Shrubs, Climbers and Leaking Drains: The Subsidence Causes That Are Not the Tree

When a crack opens up and somebody says the word subsidence, the first thing almost everyone looks at is the nearest tree.

It is a reasonable instinct. Trees are the single biggest cause. But they are not the only one, and on the surveys we attend the actual culprit is often something far less dramatic sitting much closer to the wall — a shrub that has been there for thirty years, a downpipe with a weeping joint, or a drain nobody has lifted the cover on since the house was bought.

What actually causes subsidence?

Subsidence is the downward movement of the ground beneath a building’s foundations, where that movement is not caused by the weight of the building itself. It becomes a structural problem when it happens unevenly — when one part of a footing loses support while the rest keeps it.

Zurich’s own breakdown of valid subsidence claims in a typical year is a useful picture of where the causes sit:

• Root-induced clay shrinkage — about 60%. Vegetation drawing moisture out of a shrinkable clay soil, which then contracts.
• Leaking drains and mains water pipes — about 18%. Water escaping below ground beside the foundation.
• Poor ground, infill and consolidation — about 18%. Made ground, old fill, or soil that was never properly compacted.
• Other causes — about 4%. Heave, landslip, sinkholes and historic mining.

Read that second line again. Close to one in five cases is water escaping from something that could have been found by lifting a cover — and nothing about that shows up when you stand in the garden looking at a tree.

Leaking drains: the cause people check last

A leaking drain damages the ground in one of two opposite ways, and which one you get depends entirely on what the water is escaping into.

In a granular soil — sand, gravel, or a made-up fill — escaping water washes the fine material out from between the coarser particles and carries it away down the drain run. What is left behind is a looser, more open soil, and sometimes an actual void. The ground above settles into it.

In a clay soil, the mechanism reverses. Clay swells when it takes up water, so a persistent leak lifts one patch of ground rather than dropping it. That is heave rather than subsidence, and it cracks a building just as effectively, just in the opposite direction.

Either way, the point is the same: one length of foundation stops behaving like the length next to it. Uneven support is what cracks buildings. Uniform settlement, by contrast, is usually harmless — a whole building can sit down by a few millimetres and show nothing at all.

Drains are checked last because they are inconvenient. The covers are often under gravel, planting or a patio, and lifting them is a job nobody volunteers for. But a drainage inspection chamber takes about thirty seconds to open, and what you can see down it — standing water, a partial blockage, roots coming through a joint — frequently answers the question before anyone has measured a crack.

Gutters and downpipes do the same damage above ground

Rainwater goods are the other half of the same story, and they are far easier to inspect because you can do it from the garden in the rain.

A gutter that overflows, or a downpipe joint that weeps, delivers a concentrated stream of water to one small patch of wall and one small patch of ground, repeatedly, for years. In the wall, that keeps masonry permanently wet, which lets frost break up the face and soluble salts crumble it from within. In the ground, it softens a single spot right beside the footing while everything either side stays normal.

The repair is usually a bracket, a seal, or an afternoon with a ladder clearing leaves. What it prevents is considerably more expensive. If you do only one thing after reading this, go and look at your downpipes during heavy rain.

Shrubs and hedges against the wall

A mature shrub planted hard against a house is doing a smaller version of what a tree does, but much closer to the foundation.

The mechanism is the same as the clay shrinkage figure above: the plant draws moisture out of the soil through the growing season, the clay contracts as it dries, and the ground under that stretch of footing drops. A shrub takes less water than an oak, but it is frequently within half a metre of the wall rather than ten metres away, and shallow foundations on an older property do not need much.

Three things make a shrub more of a risk than its size suggests:

• Shrinkable clay. On sand or gravel, moisture change barely moves the ground. On a firm shrinkable clay, it moves it a great deal. Soil type decides whether planting matters at all.
• Foundation depth. Victorian and earlier buildings often sit on footings a few courses deep. Modern foundations near vegetation are deliberately taken much deeper, which is why new houses rarely have this problem and old ones do.
• What it hides. A dense shrub against a wall conceals the base of the wall, the air bricks, the ground level and — very often — the gully the downpipe discharges into. You cannot inspect what you cannot see.

The instinct to rip everything out is usually wrong, incidentally. On clay, removing a long-established plant can cause the soil to recover moisture and swell, which produces heave — and heave is generally more damaging and more expensive to deal with than the slow shrinkage it replaced. Established planting near a building on clay is a decision worth taking advice on rather than a weekend job.

Climbers: what the research actually found

Ivy, wisteria and Virginia creeper attract more confident opinions than almost anything else in this field, and most of them are wrong in one direction or the other.

