Why Is There a Sewage Smell in My House? Causes & When to Call a Plumber

Quick Self-Diagnosis: Where Is the Smell Coming From?

Before you call a plumber, spend five minutes doing a basic sweep. A sewage smell in the house can range from a trivially easy fix — pouring a litre of water down an unused drain — to a structural problem requiring soil stack replacement. The smell itself gives you clues.

• Smell only in one room? Start with the P-trap on every drain in that room.
• Smell strongest near the toilet base? Check for a failed wax seal.
• Smell throughout the whole house? Likely a soil stack crack, a missing vent cap, or a shared drain issue.
• Smell comes and goes? Ventilation or trap issue — not a physical leak.
• Smell is constant and strong? Physical breach somewhere — call a plumber.

Dry P-Trap: The Most Common Cause of Drain Smell in a House

Every sink, bath, shower, basin and floor drain in your home has a P-trap — a curved section of pipe that holds a small amount of water to block sewer gases from travelling back up into the room. When a drain goes unused for several weeks, that water seal evaporates. The result is a direct open channel between your room and the sewer below.

This is by far the most common reason for a sewage smell in houses across London. It is especially common in:

• En-suite bathrooms used infrequently by guests
• Ground-floor WCs in Victorian terraces that get forgotten
• Utility room floor drains
• Second bathrooms in large Victorian conversions in boroughs like Hackney, Islington and Lewisham

How to fix a dry P-trap

1. Run water into every drain in the affected room for at least 30 seconds each.
2. Flush any unused toilets three or four times.
3. After filling the traps, add a tablespoon of cooking oil or a small amount of glycerine to each drain — this floats on top of the water and dramatically slows evaporation.
4. Wait 20 minutes and check whether the smell has gone.
5. If the smell returns within days, install a trap primer (a small device that automatically tops up the water seal) or switch to a self-sealing trap insert.

Blocked or Partially Blocked Waste Pipes

A complete blockage is obvious — the water stops draining. But a partial blockage is far more insidious. Water still drains, albeit slowly, but a pocket of rotting organic matter sits in the pipe and continuously off-gasses hydrogen sulphide and methane back into your home.

Common culprits in London homes include:

• Hair and soap scum in shower and bathroom sink and bath wastes — particularly in older Victorian cast-iron wastes that have rough interior surfaces
• Grease and food waste in kitchen sink pipes — especially in basement flats where the kitchen waste pipe has a long horizontal run before joining the stack
• Sanitary products and wet wipes in toilet soil pipes — still one of the most common call-out reasons across all 33 boroughs
• Limescale narrowing in older copper or lead pipes in properties built before 1970, common in boroughs like Kensington, Westminster and Camden

A drain smell in the house that coincides with slow drainage is almost always a partial blockage — in gardens with mature trees, root damage to Victorian clay drains causes both blockages and odour escape through structural cracks. Our drain unblocking service covers rodding, jetting and CCTV survey across all London boroughs, typically resolving the issue within the first visit.

Cracked or Leaking Soil Stack — Particularly Relevant in London

The soil stack is the large vertical pipe (typically 100mm diameter) that runs from your ground floor up through the house and exits through the roof. It carries all toilet waste and vents sewer gases above roof level. In most London properties built before 1960, this pipe is cast iron. In post-1970 builds it is uPVC.

Cast-iron soil stacks develop problems in two specific ways over time:

1. Lead-caulked joints fail. Victorian stacks used lead wool hammered into the joint socket to create a seal. Over decades, vibration (particularly relevant in London given Tube lines running under large swathes of the city) and thermal cycling cause these seals to crack and allow gases to seep into wall cavities and rooms.
2. Rust-through. External sections of cast-iron stack that face weather can rust from the outside in, eventually producing pin-holes or larger breaches.

In terraced streets across Hackney, Southwark, Lambeth and Tower Hamlets — where Victorian terraces were converted into flats during the 1980s and 1990s — it is common to find soil stacks that have never been replaced and are now 80–100 years old. A CCTV drain survey or smoke test can identify the exact breach location without breaking into walls unnecessarily.

Signs you may have a cracked soil stack rather than a simpler issue:

• Sewage smell is present in multiple rooms on different floors simultaneously
• You can smell it within wall cavities or near skirting boards
• The smell is worse in cold weather (warm indoor air pulls sewer gas from cracks) or immediately after rain
• You notice damp patches on walls near the stack run without any obvious pipe leak

Broken or Missing Vent Pipe Cap

Your soil stack extends above roof level specifically to release sewer gases safely above head height. The top of the stack either terminates open (the simplest design) or is fitted with a cage or cap to prevent bird nesting and debris ingress.

When the open stack terminal gets blocked by a bird's nest, leaves, or a collapsed cage, the ventilation function is lost. Sewer gases that would normally exit at roof level instead find the path of least resistance — back down the stack and into your home through any unsealed joint, dry trap, or loose toilet connector.

