First decide whether it is safe to investigate
If you suspect a gas escape, do not assume the smell is a drain problem. Stop this checklist, avoid flames and operating electrical switches, leave the affected area and call the National Gas Emergency Service on 0800 111 999 from a safe place. Gas Safe Register sets out the emergency precautions in its gas safety guide. Do not go into a cellar to reach a gas meter where an escape is suspected.
Smell cannot rule out carbon monoxide: it has no smell. If someone becomes unwell in the building, go outside and seek medical help; severe symptoms require 999. The NHS guidance on carbon monoxide explains the symptoms and urgent actions. Never use how quickly an odour fades as proof that the atmosphere is safe.
The checks below are ordinary observations from rooms and accessible fittings. They do not involve entering a drain, lifting a heavy chamber cover, opening a soil pipe, removing a boiler casing or putting your face over an outlet. If sewage is backing up, stop using affected fixtures and arrange help. Do not keep flushing to see whether the problem will clear.
The useful question is what changes before the smell returns
A smell which disappears before a contractor arrives can be frustrating. The solution is to make the next occurrence informative. Rather than writing “bathroom smells again”, record where you first noticed it, what had just been used and whether wastewater was draining normally. A sequence is more useful than a description of how unpleasant it was.
For example, “downstairs basin gurgles when the upstairs bath empties, then the downstairs room smells for twenty minutes” gives a professional a repeatable event to investigate. “Smells bad sometimes” does not distinguish a dirty waste fitting from a ventilation defect, an open connection or an outside source entering through a window.
Do not deliberately recreate a strong smell or a backup. Record ordinary use only, and stop if water rises in another fixture. Equally, avoid changing five things at once. Cleaning every waste, adding chemicals and leaving every window open may improve the room temporarily, but it removes the clues needed to identify which change helped.
Use a short smell diary instead of guessing at the pipe
Choose a note on your phone or a sheet beside the calendar. Use the same headings each time so that an engineer can compare episodes. You are recording observations, not trying to diagnose the drainage layout. A photograph of an accessible fitting and a simple room sketch are often enough to identify which bathroom or branch you mean.
| Record | Useful example | Why it helps |
|---|
| Time and duration | 07:40, noticed after shower; gone by 08:15 | Separates a short discharge event from a persistent opening |
| First location | Inside basin cupboard before the rest of the room | Gives the inspection a starting point |
| Recent ordinary use | Upstairs WC flushed; washing machine pumping out | Identifies a possible shared drainage event |
| Visible or audible change | Gurgle in shower waste; no standing water | Helps distinguish flow and ventilation symptoms |
| Weather and openings | Windy; bedroom window open towards rear stack | Tests whether outside entry is plausible |
| Action and result | Ran unused basin briefly; smell stayed away overnight | Records a clue without claiming a permanent cure |
Include days when nothing happens. If you use the same bathroom every morning but the smell occurs only when a washing machine also runs, the uneventful days help narrow the comparison. There is no need for weeks of records before seeking help. One clear sequence can be useful, and flooding, illness or an unexplained strong odour should not wait for a completed diary.
If it happens after a room has not been used
Start with the fittings used least often: a guest shower, utility-room sink, basement basin or floor drain. Ordinary water traps retain a water seal between the room and the drainage system. An infrequently used fitting is therefore worth checking before assuming a buried drain is broken. Some fittings use different arrangements, including waterless traps, so identify the product before taking anything apart.
Where a normal sink or shower drains freely and there is no sign of backup, run a modest amount of water through it in the normal way. Note the time and whether the smell changes. Do not leave a tap running unattended or add water to a drain which is already backing up. A returning smell after normal use is evidence for further inspection, not a reason to keep pouring water indefinitely.
If the smell stays away until another long period of non-use, discuss trap maintenance and the fitting design with a plumber. If it returns the same day after another fixture discharges, tell the plumber that too. Those patterns support different next checks. A simple water top-up cannot establish whether a seal is being lost through evaporation, a leaking fitting or pressure changes elsewhere.
Look for an obvious missing removable insert after cleaning a shower, but follow the waste manufacturer's instructions. Some inserts form part of the seal rather than merely catching hair. Photograph what is there instead of buying a generic replacement which happens to fit the opening. Do not block an overflow or ventilation opening as a permanent smell cure.
If flushing or emptying a bath starts it
Listen from a normal standing position for a gurgle in a nearby basin or shower. Note whether the toilet bowl level looks different after ordinary use and whether any other fitting drains slowly. These are observations a plumber can connect with the actual pipework. They do not, on their own, prove which valve or pipe needs replacing.
Drainage ventilation and the condition of the odour barriers need to be considered together. McAlpine describes air admittance valves as allowing air into drainage while preventing foul gases escaping. Finding a valve near the smell does not establish that it is defective; a plumber should assess the connected drainage before recommending a replacement.
Ask the attending professional to consider the branch layout and ventilation together, particularly if the problem began after a new bathroom or relocated kitchen. Merely replacing an accessible valve may leave the underlying installation issue untouched. Conversely, opening a floor before checking a visible connection can turn a small plumbing investigation into avoidable building work.
A smell after flushing can also be associated with a connection close to the WC. Keep the description specific: does it start at the back of the pan, inside boxing, in the cupboard next door or at a different waste? Do not lean on or rock the toilet to test it. A joint may require professional access even when the floor around it appears dry.
