Leaking Drains and Subsidence: The London Cause Nobody Checks First

Two ways a drain moves a house

A foundation works by spreading the weight of the wall onto ground that is strong enough to carry it. Water from a broken drain attacks that arrangement from two directions, and which one applies depends on the soil and on where the pipe runs.

Softening. Water leaking steadily into the ground beside a footing raises the moisture content of the soil beneath it. Clay that was firm becomes soft; silty and sandy soils lose strength as they saturate. The ground can no longer support the load it was carrying and the footing settles into it. This happens whether the drain is above or below foundation level, because water finds its way down, and it is the usual mechanism in London's clay.

Washout. Where a drain runs close to and below the foundation, and the ground is sandy, silty, gravelly or made up of old fill, the flow reverses: groundwater and the fine fraction of the soil wash into the broken pipe and are carried away. The ground beneath the footing loses volume, voids open, and the wall drops into them. Clay is too impermeable for this to happen on any scale, so washout is the mechanism to suspect on river terrace gravels and on filled ground, and it is the one that can leave the foundation genuinely unsupported.

Two indirect effects are worth knowing. A leak into clay that a tree has kept persistently dry does not cause subsidence at all but heave, as the clay takes the water up and swells. And a leaking drain attracts roots: they do not break a sound pipe, but they enter an existing crack or open joint, grow, and enlarge it, so a small defect becomes a large one and the tree gets blamed for a problem the pipe started.

Why London drains leak

Most London houses were built before 1970, and their below-ground drainage is salt-glazed clay pipe in short lengths with rigid joints, sealed with cement mortar. Those joints have no give: the slight seasonal movement of clay soil, a root, a heavy vehicle over a shallow run or the settlement of an extension built over the line is enough to crack the mortar or the pipe, and once cracked they leak every time water passes. Drains laid since the 1970s have flexible joints and leak far less, but broken pipes, poor laying, displaced joints and damage from later building work are still common, so a newer house is not immune. Other routes by which water gets into the ground beside a footing: downpipes discharging onto the ground instead of into a gully, soakaways built too close to the house (Building Regulations guidance keeps them at least 5 m away), gullies blocked and overflowing, and the "French drain effect", where the granular backfill around a service trench carries water along it to somewhere it was never meant to go. Leaking water mains are a separate problem with the same result, and only the water company may repair them.

What drain-related subsidence looks like

Two features set it apart from the tree-and-clay pattern. It is localised: the movement sits at the corner or length of wall nearest a drain run, a manhole, a gully or a downpipe, which in a typical terrace or semi means the rear addition where the kitchen and bathroom are, or the front corner where the downpipe and the gully sit. And it is progressive: the wall goes down and stays down, sometimes faster after wet weather, whereas clay shrinkage around a tree takes a house down in summer and part of the way back up in winter. The cracks themselves are the usual tapered diagonals from corners and openings that our guide to the signs of subsidence describes; it is where they are and how they move that points at the pipe. Supporting clues: damp brickwork low on the wall, paving sinking beside the house, a manhole whose surround has dropped, or a patch of garden that is greener than the rest.

How the leak is found

The investigation is the same one we run for any subsidence, with the drainage brought forward.

• Desktop and inspection. Manhole and gully positions from the site and from old drawings, the age of the house, and a level survey along the brick courses near the damp-proof course, which shows where the low point is before anything is dug.
• CCTV survey. A camera run through the pipes from each manhole, recording cracks, displaced or open joints, root intrusion, breaks and bellies, with the distance to each. It commonly costs low hundreds of pounds and is the single most useful test in a drain case. It is worth knowing that many old drains show defects without causing any subsidence, so a cracked pipe on the film is a suspect, not a conviction.
• Water test. The suspect length is plugged and filled, and the drop in level over a set time gives the leakage rate, which is what turns "cracked" into "leaking enough to matter". A dye test can show where the water reappears.
• Trial pit. Our specialist site team excavates beside the affected footing for the engineer to record its depth and what it bears on, and to see whether the ground is wet and soft, or voided and loose, and which of the two mechanisms is at work. See our explainer on trial pits and opening-up works.
• Monitoring. Crack widths from fixed studs and level readings every six to eight weeks. Steady one-way movement confirms a drain or soft ground; a seasonal rhythm points back to a tree.

