Why do my drains smell?

Nearly always because the water seal that normally blocks sewer gas has failed, or because something is decomposing on the inside of the pipe. The two produce different smells and need different fixes.

Every drain has a trap — the U-bend under a sink, or a built-in trap in a shower or floor gully. It holds a small volume of standing water that physically seals the pipe and stops gas from the sewer coming back up. If that water evaporates, the seal is gone. This is why a guest bathroom, an unused utility sink or a rarely used floor drain starts smelling of sewage after a few weeks. The fix is simply to run the tap for thirty seconds, and for drains you use very rarely, a small amount of cooking oil poured in afterwards slows evaporation.

A trap can also be siphoned dry by a heavy discharge elsewhere on the same branch, or lose its seal if the pipe run was installed without adequate venting. If a drain smells only after another fixture is used, that points squarely at a venting problem and is worth a plumber's look.

The other cause is biofilm. Soap, grease, hair, food residue and skin cells build up as a soft layer on the inside of the pipe just below the plughole, and bacteria feeding on it produce a sour or musty smell rather than a sewage one. It is usually strongest right at the opening and worse in hot weather. Cleaning mechanically — a bottle brush or drain snake, followed by very hot water and a biological drain cleaner — works better than pouring chemicals down.

Persistent sewage smells across multiple drains suggest a blocked or damaged soil pipe, which needs investigating properly.