Why Does My Drain Keep Blocking? Causes and Fixes

A cleared drain is not the same as a fixed drain

Plungers, rods and drain unblocker gel punch a hole through the obstruction. Water flows again, you assume it is done, and three weeks later it backs up in the same spot. That pattern is the clearest sign there is a defect or a build-up the rodding never reached.

A properly cleared drain is jetted back to the full bore of the pipe, so a 100mm pipe runs at 100mm rather than through a 30mm channel in a wall of grease. If your drain was rodded rather than jetted, expect it to close up again.

• Blocks every few weeks or months in the same place
• Slow drainage even when it is not fully blocked
• Gurgling from a toilet or bath when another appliance drains
• A persistent smell from an outside gully

The four causes we see most in Southampton

Fat, oil and grease is the biggest one. It goes down warm and liquid, then sets solid on the pipe wall as it cools, catching everything that follows. Kitchen waste pipes and the run out to the first inspection chamber are where it accumulates.

Wet wipes, cotton pads and sanitary items are second. Nothing labelled flushable actually breaks down like paper does, and one snagged wipe becomes a mat within weeks.

Root ingress is third, and very common in the older terraced and Victorian stock around Portswood, Shirley, Bevois Valley and St Denys. Those areas still have plenty of clay pipe laid in short sections with mortar joints. As the ground moves, joints open a few millimetres, roots find the moisture and grow into the pipe like a net.

Fourth is a structural defect: a cracked or displaced pipe, a collapsed section, or a bellied run where ground settlement has left a dip that holds standing water. Silt drops out in the dip and the pipe silts up from there.

Southampton specifics worth knowing

A lot of the city sits on clay and brickearth over gravel, which shrinks and swells with the seasons. That movement is what opens joints on older clay drainage in the first place, and it is why blockages often start after a very dry summer or a hard winter.

Some older parts of the network are combined, carrying both foul waste and rainwater. In heavy rain a combined system can surcharge, which shows up as water rising in a chamber or a toilet backing up during a downpour rather than in normal use. That pattern usually points at the shared or public sewer rather than your own pipework.

Responsibility matters for cost. Since 2011, shared drainage serving more than one property is generally the responsibility of the water company, in this area Southern Water, and a lateral drain beyond your boundary usually is too. The pipework inside your boundary serving only your house is yours. If a blockage is on a shared run, Southern Water will normally attend at no charge, so it is worth ringing them before booking a private job.

What to do yourself, and where to stop

Plenty of one-off blockages are genuinely fixable at home. Lift the inspection chamber lid nearest the problem first: if it is full, the blockage is downstream of it; if it is empty, the problem is between the house and that chamber. That single check saves a lot of guesswork.

Skip the caustic soda. It generates heat, can damage older pipework and plastic traps, and leaves an unpleasant hazard for whoever opens the drain next. Boiling water and washing-up liquid does more for a greasy kitchen waste than any supermarket bottle.

• Scrape fat and oil into the bin, never down the sink
• Fit a mesh strainer over the plug hole, hair is the main culprit in showers
• Bin all wipes, including the flushable ones
• Clear leaves out of gullies twice a year, autumn hits hard on tree-lined roads
• Run hot water through the kitchen sink for a minute after washing up

When a CCTV survey earns its money

If a drain has blocked three or more times in a year, stop paying to clear it and find out why. A CCTV survey pushes a camera through the line and shows exactly what is there and at what distance, so any repair is targeted rather than speculative.

As a rough guide, a straightforward clearance is typically in the region of 80 to 200 pounds depending on access and how long it takes, and a domestic CCTV survey with a report commonly lands somewhere around 150 to 350 pounds. Repairs vary enormously: a patch lining or root cutting on one joint is a different order of cost to excavating a collapsed section under a driveway. Anyone quoting a repair figure before seeing the footage is guessing.

A survey report is also useful evidence for a buildings insurance claim, as many policies cover accidental damage or collapse of underground pipes, and for the homebuyer paperwork if you are selling.