Fixing the drain, then waiting

Where the pipe is otherwise sound, a localised defect can be patch-lined from inside without excavation; a run with several defects can be relined along its full length with a cured-in-place liner; a collapsed or badly displaced run is dug up and relaid, with flexible joints, and the old line sealed. Roots are cut out first. Where the leaking run is shared with a neighbour, or lies beyond your boundary, it is usually the water company's pipe and its repair (see the responsibility question below). Drainage work is controlled under Part H of the Building Regulations and, where you are replacing or rerouting drains, Building Control will want to see it.

The repair stops the water. It does not immediately stop the movement. Ground that has been softened has to dry out and consolidate again under the weight of the wall, and that can take up to a year, during which the readings keep moving in the same direction at a slowing rate. So the sequence is: repair the drain, keep monitoring, and only when the readings have gone flat carry out the crack stitching, repointing and redecoration. Doing the cosmetic work straight after the drain works is the commonest way to pay for it twice.

Where the trial pit found voids or loosened ground beneath the footing, repairing the pipe is not enough on its own, because the ground will go on settling into the gaps. The options are pressure grouting of the loosened zone, which suits granular ground and hardcore, or underpinning of the affected length, usually partial. That is the minority of drain cases; most are solved by the pipe and patience.

Who pays

The insurance position has three parts, and the wording of your own policy governs each. Damage to the house caused by ground movement from a leaking drain is subsidence and is claimed as such, with the usual excess of around £1,000; some insurers handle it under the escape of water section instead. The drain itself is different: replacing drains that have simply worn out is the owner's cost and not an insured loss, but drains damaged by something external, roots, ground movement, a vehicle over a shallow run, are commonly covered under the accidental damage to underground services section with its own, smaller excess, and insurers generally pay for the drain repairs that are needed to stabilise a building under an accepted subsidence claim. Neglect matters: a drain known to be leaking and left can reduce what the insurer pays, because maintaining the property is a condition of cover. Our guide to subsidence insurance claims covers the process in full, and our repair cost guide puts figures on each stage.

Before it happens

A few habits prevent most drain-related movement. Have a CCTV survey done when you buy a pre-1970 house, and again before an extension is built over or near a drain run, because an extension's foundations are the commonest thing to break one. Keep downpipes connected to gullies and gullies clear, and never let a downpipe discharge onto the ground beside a wall. Keep soakaways well away from the building. Cut roots out of a drain and reline it as soon as they appear, rather than rodding it clear every year. And if paving or a manhole surround beside the house starts to sink, treat it as a drain question before it becomes a foundation one.

Where EMA Structures fits

We investigate drain-related movement as part of a subsidence assessment, £495 to £895 + VAT depending on the size of the property, which covers the inspection, a record of every crack, a level survey to find the low point, a diagnosis and a written report. The CCTV survey and water test, carried out through a drainage specialist, trial pits by our own site team, and any crack and level monitoring (£1,200 to £2,000 + VAT for the set-up and three seasonal visits) are not included in that fee; each is quoted separately once the assessment shows it is needed, so that the diagnosis rests on evidence rather than on the nearest tree. Where the answer is the pipe, we say so and time the repairs correctly. Where it is grouting or underpinning, we design it, deal with Building Control, carry it out or oversee it, and certify it. Send us photographs of the cracks and a note of where the drains and downpipes are, and we will give you an honest first view.

General guidance on UK practice as at September 2026. Insurance cover, excesses and drain ownership vary by policy and by property and should be checked with your insurer and water company. Fees exclude VAT.