Why Do My Drains Smell Worse in Hot Weather (and After Rain)?

Reviewed by the DCI Drainage team · Last updated September 2026

Every warm spell and every proper Cornish downpour brings the same phone calls: the drains were fine last week, and now the garden, the bathroom or the kitchen smells like something died in the pipes. Usually nothing new has gone wrong. The weather has simply exposed a weakness that was already there. This guide explains what heat and rain each do to your drains, the quick fixes worth trying today, and how to tell a harmless seasonal whiff from a smell that is telling you about a real problem.

Why hot weather makes drains smell worse

Three things happen to a drainage system in a heatwave, and each one turns the smell up.

The bacteria work faster

Your drains always contain a film of fat, soap, food residue and waste being broken down by bacteria. That process produces gases, including hydrogen sulphide, the classic rotten egg smell. Bacterial activity roughly doubles with every several degrees of warmth, so the same coating of gunge that was odourless in March becomes noticeably ripe in a July hot spell. Nothing new has entered the drain; the existing residue is simply decomposing faster.

The water seals dry out

Every sink, shower, toilet and outside gully has a trap: a bend holding standing water that acts as a plug between your home and the gases in the drains. In hot, dry weather that water evaporates, and in little-used fittings, the guest bathroom, the utility sink, the gully on the shady side of the house, the seal can disappear entirely. Once the trap is dry, there is an open pipe between the sewer and your nose.

There is less water moving

Households often use water differently in summer, and long dry spells mean nothing is rinsing the pipes through from above. Waste sits in the runs for longer, drying and sticking rather than flushing away, which gives the bacteria even more to work on.

Why drains smell after heavy rain

Rain does something different. When a storm dumps a month of rain in an afternoon, which the South West manages regularly, the drains and sewers fill fast. All that incoming water compresses and displaces the air and gas sitting in the pipes, and that gas has to go somewhere. It gets pushed out through gullies, manhole covers and any trap whose seal is weak or shallow, which is why the smell often appears indoors at the fittings you use least.

Heavy rain also stirs the pot. Silt, debris and stagnant water that have sat quietly in low spots and bellies along the pipe get churned up and moved, releasing smell as they go. And where a drain is already partially blocked, a storm can push the system to the edge of surcharge, with waste water backing up towards gully grates, bringing the smell with it. A smell that arrives with every downpour and fades within a day is usually gas displacement and nothing more. A smell that arrives with rain and then stays is a sign the storm has exposed a blockage or a damaged pipe.

Working out which problem you have

The pattern of the smell tells you most of what you need to know.

Quick fixes that actually work

Before calling anyone, these steps clear the majority of seasonal drain smells and cost almost nothing.

• Top up every trap. Run water into every sink, shower, bath and floor drain in the house for thirty seconds, including the ones nobody uses. Pour a bucket of water into each outside gully. In a heatwave, repeat weekly.
• Clean the gully pots. Lift the grates and scoop out leaves, silt and grease. A gully full of rotting debris will smell in any weather, and doubly so in July.
• Flush the kitchen run with hot water and washing up liquid. A kettle of hot (not boiling) water with a good squirt of detergent, run through weekly, keeps the fat film down. Skip the pouring-fat-down-the-sink habit entirely; it is the raw material for most kitchen drain smells.
• Refresh seldom-used toilets. Flush them a couple of times a week so the pan and trap stay full.
• Do not rely on bleach. It masks the smell for an evening and does very little to the fat and residue producing it. Moving water and physical cleaning beat chemicals here.

If drain smells are a recurring theme at your place in any season, not just summer, the causes run wider than the weather, and we cover the full picture in our smelly drains guide covering kitchens, bathrooms, showers and outside drains.

When the smell means a real problem

Call a professional when the smell survives the fixes above, keeps returning within days, comes with gurgling or slow drainage, or when you can smell it strongly through a wall or floor rather than at a plughole. Those patterns point to fat and scale coating the runs, a partial blockage holding stagnant waste, or a cracked pipe leaking into the ground, none of which a bucket of water will fix.

The right response depends on the cause. A fouled but sound pipe needs a proper clean-through, which is exactly what a scheduled drain cleaning and maintenance visit deals with, and jetting the run clears years of built-up fat and silt in one go. A smell with no obvious blockage behind it is often a job for a CCTV drain survey, because a camera finds the cracked joint or dead leg producing the smell far faster than digging on a hunch. Either way, a persistent drain smell is your system telling you something, and it is always cheaper to listen early.

Frequently asked questions