a normal home, a sewage smell is mostly unpleasant rather than dangerous, but a strong or lasting smell shouldn't be ignored. Sewer gas includes hydrogen sulphide, which gives it the rotten-egg smell, and methane. If the smell is overpowering, open the windows, avoid naked flames and get it checked.

How a plumber finds the cause

A plumber checks the traps and the vent first, then runs a camera through the line to look for a blockage, a crack or a displaced joint. A CCTV inspection typically costs $200–$550 in Sydney. Block Drains' inspection is $249 fixed, with a written report showing where any fault is and how deep.

Questions

Why does my bathroom smell like sewage only sometimes?

Often it's when another fixture drains and pulls water out of a trap, usually with a gurgle, or when wind blows across the roof vent. Both point to a venting problem or a partial blockage.

Why does the house smell like sewage after rain?

Heavy rain can overload the sewer, and a partly blocked or cracked line can let gas or water back up. If it happens every storm, get the line checked with a camera.