7 Warning Signs You Have a Blocked Drain

A blocked drain rarely announces itself all at once. In most cases, the warning signs build gradually over days or weeks — and the earlier you catch them, the cheaper and simpler the fix. We spoke to drainage engineers across England about the symptoms they see most often, and distilled their advice into seven clear signs that something is wrong with your drains.

1. Water Drains Slowly

This is the single most common early warning sign. Your kitchen sink takes noticeably longer to empty. The shower tray fills up around your ankles. The bathroom basin holds water for 30 seconds after you pull the plug instead of draining in a few seconds.

What it usually means: A partial blockage is forming — typically a build-up of grease, soap scum, or hair that's narrowing the pipe. At this stage, the blockage is often close to the plughole and may respond to DIY methods like a plunger or drain snake.

When to worry: If multiple fixtures are draining slowly at the same time (for example, both the kitchen sink and the downstairs toilet), the blockage is likely further down the system and you should call a professional.

2. Gurgling Sounds

A gurgling noise from your plughole, toilet, or external drain is caused by trapped air being forced through water. When a pipe is partially blocked, water has to squeeze past the obstruction, displacing air and creating that distinctive bubbling or gurgling sound.

What it usually means: A blockage is developing but hasn't yet fully sealed the pipe. You might hear gurgling from one fitting when you use another — for example, the toilet gurgles when you run the bath. This is a classic sign of a shared waste pipe starting to block.

When to worry: Gurgling that gets louder or more frequent over a few days suggests the blockage is growing. Don't ignore it — this is the ideal time to act, before the pipe blocks completely.

3. Bad Smells

A foul smell coming from a drain, plughole, or outside manhole cover is a strong indicator of a blockage. Waste material trapped in the pipe decomposes, producing hydrogen sulphide — the rotten egg smell that's impossible to miss.

What it usually means: Organic matter (food waste, grease, or sewage) is stuck in the pipe and rotting. External drain smells often point to a blockage in the underground pipework between your house and the public sewer.

When to worry: If you can smell sewage inside your home — particularly in rooms without obvious drain access like bedrooms or hallways — the blockage may be causing sewage to back up. This is a health hazard and warrants an urgent call to a drainage company.

4. Raised Water Levels in the Toilet

After flushing, the water in your toilet bowl should drop to its normal resting level within a few seconds. If it rises higher than usual, sits high for an extended period, or nearly overflows before slowly subsiding, the waste pipe downstream is restricted or blocked.

What it usually means: The toilet's soil pipe is partially blocked, often by a build-up of toilet paper, wet wipes (even “flushable” ones), or sanitary products. In older properties with cast-iron soil stacks, internal corrosion can narrow the pipe and trap debris.

When to worry: If the water rises to within a centimetre of the rim, stop using that toilet immediately and call for help. One more flush could send sewage onto your bathroom floor.

5. Overflowing or Raised Manhole Cover

External manholes (also called inspection chambers) are the access points to your underground drainage. Most residential properties in England have at least one, usually in the back garden or side passage. If you can see water sitting at or above the level of the manhole cover — or worse, overflowing onto the ground — the drain downstream is blocked.

What it usually means: A significant blockage in the main drain run, often caused by tree root ingress, a build-up of fat and debris, or a collapsed pipe section. This is almost never a DIY fix.

When to worry: An overflowing manhole is a sewage spill. It poses a health risk, especially if you have children or pets using the garden. Call a drainage company the same day. If the manhole serves multiple properties, contact your water company as it may be their responsibility.

6. Damp Patches or Wet Spots on Walls and Floors

Unexplained damp patches near ground level on external or internal walls can indicate a leaking or overflowing drain below the surface. You might notice staining, bubbling paint, or a musty smell. In gardens, you might see an area of unusually green, lush grass — it's being fertilised by leaking sewage.

What it usually means: A cracked or displaced pipe is leaking wastewater into the surrounding ground. Over time this can undermine foundations and cause subsidence. It's not always a blockage per se, but blockages cause pressure build-up that cracks older clay and pitch fibre pipes.

When to worry: Any unexplained damp near a drain run should be investigated. A CCTV drain survey (typically £100–£200) will show exactly what's happening underground without any digging.

7. Fruit Flies or Drain Flies

Small flies congregating around sinks, shower drains, or external drain covers are often drain flies (also called moth flies). They breed in the organic sludge that coats the inside of partially blocked pipes. If you're seeing clusters of tiny, fuzzy-winged flies that weren't there before, there's likely a build-up of organic material in your drains.

What it usually means: The pipe has a layer of decomposing grease, food waste, or biofilm that's providing a breeding ground. The drain may not be fully blocked yet, but it's heading that way.

When to worry: Drain flies themselves are more of a nuisance than a health risk, but they're a reliable indicator that your drains need cleaning. If the problem persists after cleaning the plughole and trap, the build-up is deeper in the system.

When to Call a Professional

Not every symptom requires an emergency call-out. Here's a simple rule of thumb:

• One fixture, draining slowly: Try a plunger or drain snake first. See our DIY vs professional guide for step-by-step instructions.
• Multiple fixtures affected, or external signs (manhole, smell, damp): Call a professional. The blockage is in the main drain run and needs proper equipment to clear.
• Sewage backing up into your home: Call immediately. This is a health hazard. See our emergency drain clearance guide for what to do while you wait.

A standard drain clearance for a residential property typically costs £100–£170. Catching the problem early almost always means a simpler, cheaper job — so if you're noticing any of the signs above, don't wait for it to get worse.