Sewage Smell in Your House? 7 Causes & How to Fix Each One

Q: Is a sewage smell in the house dangerous?

A sewage smell indicates hydrogen sulphide gas (H2S). At low levels it causes unpleasant odour and headaches; at higher concentrations it can be harmful. If the smell is strong and sudden, open windows and leave. If the smell is near gas appliances, call 0800 111 999 immediately.

Q: Why does my bathroom smell like sewage at night?

Smells that appear mainly at night often indicate a P-trap that partially refills during daytime use but dries overnight. It can also indicate a vent stack issue — when wind pressure changes at night, it draws sewer gas back down through drains rather than out through the roof vent.

Q: What is a P-trap and why does it stop sewage smells?

A P-trap is the U-shaped pipe section below every sink, bath, and shower drain. It holds a small volume of water at all times that acts as a physical barrier preventing sewer gases from rising back through the drain into your home. If a drain isn't used for several weeks, that water evaporates and the barrier is lost.

A sewage or rotten egg smell inside your London home is unpleasant and potentially hazardous. Here are 7 causes with specific fixes — including a gas safety warning and London-specific drain advice.

A sewage smell inside your London home is more than unpleasant — it signals that sewer gases, including hydrogen sulphide (H2S), are entering your living space. We deal with this call regularly across London properties, and in the majority of cases the fix takes less than five minutes. But some causes are genuinely serious and indicate structural problems with your drainage system.

London's housing stock — Victorian cast iron soil pipes, mid-century clay drains, Edwardian terraces with shared drain runs, modern flat conversions with complex waste layouts — makes this a surprisingly common and varied problem. Here are the seven causes we diagnose most frequently, with the specific fix for each.

⚠️ If the smell is strong, sudden, or near gas appliances: Open windows, leave the property, and call the National Gas Emergency line: 0800 111 999. A natural gas leak has a different (sulphur/rotten egg) smell and is a different, more urgent emergency. Do not operate light switches or electrical appliances if you suspect gas.

1. Dry P-Trap (Most Common — Check This First)

A P-trap is the U-shaped pipe section below every drain in your home — every kitchen sink, bathroom basin, bath, shower, bidet, and floor drain. This curve holds a small volume of water at all times that forms a physical barrier preventing sewer gases from rising back through the drain into your home.

If a drain hasn't been used for several weeks — a guest bathroom, a floor drain in the utility room, the corner basin no one uses — the water in the P-trap simply evaporates. Once it does, sewer gas flows freely up through the drain.

💡 Quick fix — takes 3 minutes: Run water down every drain in the property for 30 seconds each. For rarely-used drains, add one tablespoon of cooking oil after the water — it floats on the water surface and significantly slows evaporation. This is the permanent solution for drains that are genuinely never used.

If the smell disappears within 30 minutes of running all the drains, a dry P-trap was your culprit. We've had Islington residents convinced they had a major drain fault that resolved itself with this single step.

2. Blocked or Slow Drain

A partial blockage allows organic material — food debris, hair, soap scum, grease — to sit and decompose inside the pipe. Decomposing organic matter produces sulphurous compounds. The smell is usually strongest when you run water or use the affected drain.

Signs: slow drainage, smell localised to one drain, gurgling when water drains. Fix: Clear the drain using boiling water, baking soda and vinegar, or a drain snake. See our complete guide: How to Unblock a Drain.

If the smell returns after clearing, the blockage may be deeper in the pipe where DIY tools can't reach. Our blocked drains London team uses high-pressure water jetting to clear blockages at any depth, including the main sewer line.

3. Blocked or Partially Blocked Soil Pipe

The soil pipe is the main vertical pipe running from your toilets down through the property to the sewer. If it becomes partially blocked — by accumulated waste, build-up at a joint, or a partial collapse — gases back up through the system and emerge from multiple drains simultaneously.

Signs of Soil Pipe Blockage	What It Looks Like
Smell throughout the house	Not isolated to one bathroom — the smell travels
Multiple drains slow at once	Kitchen, bathroom and WC all draining sluggishly
Toilet gurgling	Bubbling or gurgling when other drains are used
Slow flush	Toilet requires 2–3 flushes or drains slowly

Fix: Professional high-pressure drain jetting followed by a CCTV inspection. Not a DIY job — the soil pipe requires proper professional equipment to clear safely.

4. Cracked or Damaged Soil Pipe

This is particularly common in older London properties — and one we find regularly when surveying properties in older boroughs like Newham, Lewisham, and Tower Hamlets. Victorian and Edwardian houses typically have cast iron soil pipes that are now 100–130 years old. These corrode from the inside, develop hairline cracks, and allow sewer gas to escape into wall cavities and floor voids before finding its way into living spaces.

You might smell it in rooms nowhere near a bathroom. Post-war properties may have clay drain pipes that crack under ground movement — London's clay soil shrinks and swells significantly with seasonal changes, stressing underground pipework.

💡 CCTV drain survey: Around £150–£250 in London, a CCTV survey sends a camera down the pipe to identify exactly where a crack or break is located. This is often the most cost-effective diagnostic for smells that persist despite trying everything else.

