Drain smells bad — finding and fixing the cause

A bad smell from a drain can come from several sources: a dry trap (the U-bend has evaporated), decomposing organic matter in the waste pipe, a cracked drain pipe, or a nearby sewer vent. Identifying the source is the first step.

If the smell comes from a specific sink, bath, or shower that is not used frequently: the U-bend has dried out and is no longer blocking sewer gases. Run the tap for 30 seconds to refill the trap. For floor drains or seldom-used fixtures, pour a cup of water down the drain weekly to maintain the water seal.

If the smell comes from a regularly used drain: run the hot tap and add half a cup of bicarbonate of soda followed by half a cup of white vinegar. Leave for 15 minutes, then flush with hot water. This breaks down the biofilm inside the trap and waste pipe that produces hydrogen sulphide (the rotten egg smell).

If the smell is in the bathroom generally and all fixtures are regularly used: check the trap on the bath or basin waste — some pop-up waste mechanisms accumulate hair and slime around the plug mechanism. Remove the plug and clean underneath.

For outside drain smells: open the inspection chamber and check it is not full. If it is full or slow-draining, there is a downstream blockage. An empty chamber with a smell suggests a cracked pipe section drawing in ground air — a drain survey is needed.

A persistent smell that worsens in warm weather and cannot be traced to a specific fixture may be from a cracked soil stack or underground drain. A CCTV drain survey (£100–£300) will identify the source. Cracked clay or concrete pipes in older properties are a common cause.