Why Drains Smell Worse in Summer: The Science We Find Fascinating (You Won't)

Why Drains Smell Worse in Summer: The Science We Find Fascinating (You Won’t)

Every July, phones across TW1 to TW20 start ringing with the same question. Not “how do I fix my drain?” but a more existential “WHY does it smell like that?” The answer is genuinely interesting, at least to us. Bear with it, because understanding the cause is what leads you straight to the fix.

The Bacteria Are Having a Brilliant Summer

Your drains are never sterile. They contain a thriving community of bacteria that breaks down organic waste, grease and soap scum as it passes through. In winter, this community ticks along quietly. In summer, it absolutely goes off.

Bacteria roughly double their activity rate for every 10°C rise in temperature. So a drain that produced a faint background whiff in February is producing four times the odorous gas by the time July arrives. The gases in question are hydrogen sulphide (the classic rotten-egg smell), ammonia, and a cheerful family of compounds called volatile fatty acids. None of them are pleasant. All of them increase dramatically with heat.

The drains themselves are not broken. The biology is just working faster.

Dry Traps: The Most Common Culprit Nobody Thinks Of

Under every sink, basin, bath and shower tray is a U-shaped bend called a trap. That bend holds a small plug of water at all times, and that plug of water is the only thing physically separating your kitchen from the sewer gas network beneath it.

In summer, if a drain is used infrequently, that water plug evaporates. Once it goes, there is nothing between you and the full aromatic output of the sewer. This is why spare bathrooms, utility sinks, outside gullies and patio drains often smell worst in a heatwave. They are not blocked. The trap has simply dried out.

The fix is almost embarrassingly simple: pour a litre of water down the drain. If the smell clears within a few minutes, that was your problem. Job done.

Low Flow and the Grease Ledge Problem

Summer also changes how much water moves through domestic drains. People shower rather than bath, run dishwashers less, and generally use less hot water. Lower flow means grease and soap deposits that would normally be flushed along the pipe sit in place for longer. In the heat, they go rancid faster.

This is particularly noticeable in kitchen drains, where cooking grease accumulates on pipe walls over time. The technical term is a grease deposit or, in its more dramatic form, a fatberg (a solid mass of congealed fat, wipes and debris that blocks the pipe entirely). A summer fatberg is a thing of horrible, fascinating grandeur. We have seen specimens in Shepperton and Sunbury that deserved their own nature documentary.

You do not want one. They cost between £150 and £450 to clear professionally, depending on size and access, and they only get worse with time.

Outside Drains and the Gully Situation

Outdoor drains, particularly patio gullies and channel drains, collect leaf debris, soil and garden waste all year round. In winter, rain keeps this material moving. In summer, it sits, warms up, and ferments. The smell from a blocked or slow outdoor gully in July can be remarkable.

These drains are also the ones most likely to have dry traps, because nobody is running water through them regularly. If your outside drain smells, check two things in this order: first, pour water in to refill the trap; second, remove the grate and look for a visible blockage of debris. Both are things you can do yourself in five minutes.

The Checks That Fix 80% of Summer Drain Smells

Run through these before calling anyone:

• All indoor floor drains, spare bathrooms and utility sinks: pour one litre of water down each one. Wait five minutes and check.
• Outside gullies and patio drains: refill with water, then lift the grate and remove any visible leaf or debris build-up.
• Kitchen sink: run hot water for two minutes. If the smell is greasy or sulphurous rather than sewage-like, pour a kettle of hot (not boiling) water down the drain, followed by a cup of bicarbonate of soda and a cup of white vinegar. Leave for twenty minutes, then flush with more hot water.
• Check all plug holes for hair and soap build-up. A partial blockage holds organic material in the warm pipe, which accelerates the smell problem significantly.

If the smell persists after all of the above, or if it is coming from multiple drains at once, you likely have a partial blockage further down the line, a problem with the main soil stack (the large vertical pipe that carries waste from upper floors), or an issue with the sewer connection itself.

When to Call Us

A smell from one dry trap is a self-fix. A smell from every drain in the house at once is a sign of a blocked or venting-impaired main drain, and that will not improve on its own. Left long enough, a partial blockage becomes a full one, usually at the worst possible moment.

We cover the full TW postcode area, including Shepperton, Sunbury, Hampton, Twickenham and everywhere between, and we arrive within the hour for emergencies. A CCTV drain survey (a small camera sent through the pipe to show exactly what is in there and where) costs between £80 and £150 for most residential jobs and removes all the guesswork.

If the smell is still there after the checks above, or if you have slow-draining water anywhere in the house, call us on 07700 137648. We are available 24 hours a day, seven days a week, and we will tell you honestly over the phone whether it sounds like a self-fix or something that needs us.