Historic England ran a multi-year study on ivy growing on walls, and its findings are more interesting than either camp expects. Ivy cannot force its way into sound masonry. The aerial rootlets grip the surface and were not found to penetrate stone or mortar at all — the plant has no mechanism for boring into a wall that is in good condition.

What it does instead is exploit defects that already exist. Stems work into recessed or decayed joints, behind loose pointing, and into open cracks. The case that genuinely matters is a stem that finds its way into a crack or a cavity while it is thin and flexible, and then thickens there over a decade or two. A woody stem expanding inside a joint can loosen blocks and lift copings, and that is real structural damage.

The same research also found ivy can be actively protective. A cover buffers temperature and humidity swings, reduces the frequency and severity of frost cycles, traps airborne pollutants, and shelters the wall from driving rain. Monitoring at a test wall showed no increase in weathering beneath the ivy over several years.

Two practical points follow from that. The first is that cutting ivy off at the base is no longer automatically the right move: the study warned it can prompt stems already sitting in cavities to root in properly, which is the most destructive behaviour of all. The second is the one rule that is not negotiable — keep it off roofs and out of gutters. A climber in the gutter line causes overflows, and overflows bring you straight back to the section above.

What we look at on site

A subsidence survey that only examines the crack has not really started yet. The crack is the symptom, and reading it well narrows the field — our guide to how to read a crack in a wall covers what the shape and direction suggest — but establishing the cause means looking at the things around the building.

In practice that means the drainage runs and inspection chambers, the gutters and downpipes and where they discharge, the ground levels against the wall, the planting and its distance from the foundation, the soil type, and the history of what has been built, removed or altered nearby. Where the cause is still not clear, a trial pit establishes what the foundations actually are and what they are bearing on, and where the structure itself needs opening up, a structural investigation takes the finishes off.

Where a tree is genuinely implicated, the distances and species matter a great deal, and trees and subsidence: how close is too close goes through that in detail.

When it turns out not to be subsidence at all

A good proportion of the cracks we are called to are not subsidence, which is usually the best news anyone gets that week.

Thermal movement, drying shrinkage in newer plaster, a lintel that was never adequate, sulfate attack in old mortar, wall tie failure in a cavity wall, and straightforward seasonal movement all produce cracking that can look alarming. So does a building that settled decades ago, stopped, and has been stable ever since. Telling live movement from historic movement is one of the more valuable things an engineer does, because the answer decides whether you need underpinning or a bag of filler.

Frequently asked questions

Can a shrub cause subsidence?

Yes, on a shrinkable clay soil and where foundations are shallow. A shrub takes less moisture from the ground than a tree, but it is usually far closer to the wall, and older properties often have footings only a few courses deep. On sand or gravel soils, planting has very little effect.

Do leaking drains cause subsidence?

They are a leading cause — Zurich attributes roughly 18% of valid subsidence claims in a typical year to leaking drains and mains water pipes. Water escaping into granular soil washes out fine material and leaves voids; water escaping into clay makes it swell, producing heave instead.

Does ivy damage walls?

Not sound ones. Historic England’s research found ivy cannot penetrate intact stone or mortar, and that it can even protect a wall by reducing frost cycling and sheltering it from driving rain. It becomes damaging where it exploits existing cracks, cavities and decayed joints, and where stems thicken inside them over years.

Should I pull the ivy off my house?

Not reflexively. Cutting stems at the base can prompt stems already in cavities to root in, which is worse than leaving them. Removal should be a considered decision based on the condition of the wall behind. Keeping it out of gutters and off the roof, however, is always correct.

Should I remove a shrub growing against my wall?

Get advice first if you are on clay. Removing long-established planting lets the soil recover moisture and swell, which can cause heave — often more damaging than the shrinkage it replaces. Managing the plant’s size is frequently the better route.

How do I know whether the crack is still moving?

Crack monitoring over a period of months settles it where the evidence on site is ambiguous. Much of the time, though, the condition of the joint behind the finish and the pattern of the cracking are enough for an engineer to reach a view on the first visit.

What to do next

If a crack has appeared, start with the free checks. Look at your downpipes while it is raining. Lift the drain covers you can get to. Look at what is planted against the wall and how close it is. That is twenty minutes of work and it very often points straight at the answer.

If your insurer has asked for a report, or the crack has come back after being filled, the next step is a survey that establishes the cause rather than describing the symptom — and, where remedial work follows, a certificate of structural adequacy once it is complete.

Ora Labora Structural Engineers carry out subsidence surveys, structural investigations and structural reports for insurance companies as structural engineers in Peterborough and structural engineers in London. Call +44 1733 602 844 or book a consultation and we will come and find the cause.

Sources

Subsidence claim causes: Zurich, An in-depth look at subsidence. Ivy research: Historic England, Ivy on Walls.