This is a particularly common issue in London's terraced housing stock where roof access is awkward and stack terminals have not been inspected in years. A blocked vent stack often presents as:

• Intermittent gurgling from toilets and waste pipes after flushing
• Slow draining combined with sewage smell
• Smell that worsens when using any water appliance (washing machine, dishwasher, shower)

Failed Wax Seal on Toilet Base

The joint between your toilet pan and the soil pipe beneath it is sealed with either a wax ring or a rubber push-fit connector. This seal prevents gases escaping at floor level. When the seal fails — usually because the toilet rocks slightly on an uneven floor, or simply through age — sewer gases leak out continuously at the base of the toilet.

Signs of a failed toilet wax seal:

• Sewage smell strongest directly around the toilet base
• Visible moisture or a brown stain on the floor around the pan
• Toilet rocks slightly when you sit on it
• Smell is consistently present rather than intermittent

Replacing a wax seal is a same-day repair. The toilet is disconnected from the water supply, the pan lifted, the old seal removed and a new wax ring or rubber connector fitted. If you also have a blocked toilet that has caused the pan to move through repeated plunging, it is worth addressing both issues in the same visit.

Shared Drain Issues in London Flats

London's housing density creates a drainage complication that rarely applies elsewhere in the UK. In a converted Victorian terrace split into four flats, all four properties share lateral drain runs before connecting to the main sewer under the street. Before 2011, these shared private drains were technically the collective responsibility of the properties connected to them.

In October 2011, Thames Water took ownership of these shared sewers and lateral drains under a transfer that applied across London and the rest of England and Wales. This means:

• If a drain smell is traced to a shared lateral pipe (one that serves more than your property), Thames Water should investigate and repair it at no cost to you.
• Drains within your property that serve only your home remain your responsibility.
• Thames Water has a 24-hour sewer enquiry line and is obliged to respond to reports of blocked or damaged shared sewers.

In practice, a qualified emergency plumber can perform a CCTV survey to identify whether the issue is inside your property or in the shared section, then advise whether to contact Thames Water or proceed with a private repair.

Saniflo Macerators in Converted London Flats

Thousands of London flats — particularly those converted from larger Victorian and Edwardian houses in the 1980s and 1990s — have Saniflo macerator units installed to allow a bathroom or WC to be added below or remote from the main soil stack. A Saniflo grinds waste and pumps it horizontally or upward to the nearest soil connection.

When a Saniflo macerator's rubber seals fail, the result is one of the most potent sewage smells you will encounter inside a home. The macerator sits in the room (typically inside a cupboard or behind a panel) and any seal failure allows raw sewage odour to escape continuously. Common causes of seal failure include:

• Age — most Saniflo seals are rated for 8–12 years under normal use
• Wet wipes or non-macerator-safe products being flushed
• Running the unit dry (no water flow) which burns out the rubber seal
• Limescale buildup that prevents the seal from closing fully

Fatbergs and Grease Buildup Downstream

London has a well-documented fatberg problem. The city's ageing Victorian sewer network — much of it designed for a population of two million, now serving nine million — accumulates solidified deposits of cooking grease, wet wipes, and other non-flushable material at bends and junctions. These deposits partially block sewer flow, causing sewage to back up and gases to pressurise into connected properties.

You are more likely to experience fatberg-related odour issues if:

• You live near a commercial kitchen, takeaway or restaurant with a shared drain connection
• Your property is at the low end of a terrace where the drain gradient is shallow
• You are in an older part of inner London (City of London, Southwark, Lambeth, Tower Hamlets) where the Victorian sewers are most congested

If Thames Water has confirmed a clear shared sewer but you still experience odours, the issue is likely within your private drain run. High-pressure jetting can clear grease accumulation in private laterals effectively. If an outside drain is the source of the smell, our guide to blocked outside drains in London covers inspection chamber access, gully clearing and when to involve Thames Water.

Why Summer Makes Sewage Smells Worse

Many London households notice drain smells specifically in June, July and August. This is not coincidence — there are three reinforcing mechanisms at work:

1. Faster trap evaporation. At 28°C (typical London summer temperature), a P-trap can lose its water seal in as little as two weeks compared to six or more weeks in winter.
2. Accelerated bacterial decomposition. Organic matter in partial blockages decomposes much faster in warm conditions, producing more hydrogen sulphide gas per day.
3. Thermal expansion. Cast-iron soil stack joints that are borderline can open up slightly in heat, allowing gas to escape that would otherwise stay sealed in cooler months.

The practical advice: if your property has any known drain issues — slow drainage, occasional smells, an old soil stack — address them in spring before heat amplifies the problem. A call-out in April is cheaper, less urgent and easier to schedule than an emergency call in August.

Intermittent vs Constant Smells: What the Pattern Tells You

The behaviour of the smell is one of the best diagnostic signals available without any tools.

London 2026 Repair Costs for Sewage Smell Causes

All prices are indicative for London 2026. Final cost depends on access, property type and whether parts are needed. Our £60 call-out fee is waived on jobs that run over 2 hours — meaning most multi-cause investigations carry no separate call-out charge.

Frequently Asked Questions