Smelly Drains: Why They Smell and How to Fix the Odour

Why Does My Drain Smell?

A bad smell rising from a drain is one of the most common plumbing complaints — and one of the most misunderstood. The good news is that most causes are straightforward once you know what you are looking for. The less good news is that a handful of causes do need professional attention before they get worse.

Here are the main culprits.

---

1. A Dry Trap

Every sink, bath, shower, and basin has a U-shaped section of pipe beneath it called a trap. That curve holds a small pool of water, which acts as a seal to stop sewer gases drifting back into your home.

If a fitting is rarely used — a guest bathroom sink, a utility room floor drain — the water in the trap can simply evaporate. When it does, there is nothing between you and the sewer.

Fix: Run the tap for 30 seconds to refill the trap. For floor drains, pour a cup of water down periodically. If the smell returns quickly, the trap may be damaged or incorrectly fitted.

---

2. Biofilm Buildup

Hair, soap scum, toothpaste, and grease gradually coat the inside of drain pipes and the underside of plughole covers. Bacteria feed on this layer — called biofilm — and produce hydrogen sulphide gas, which smells like rotten eggs.

Fix:

• Remove and scrub the plughole cover or strainer.
• Pour a kettle of boiling water down the drain (not for plastic pipes — use very hot tap water instead).
• Follow with a mixture of bicarbonate of soda and white vinegar, leave for 15 minutes, then flush again.
• A long flexible drain brush can reach further into the pipe.

Repeat monthly to keep biofilm under control.

---

3. A Partial or Full Blockage

Food waste, fat, wet wipes, and hair accumulate over time and create a partial blockage. Water drains slowly, sits in the pipe longer, and the decomposing material smells.

Fix: Try a plunger first. If the drain is slow but not completely blocked, a drain unblocker product (used according to the manufacturer’s instructions) may shift the clog. Avoid overusing caustic products — they can damage older pipework and are not always effective on solid blockages.

If plunging and cleaning do not resolve the smell within a day or two, the blockage is likely deeper in the system and needs professional clearing — the kind of job our sister firm does when a blocked drain is beyond the reach of a plunger.

---

4. A Faulty or Missing Trap

Sometimes traps are incorrectly installed, cracked, or missing altogether — particularly in older properties or after DIY work under a sink. A damaged trap cannot hold water and will allow sewer smell to pass through continuously.

Fix: This needs a plumber. Replacing a trap is a straightforward job, but it does require disconnecting pipework and ensuring the new trap is correctly fitted and sealed.

---

5. Venting Problems

Soil stacks and drain systems rely on vent pipes to equalise pressure. If a vent pipe is blocked (by debris, a bird’s nest, or frost) the system can siphon water from traps or push sewer gases back through them. You may notice a gurgling sound alongside the smell.

Fix: Vent blockages are not a DIY job. A plumber can inspect and clear the vent stack safely.

---

6. Cracked or Leaking Drain Pipes

If sewer smell is noticeable throughout a room or the whole house — not just near one fitting — there may be a cracked pipe within the floor or wall. This is more serious and can cause damp as well as odour.

Fix: Call a plumber. CCTV drain surveys can pinpoint cracks without unnecessary excavation.

---

When Is a Smelly Drain Serious?

Most drain smells are a nuisance rather than a danger, but contact a plumber promptly if:

• The smell is throughout the house, not just one room.
• You can hear gurgling from multiple fittings at once.
• Drains are backing up or overflowing.
• You suspect a cracked or leaking pipe.
• The smell returns quickly after you have tried the DIY fixes above.

Prolonged exposure to sewer gases is not pleasant and, in enclosed spaces, can be harmful. It is always better to get a persistent smell investigated sooner rather than later.

---

Quick Maintenance Tips to Prevent Drain Smells

• Pour water down infrequently used drains every couple of weeks.
• Fit hair-catcher strainers in showers and baths.
• Never pour cooking fat or grease down the sink.
• Clean plughole covers weekly.
• Flush kitchen drains with hot water after washing up.

---

If you have worked through the steps above and the smell persists — or if your drains are backing up — do not leave it to chance. Call our drainage line on 07700 137648, any time of day or night. We cover the TW postcode area and can diagnose and fix drain problems quickly, with no call-out charge for the visit itself.