You flush the toilet and it gurgles. Or you run the shower and air bubbles up through the toilet bowl. Maybe the washing machine drains and the toilet starts making that low, hollow sound. You're not imagining it - and it's not just a quirk of old plumbing.

This is one of the more common things we get called out to across Croydon. People describe it differently - "the toilet sounds like it's breathing," "there's air coming up when the bath drains," "it gurgles every time it rains heavily." Same problem, different words.

What it might mean

There's no single answer here, and anyone who tells you otherwise without looking at the drain first is guessing.

What this symptom usually points to is some kind of restriction in the system - but where exactly, and what's causing it, takes a proper look to establish. It could be a partial blockage in the waste run. Fat, wipes, scale, silt - we see all of these. It could be a restricted soil pipe or a blocked vent stack, which stops air moving through the system properly and forces it back up through the nearest fixture (often the toilet). Or the restriction might be further along the main drain, especially if multiple fixtures are affected at once.

In older Croydon terraces and converted flats, there's another layer to this. A lot of the drainage around central Croydon and Thornton Heath is salt-glazed clay pipe - Victorian-era, brittle, prone to root ingress and joint displacement. In post-war semis and maisonettes across Purley and Coulsdon, pitch fibre pipe is common. That stuff deforms over time, going from round to oval, and the resulting restriction gets mistaken for a blockage when it's actually the pipe collapsing in on itself. Rodding it out doesn't fix that. It just delays the next call.

Edwardian conversions - the semis split into two or three flats - often have soil stacks that were never properly redesigned at conversion. Undersized stacks, shared gullies, no clear venting. The gurgling is frequently a sign the system's struggling with more than it was designed to handle.

How urgent is it?

Depends what's happening.

If it's just an occasional gurgle on one fixture and the water's draining away normally, it's worth getting looked at soon - but it's not a same-day emergency. Yet.

If more than one fixture is affected, if water is rising in the toilet bowl rather than draining, or if you've got an outside drain that's also backing up - treat that as urgent. That pattern suggests the restriction is further down the system and wastewater has nowhere to go. When that happens, it doesn't stay in the pipe. It comes back up - through toilets, through shower trays, through low-level gullies. We've seen it come back through kitchen sinks in ground-floor flats where the stack drainage was shared and no one realised.

Don't wait to see if it clears itself. Partial blockages don't tend to get better on their own.

What you can safely check

First - is it one fixture or several? Run the bath. Run the washing machine. See if the gurgling happens every time or only from one specific point. That information is genuinely useful and helps narrow things down quickly.

If the water level in the toilet bowl is rising, stop flushing. Every additional flush adds more water to a system that's already struggling to move it.

Do not lift inspection chamber covers unless you're confident it's safe to do so. If there's already backflow happening, lifting the cover can release it. And if you're not sure where the chamber is or what you're looking at, leave it.

Beyond that - don't put anything down the drain trying to "shift it." Supermarket unblockers might mask the symptom for a day or two, but they won't touch a structural restriction, a collapsed pipe, or a blockage further along a shared run.

What we check

We start at the inspection chamber - if there is one accessible. In a lot of Croydon properties, especially terraced houses with rear extensions or flats where the drainage was never redesigned, access is limited or in an awkward spot. We've worked on drains in south Croydon where the chamber was buried under decades of paving with no obvious lid.

From the chamber we can usually tell straight away whether there's standing water, where the flow's going, and whether the issue is upstream or downstream. A flow test - running fixtures and watching what happens - tells us more.

If the problem keeps recurring or we can't see the cause from the outside, we put a camera down. A CCTV drain survey shows us exactly what's there - root ingress, a deformed pipe, a displaced joint, a section that's dropped and is holding water. You can't diagnose that by looking at it from above, and you can't fix it properly without knowing what you're dealing with.

In shared drain runs - which are very common in the dense terraced streets around central Croydon - it also tells us whose drain the problem is actually in. That matters more than people realise.

How it's fixed

If it's a blockage - fat, wipes, scale, accumulated silt - the fix is usually drain rodding or high-pressure water jetting, depending on how compacted it is and where it's sitting. Jetting is more thorough, especially in longer runs or where grease has built up along the pipe walls. You can read more about that process on our drain unblocking page.

If the CCTV survey finds something structural - a deformed pitch fibre pipe, a cracked clay joint, a root that's pushed its way through - rodding it out isn't the answer. That's a repair job, not a cleaning job. Getting the wrong fix means you're back to square one in six months, sometimes sooner.

Drain lining is one option for pipes that are damaged but still intact enough to line - it avoids digging and works well in sections that are hard to access. But it's not always the right call, and we won't push it where it isn't needed.

The gurgling tells you something's wrong. What it doesn't tell you is how wrong - and in Croydon's older housing stock, the gap between a simple blockage and a failing pipe can be significant.

Worth knowing before it gets worse

If one fixture is affected and water's draining - get it looked at this week.

If multiple fixtures are gurgling, water's rising in the bowl

Toilet bubbling when another fixture drains in Croydon