Why Do Drains Smell in Hot Weather? Causes and Outside Drain Fixes
Yes, drains smell worse in hot weather: heat speeds up the bacteria breaking down grease and silt, and dry traps let sewer gas in. How to stop a smelly outside drain, and when it needs a jet wash.
Drain smells worsen in summer because heat accelerates bacterial decomposition in grease and organic matter. In London, old clay pipes, fat deposits (fatbergs) and low water use in unoccupied properties (dry traps) are the main culprits.
Key Takeaways
- Summer heat speeds up bacterial decomposition of grease and organic waste, intensifying drain smells in London homes.
- Dry P-traps — common in holiday-empty properties and guest bathrooms — let sewer gas rise directly into living spaces.
- London's Victorian-era clay pipe network is especially vulnerable to fatberg build-up and cracked joints that allow smells to escape.
- Most summer drain odour problems can be fixed without major works: pouring water into traps, cleaning waste with bicarbonate, or calling a drain specialist for a jet-wash.
Why Heat Makes Drains Smell Worse in Summer
If you've noticed drain smells getting worse in summer across your London home, you're not imagining it. The science is straightforward: bacteria responsible for breaking down organic waste in your drains and sewer pipes work faster in warm conditions. When ambient temperatures climb above 20°C — a regular occurrence in London between June and August — bacterial activity can double or triple, producing hydrogen sulphide (the rotten-egg gas) and methane at a much higher rate than in winter months. The result is that smells you'd barely notice in January become genuinely unpleasant by July.
This isn't just a surface issue. London's sewer pipes sit between one and six metres underground, where they absorb and retain heat throughout the summer. Thames Water's combined sewer system — which carries both rainwater and sewage — can reach elevated temperatures in dry spells, since there's less cold rainwater diluting the wastewater flow. Less dilution means a higher concentration of decomposing organic material sitting in the pipe, and more gas produced per litre of waste.
There's a secondary factor too: evaporation. The water seals in your P-traps (the curved section of pipe under sinks, baths and toilets) rely on a small standing pool of water to block sewer gases from rising into your home. In summer, that water evaporates faster — especially in bathrooms that aren't used daily, such as en-suites or guest bathrooms. Once the trap runs dry, there's nothing between your living space and the sewer below.
London-Specific Causes of Summer Drain Odour
London has a particular set of drain and sewer characteristics that make summer smells worse here than in many other UK cities. Understanding them helps you target the right fix rather than masking the symptom with drain deodorant.
Victorian clay pipe infrastructure. A large proportion of London's residential drains — particularly in Victorian terraced houses across boroughs like Islington, Hackney, Lewisham and Wandsworth — run through original clay pipes that are 100 to 150 years old. Clay is porous and develops hairline cracks over decades of settlement, root intrusion and freeze-thaw cycles. In summer, gases escape through these micro-fractures at ground level and can permeate through patio slabs or rear garden soil before entering the property through airbricks or floor voids. Our guide on tree root damage to London drains covers how root ingress worsens this problem in older pipe runs.
Fatbergs and grease accumulation. London's sewer system is the worst in the UK for fatberg formation. Thames Water removes thousands of tonnes of solidified fat, oil and grease from London sewers every year. During summer, higher ambient temperatures mean these deposits soften and partially liquefy, releasing trapped volatile compounds and intensifying the smell at surface level. Drains serving kitchens — both domestic and the takeaways and restaurants common in London high streets — are especially prone to grease-driven summer odour.
Holiday-empty properties. London has a high proportion of rental flats and second properties that sit unoccupied during summer. When no one is running taps or flushing toilets, every trap in the property gradually dries out. Landlords and short-let hosts often return in September to discover a property that smells strongly of sewer gas — which is entirely preventable. See our full article on sewage smell inside the house for a room-by-room diagnostic. If the smell persists after every trap has been topped up, our drain smell investigation service can find where the gas is getting in.
