Blocked or broken? How to tell the difference before you pay twice

The two problems look identical from above

A drain that will not take water looks the same whether the cause is a mass of wipes sitting at a bend or a section of clay pipe that has cracked and dropped out of line. Both give you standing water, slow fittings and a smell. The difference only shows once you know where to look, and it matters because the two need completely different work.

A blockage is material in an otherwise sound pipe. Remove the material and the drain works again. A structural fault is the pipe itself failing, and no amount of clearing changes it. You can jet a collapsed drain clear and it will collect the same debris within weeks, because the ledge that caught it is still there.

The single most useful clue: how often

If a drain has blocked once in fifteen years, it is almost certainly a blockage. Something unusual went down it, or a slow build-up finally closed. Clear it and move on.

If it has blocked twice in a year, treat that as a structural problem until proven otherwise. Repeat blockages in the same place are the clearest signal there is that something physical is catching material: a displaced joint, a partial collapse, roots through a crack, or a section laid with too little fall.

The second clearance is the moment to stop paying for symptoms. A CCTV drain survey costs less than three more clearances and tells you whether there is anything to repair.

Other signals worth noting

• A sinkhole or soft patch in the garden or drive along the line of a drain. Soil is being washed into a broken pipe.
• Unusually lush growth directly over the run. Roots have found moisture, which means the pipe is leaking.
• Damp at low level inside, particularly on a wall near a drain, often mistaken for rising damp.
• Rats indoors near ground level. They get in through broken pipework more often than people realise.
• The drain clears easily but returns fast. Classic sign that the obstruction was material caught on a defect rather than the defect itself.

What London’s housing stock makes more likely

Most of the capital drains through vitrified clay laid in short jointed lengths. Those joints are the weak point, and a century and a half of ground movement has opened plenty of them. In the terraced streets of Wandsworth, Fulham and Balham, root ingress at clay joints is the single most common finding on camera.

South London adds clay subsoil, which swells and shrinks with the seasons and gradually pulls rigid pipework apart. That is why Dulwich and Wimbledon produce so many displaced joints, often under mature gardens where nobody wants to dig.

What each answer costs you

Clearing an outside drain is a fixed £144 plus VAT. A survey is £300 plus VAT including the footage and a written report. A repair is quoted once the survey shows what is there.

Put like that, the arithmetic is straightforward. If you are on your second or third clearance for the same drain, you have already spent survey money without learning anything. See our full price list for what each job costs.

If the fault turns out to sit beyond your boundary, it is usually the water company’s to repair rather than yours, which is another reason to establish the location before anyone starts work.