Drain Odours: Causes and Prevention

The main causes of sewer smells at home — dried traps, faulty floor drains, stack venting, leaking joints — and what to do about them during a renovation.

Sewer smells are among the most frustrating complaints after a renovation because the source is often invisible. New tiles, new sanitaryware, everything clean, yet an unpleasant odour keeps appearing in the bathroom or kitchen. The causes are usually found at a handful of typical points, and most can be prevented at renovation stage.

An odour is not just a nuisance. Sewer gases contain unpleasant-smelling compounds as well as gases you would not want accumulating indoors. A smell is also often the first sign of a hidden leak or damp, and investigating it promptly can save floors and walls from damage.

The water seal: the main barrier

To stop sewer gases entering a room, every fixture outlet has a trap. The trap always holds some water, which forms a seal between the pipe and the room. When that seal is broken, smells come straight in.

Typical reasons the seal fails:

evaporation: water dries out in a rarely used guest bathroom, a second floor drain or a shower that has not been used for a while;
siphonage: when a large volume of water flows from another fixture and the system lacks air, the water in the trap is pulled out;
no trap at all: for instance, a washing machine drain hose pushed straight into the waste pipe without a trap;
poorly installed trap: too shallow a seal or damaged.

For rarely used fixtures there are floor drains and traps with a dry seal that closes with a mechanical valve when there is no water.

Floor drains and showers

A floor drain set under tiles is one of the most common sources of bathroom odours. Causes include a poor joint between drain and waterproofing, a floor falling away from the drain instead of towards it, and a trap section that cannot be removed for cleaning. When choosing a drain, check its seal depth and which parts come out for cleaning.

Hair and soap residue build up quickly in showers. If the trap is not cleaned, decaying residue becomes a source of odour itself. That is why a drain cover and trap cup that lift out easily matter in practice.

The stack and venting

In an apartment building, every flat's drainage connects to a shared stack that is vented at roof level. If that venting fails, for example because someone on an upper floor has blocked the roof outlet, negative pressure develops and water is sucked from traps. The source of such a problem is not inside the flat and has to be resolved with the building management.

In a private house, the vent pipe must be included in the design. Its outlet is placed away from windows and air intakes.

Leaking joints and damaged pipes

A dislodged rubber seal, a pipe not fully inserted in its socket, a cracked old cast iron pipe: these also cause odours. Sometimes the smell appears only when a particular fixture drains, which points to a joint problem on that line. The connection between an old stack and new plastic pipe needs a dedicated adaptor and seal; if it is not airtight, smells escape through the plasterboard boxing into the room.

The connection between a toilet and its floor or wall outlet also needs attention. A leak there causes both odours and damp under the floor.

Measures to take during renovation

check that every fixture connects through a trap, and give the washing machine and dishwasher their own trap or trapped connection;
choose dry-seal traps for rarely used drains and fixtures;
inspect and sign off the drain-to-waterproofing joint and floor falls;
use quality adaptors at stack connections;
keep access to the stack's inspection point;
run water through the pipes to test them before they are enclosed.

If the smell is already there

The first step is simple: pour water into every trap, especially rarely used ones. If the smell goes, evaporation was the cause. If it appears only when water drains in another flat, the stack venting should be checked. If the smell is constant and comes from one spot, a hidden joint may be leaking; access hatches and photos of concealed work make the search far easier.

Zaferoğlu İnşaat carries out plumbing and drainage in bathroom and kitchen renovations together with testing of concealed work. Note in which room, at what time and after which appliance the smell gets worse; tracing the source starts much more easily with those notes.

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