If rain, wind or an open window seems to control it
Compare inside and outside observations from safe, ordinary access. Is the smell already present in the garden before it enters the room? Do neighbours notice it at the same time? Is one open window near an external pipe termination? A weather association is a clue to investigate, not proof that the public sewer or the roof vent is responsible.
Thames Water describes how heavy rain can overload sewers and cause wastewater to back up in its sewer-flooding guidance. However, an indoor smell during rain can still involve an indoor defect. A compromised seal and an outside pressure event may coexist. Reporting the external conditions does not remove the need to inspect the room where the smell enters.
When reporting an outside incident, give the exact location and whether there is visible sewage, a blocked public drain or affected neighbouring properties. Avoid describing ordinary rainwater as sewage unless you have evidence. Photographs taken from a safe distance and the date and time give the water company something it can match to other reports.
Do not climb onto a roof, remove a vent termination or seal it with a bag to see whether the smell changes. Those experiments can introduce a drainage fault or fall risk. If the pattern suggests an outside vent, a competent drainage professional can assess its position and condition using suitable access.
If it happens when the washing machine or dishwasher pumps out
Record whether the smell is strongest at the appliance door, a standpipe, the cupboard under the sink or another room. An appliance cleaning problem and a waste connection problem need different work. Follow the appliance's own maintenance instructions for accessible filters and seals; do not disconnect a hose simply because an online checklist says to check behind it.
A useful booking description is “odour begins during the drain part of the wash cycle, the kitchen sink gurgles, and no water appears under the machine”. That gives the plumber a specific operating stage to observe. If water is visible, stop using the appliance and describe whether it is clean, soapy or foul without touching or tasting it.
Do not push a waste hose deeper into a standpipe or improvise an airtight seal. Connection heights and arrangements depend on the appliance and plumbing design. A photograph showing the existing arrangement lets the installer check it against the correct instructions. If the connection is concealed, agree how it will be accessed and reinstated before cupboards are cut.
A smell alone does not establish a pressurised water leak
Wastewater pipework carries used water away. A mains or heating leak is a different circuit. That distinction matters because a drain survey, a plumbing inspection and tracer-gas investigation of a water pipe answer different questions. Paying for the wrong type of visit can produce a technically competent report which never addresses the smell.
If the smell is accompanied by a damp ceiling, expanding stain, moving water meter with ordinary use excluded, or repeated boiler pressure loss, mention those as separate symptoms. They may justify a water-leak investigation alongside the drainage checks. Do not assume one diagnosis explains everything just because the symptoms started in the same week.
Our guide to a bathroom drain smell and when a leak is involved covers that distinction. For a persistent odour across several rooms, see the separate whole-house sewage-smell guide. This page focuses on the timing of an intermittent problem, which is information to take into either investigation.
What to ask before paying for an investigation
Send the diary, two or three relevant photographs and a short description of the property. Say whether it is a flat, whether there is a shared stack, and what work preceded the first occurrence. Ask the firm whether it investigates drainage odours specifically, rather than assuming every business advertising leak detection provides the same services.
- Which visible fittings and connections will be inspected first?
- Can the investigation cover the event recorded in the diary?
- What would justify a CCTV drain survey rather than a plumbing inspection?
- Would any testing require coordination with neighbours or building management?
- Will the findings distinguish confirmed defects from possible causes?
- Who repairs the fault, and who reinstates any access opening?
If specialist smoke testing is proposed, let the contractor plan it with the building's occupants and relevant alarm arrangements. It is not a DIY experiment with a smoke bomb. If CCTV is proposed, ask which accessible run will be surveyed and what limitations remain. A camera view of one drain does not automatically exclude a concealed joint above it.
For a flat, inform the managing agent where a shared stack or another dwelling may need inspection. Keep a record of access requests and reports. Do not describe a neighbour as responsible solely because their bathroom is above the strongest smell: odour can travel through a service duct, and responsibility needs the actual defect and relevant documents.
After a repair, check the same event again
Ask the contractor what was found, what was changed and how the result was checked. “Smell gone on departure” is less useful than a record that a specific disconnected joint was repaired and the affected fittings operated normally afterwards. Where a cause remains unconfirmed, the report should say so and identify the next observation or test.
Return to the diary headings during normal use. If the original trigger was an evening bath, a quiet morning is not a meaningful comparison. If the trigger was a long period of non-use, keep the maintenance advice and note when the fitting is used. If symptoms return, send the earlier findings with the new record so that the next visit builds on the first.
Retain photographs of access points before they are closed. Do not leave an unsafe opening accessible, but ask for the pipe or fitting to be documented before reinstatement. That small record can prevent repeated exploratory opening if a different problem later appears nearby.
When Leak Fix London is the appropriate next call
If you have evidence of escaping water alongside the smell, send us the symptoms and photographs. Tell us where water appears, whether it follows a particular fixture's use, and whether a plumber has already identified a damaged water pipe. We can discuss the water-leak investigation and repair and any access or making good the work may require.
Our rate is £150 per hour; the total depends on the time involved. Our no-find-no-fee commitment concerns time spent looking for a water leak: if we do not find the leak, you do not pay for that search time. It is not a promise to identify every unexplained odour without charge. See the pricing and scope before booking.
For a blocked drain, sewage backup or a smell-only drainage problem, book the appropriate drainage or plumbing specialist; drain unblocking is not our service. Explaining that boundary at the start helps you spend the first visit on the problem you actually have.