Inside vs Outside Drain Smells — What's Different
The source of a summer drain smell determines the fix, so it's worth distinguishing between odours that originate inside the property and those coming from external drainage.
| Smell Location | Most Likely Cause | Typical Fix | DIY Possible? |
|---|---|---|---|
| Under kitchen sink | Grease build-up in waste trap or pipework | Clean trap, enzyme treatment | Yes |
| Bathroom sink or shower | Dry or dirty P-trap, hair/soap build-up | Run water, clean or replace trap | Yes |
| Toilet area | Dry wax seal, cracked pan collar | Reseat toilet, replace wax ring | No — call a plumber |
| Outside drain cover | Partial blockage, grease/debris build-up | Clear debris, jet wash | Partial |
| Garden / patio area | Cracked clay pipe, root intrusion | CCTV drain survey, pipe repair | No — call a plumber |
| Whole-property / ground floor | Blocked or collapsed main drain | Emergency drain clearance | No — call immediately |
A smell that appears only in one room and disappears after you run the tap for thirty seconds is almost certainly a dry trap — a minor issue you can fix in minutes. A smell that persists throughout the property, especially on the ground floor, or one that gets worse after heavy rain, points to a problem in the main drain run or the sewer connection and needs professional assessment. For background on what typically causes blockages in the first place, read our article on blocked drain causes in London.
Smell From an Outside Drain: Finding Where It Comes From
A drain smell outside the house nearly always comes from one of five places. Walk round the property on a still day and check each one before you decide what needs doing, because the fix is different for each.
The gully by the kitchen or back door
This is the open grid where the kitchen waste pipe discharges. It has a water-filled trap that stops sewer gas coming up, and in hot, dry spells that water can evaporate. Grease and food waste from the sink also settle in it and start to rot. Lift the grid, scoop out what you can with a gloved hand, scrub the trap and then pour a full bucket of water in to restore the seal. If the water sits there and drains away slowly, the pipe beyond the gully is partly blocked.
The manhole or inspection chamber cover
Lift the cover (they are heavy, so use a screwdriver at each corner rather than your fingers) and look at the channel in the bottom. A clean channel with water running through means the smell is escaping round a cracked or badly seated cover, which is a simple replacement. A chamber holding standing waste means the drain is blocked somewhere downstream, and that is a job for rods or a jetter. Our guide to blocked outside drains explains how to tell your drain from Thames Water's before you book anyone.
The soil vent pipe
The tall pipe that runs up the outside wall and ends above the gutter is meant to release drain gas at roof level. On a still, warm day that gas can drift down into a garden or back in through an upstairs window, which is normal in small doses. If the smell is strong and constant, or the vent ends close to an opening window, the vent may need extending. An air admittance valve (the plastic cap on some internal soil stacks) that has stuck open can also cause a smell that seems to come from outside.
Rainwater gullies and downpipes
A gully that only takes rainwater should not smell. If one does, the trap may have dried out, or in many older London houses the rainwater runs into the same combined drain as the toilets, so sewer gas can reach it. Refilling the trap usually settles it. A persistent smell here is worth a camera check, because it can also point to a cracked pipe underground.
The street or the neighbours
If the smell is strongest at the pavement, a road gully or a sewer grating, it is coming from the public sewer and you should report it to Thames Water rather than pay for a plumber. The same applies to a shared drain beyond your boundary. Drains that serve only your home are your responsibility.
If the source is your own drain and cleaning the gully and trap has not solved it, we can clear the line with high-pressure drain jetting, or find a hidden crack or root intrusion with a CCTV drain survey. For smells that come and go inside the home as well, see our drain smell investigation service.
Outside drain still smelling after you have cleaned it?
Tell us where the smell is strongest and whether the manhole is holding water. That tells us whether to bring rods, a jetter or a camera.
📞 Call 020 7870 3200Fixes: DIY Steps and When to Call a Professional
Most summer drain smell problems in London homes fall into one of two camps: things you can fix yourself in under an hour, and things that need a qualified drainage engineer. Knowing the boundary saves you time, money and a wasted trip to the hardware shop.