Fix options: Pipe lining (internal resin sleeve inserted without excavation), patch repair, or section replacement. Our drainage team provides CCTV surveys and all repair options.

5. Failed Toilet Wax Seal

As covered in our toilet leaking at the base guide, the wax ring seal between the toilet pan and the floor drain prevents sewer gases from escaping around the toilet base. When this seal fails partially, you can get a sewage smell without a visible water leak — especially in a toilet that's not used frequently.

Signs: Sewage smell strongest in the bathroom, potentially at floor level near the toilet. May be accompanied by slight toilet movement when sat on. Fix: Wax ring replacement — possible DIY but more reliably done by a plumber in older properties with original floor flanges.

6. Blocked Drain Vent Stack

Your plumbing system relies on a vent pipe — a vertical pipe running from the drainage system up through the roof — to equalise air pressure and allow sewer gases to escape harmlessly into the atmosphere. If this vent pipe becomes blocked (bird nests, leaf accumulation, collapsed pipe top), the system can't breathe properly.

Instead of gases exiting through the roof, they take the path of least resistance — back through your drain P-traps and into the building. The telltale signs:

Drains that gurgle when other drains or the toilet are used
Toilet slow to refill after flushing
Sewage smell appearing across multiple rooms without an obvious drain source
Smell that worsens when it's windy outside

Fix: Vent stack clearance from roof level — requires ladder access and professional equipment. Not suitable for DIY.

7. Sewer Line Problems (Tree Roots, Bellied Pipe, Collapse)

The most serious cause on this list. In London, the main sewer line from your property to the public sewer can be affected by several conditions:

Tree root ingress: London's mature street trees send roots toward moisture. Your drain is an attractive target — roots enter through joints and cracks, accumulating debris and causing partial blockages. This is especially common under garden trees in areas like Dulwich, Richmond, and Highgate.
Bellied pipe: Ground settlement causes a section of pipe to sag, creating a low point where waste accumulates rather than flowing through.
Collapsed section: Pipe has completely failed, allowing soil ingress and sewage to accumulate.

These conditions allow sewer gas to accumulate and back up into the property, usually accompanied by very slow drainage across the entire property. Fix: CCTV survey to diagnose the exact issue, followed by root cutting and jetting, pipe patching, or section replacement.

Sewage Smell You Can't Find or Fix?

Our London drainage engineers use CCTV surveys, smoke testing, and pressure testing to find the exact source of any drain odour. We cover all 32 London boroughs — same-day appointments available.

Call 07456 975436

Hydrogen Sulphide (H2S): Understanding the Safety Risk

Sewage smell is caused by hydrogen sulphide gas (H2S), produced by bacteria breaking down organic waste. At the low concentrations typical of a dry P-trap or minor drain issue, it's unpleasant but not dangerous. At higher concentrations from a serious sewer line fault or major blockage, it can cause headaches, nausea, and irritation.

At very high concentrations (which would be unusual in a domestic setting unless there's a major collapse), H2S is toxic. If you notice the smell is getting significantly stronger over time, or you feel unwell when near the affected area, ventilate thoroughly and call a professional to investigate before spending more time in the space.

The key safety note: do not confuse sewer smell with natural gas or a carbon monoxide issue. Natural gas has a distinctive sulphur/rotten egg smell added to it. If the smell is near your boiler, gas meter, or gas appliances — not near drains — leave immediately and call 0800 111 999.

Frequently Asked Questions

Why does my house smell like sewage but I can't find where it's coming from?

The most likely cause is a dry P-trap in a rarely used drain — run water in every sink, bath, shower and floor drain for 30 seconds each. If the smell persists after 30 minutes, it may be a cracked soil pipe or blocked vent stack. A CCTV drain survey is the most reliable way to locate the source without guesswork.

Is a sewage smell in the house dangerous?

Hydrogen sulphide from sewage is unpleasant at low levels and can cause headaches. At higher concentrations it's potentially harmful. If the smell is very strong and sudden, open windows and ventilate. If near a gas appliance rather than near drains, call the National Gas Emergency line: 0800 111 999 immediately and leave the property.

Why does my bathroom smell like sewage at night?

Smells that intensify at night typically indicate a P-trap that partially refills during daytime use but dries overnight, or a vent stack issue where changing wind pressure at night draws gas back through drains rather than out through the roof vent. Both are fixable — the first by running water and adding oil, the second by a drain engineer.

What is a P-trap and how do I know if mine is dry?

A P-trap is the U-shaped pipe beneath every drain that holds water as a barrier against sewer gas. If a drain hasn't been used in several weeks, that water evaporates. You can tell it's dry simply by the smell disappearing after running water down the drain for 30 seconds. If the smell returns within days, the drain is evaporating too quickly — add cooking oil after each use to slow evaporation.

How much does a CCTV drain survey cost in London?

A professional CCTV drain survey in London typically costs £150–£250 for a standard residential property. This is often the most cost-effective way to diagnose a persistent smell — the camera identifies exactly where any cracks or blockages are, avoiding expensive guesswork and unnecessary excavation.