DIY fixes that work. For dry traps, run every tap and flush every toilet in the property — this refills the water seals immediately. For kitchen waste smells, mix 200g of bicarbonate of soda with 200ml of white vinegar, pour it down the drain and follow with a full kettle of just-boiled water after five minutes. This breaks up grease and neutralises the bacterial activity causing the smell. Repeat weekly during hot weather. For outside gully drains, lift the cover, remove any debris by hand (wearing gloves), then flush with a garden hose on full pressure. Our guide to clearing a blocked outside drain walks through this step by step. A biological drain treatment poured in weekly will suppress odour throughout summer.
When to call a drain specialist. If a smell persists after you've run all traps and cleaned accessible waste pipes, the problem is likely further down the line. A partial blockage 10–20 metres into the drain run will sit and ferment through summer, producing a consistent background odour that no amount of drain cleaner will fix. Our blocked drains London team uses high-pressure water jetting to clear grease, debris and partial fatberg deposits that are too deep for DIY access. We also carry CCTV drain cameras for properties where cracked or root-damaged pipes are suspected — common in pre-1960s London terraces. Where nobody knows which drain runs under the smelly area, drain tracing maps the route first.
Cost guide for professional drain work in London (2026). A standard drain jet wash costs between £80 and £150 depending on drain length and access. A CCTV drain survey runs £150–£300. Pipe relining for a cracked section — which avoids excavation — typically costs £500–£1,500 depending on length. Emergency call-outs for complete blockages start at around £120 for the first hour.
Thames Water responsibility boundary. It's worth knowing that Thames Water is responsible for the public sewer that runs under your street, but the private drain serving your property — from your home to the boundary — is your responsibility (or shared with neighbours if it serves multiple properties). If your drain smells are coming from a shared drain, Thames Water has a free reporting service, but for the private drain run, you'll need your own plumber.
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📞 Call 020 7870 3200Frequently Asked Questions
Why do my drains smell more in summer than winter?
Warmer temperatures accelerate the rate at which bacteria break down organic matter — grease, food waste and soap scum — inside your pipes and drain channels. The resulting hydrogen sulphide and methane gases are produced faster and in greater volume, making the smell noticeably worse during hot spells. London's dense sewer network also traps heat underground, compounding the effect.
Is a drain smell in summer dangerous?
Hydrogen sulphide at low concentrations (the 'rotten egg' smell) is unpleasant but not immediately dangerous in a well-ventilated home. However, high concentrations in confined spaces can cause headaches, nausea and, in extreme cases, loss of consciousness. If the smell is overwhelming or persists after ventilating, leave the property and call a plumber. Never use a naked flame near a strong sewer odour.
How do I stop my outside drain smelling in hot weather?
Start by removing the drain cover and clearing any visible debris or grease build-up. Pour a kettle of hot water mixed with washing-up liquid down the drain, followed by a cup of bicarbonate of soda and white vinegar to neutralise bacterial activity. If the smell returns within a few days, the drain likely has a partial blockage deeper in the line that needs a professional high-pressure jet wash to clear.
Why does my outside drain smell even when it is not blocked?
The usual reasons are a gully trap that has dried out in warm weather, a layer of grease and silt sitting in the trap or the inspection chamber channel, or a loose or cracked manhole cover letting gas out around the frame. Topping the gully up with a bucket of water and scrubbing the trap clears the first two. If the chamber channel is holding standing water, there is a partial blockage further down the drain and it needs rodding or jetting.
Who do I call about a drain smell outside my house in London?
If the smell comes from a gully, manhole or pipe that serves only your property, it is your private drain and a plumber or drainage engineer can clear it. If it comes from a drain shared with neighbours beyond your boundary, or from a road gully or sewer grating in the street, report it to Thames Water, who are responsible for public sewers in London.