The cause
Why foundations move
The house follows the ground, and the ground follows water. Uneven moisture under the building is behind most movement.
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Written by HyreFoundation Research Desk Primary-source research and fact checking
The short answer
Foundations move because the soil under them moves, and soil moves with water.
Expansive clay swells when wet and shrinks when dry, so symptoms often appear in a dry late summer and ease in a wet spring.
The four main causes are expansive clay, settlement or heave, poor drainage, and trees or drought drying the soil unevenly. Uneven moisture does the damage.
Key takeaways
— one is the structure going down, the other is soil pushing part of it up — and they crack a wall in broadly similar ways.
The actual multiplier is 0.2× to 1.25× of mature height, depending on species and foundation depth.
as the soil it was drying rehydrates and swells. That is a distinct failure mode from the one everyone worries about.
We do not publish it; the FAQ explains why.
How to find out what is under your own house
Most of this is free and none of it requires a contractor.
Free, official, and works by address. Define your property, then check shrink-swell potential and linear extensibility. Professionals use the same data, mapped at roughly 1:24,000. Then run our tree distance check.
It is real, from 1989, at a scale of 1:7,500,000, and it classifies by bedrock rather than soil survey data. It is a national overview, and using it to judge one address is a misuse.
Whether symptoms track the seasons is one of the most informative things available, and only you can observe it. Cracks that open in a dry August and close in a wet April behave like clay cycling, not like progressive failure.
A tree removed or planted, a new patio, altered irrigation, a repaired main, excavation next door. Foundations respond to changes in water, and changes in water are usually recent and usually human.
Fill placed before modern compaction standards, a post-tension slab, a pier-and-beam house with a crawl space — each behaves differently and each has its own characteristic problems.
A leaking supply or drain line under a slab introduces water exactly where it does most harm, and it is both a common cause and a commonly missed one.
Trees and foundations, with the actual numbers
Britain codified this because London clay forced the issue. NHBC Standards Chapter 4.2, the mandatory warranty standard for UK new homes, quantifies building near trees. The IRC has no tree provisions at all. What follows is UK guidance, and the best-evidenced material available.
"Trees, hedgerows and shrubs take moisture from the ground and, in cohesive soils such as clay, this can cause significant volume changes resulting in ground movement." Note the condition: in cohesive soils. On sand or gravel this is largely a non-problem.
NHBC classes all elms, eucalyptus, hawthorn, oaks, poplars and willows as high water demand. Other broadleaf species are moderate; birch and hazel are low. The standard instructs that where the species has not been identified, high water demand should be assumed — which is also sound advice for a homeowner.
The no-planting zone is a multiple of the tree’s mature height: high water demand 1.0× to 1.25×; moderate 0.5× to 0.75×; low 0.2× to 0.5×.
The higher figure applies to shallower foundations. So "as far away as it will grow tall" is the high-demand case: right for an oak, about double what a moderate species needs.
NHBC sorts by Modified Plasticity Index: 40% and above is high volume change potential, 20% to under 40% is medium, 10% to under 20% is low.
Shrinkable soil is defined as over 35% fine particles with a Modified Plasticity Index of 10% or more. Where the potential is unknown, the standard says assume high.
Stated plainly in the standard: "Root barriers are not an acceptable alternative to the guidance given." Worth knowing before buying one as a fix.
It says the depths given "are not those at which root activity, desiccation and ground movement are non-existent, but they are intended to provide an acceptable level of risk", and that beyond 2.5 m "conditions exist where prescriptive guidance is less reliable". That is the register the whole subject deserves.
The four things actually moving your house
Four causes account for most residential foundation movement. Cracks are the effect, and the same effect can come from any of them.
| Option | What it means for you |
|---|---|
| Expansive clay | Certain clays change volume with moisture content, swelling when wet and shrinking when dry, cycling seasonally. The measured property is linear extensibility — the reversible volume change between field capacity and oven dryness — and it is mapped nationally at address level in the USDA soil survey. |
| Settlement vs heave | Settlement is the structure going down, into poorly compacted fill, soil that has consolidated, a void from erosion, or clay that has shrunk. Heave is soil swelling and pushing part of the structure up. They are corrected in opposite directions, so the distinction has to be made before anything is installed. |
| Water management | Ground sloping toward the house, downspouts discharging at the wall, irrigation against the foundation, beds that hold water, paving pitched the wrong way. The most common contributing cause, and by far the cheapest to correct — see drainage and grading. |
| Vegetation and drought cycles | Trees and large shrubs extract moisture from soil, and in clay that means localized shrinkage. Extended dry periods followed by heavy rain produce the shrink-swell cycle that does the damage. Removing a mature tree reverses the process and can cause heave. |
Less common but real: plumbing leaks under a slab, mine subsidence, sinkholes in karst terrain, land subsidence from groundwater withdrawal, landslide movement on hillsides, frost heave, and sulfate heave where lime stabilization has reacted with sulfate-bearing soil. These are geographically concentrated, and a local engineer will know whether yours is one of them.
About that statistic every foundation company quotes.
The claim that "expansive soils cause $2.3 billion of damage a year" comes from Jones and Holtz, Expansive Soils — The Hidden Disaster, a three-page article in ASCE’s Civil Engineering magazine in 1973.
It is in 1973 dollars, with no retrievable method. We publish none of the national figures; the FAQ traces the rest.
What to ask before you sign
The full checklist is on choosing a foundation contractor.
Where this goes wrong
Enormous numbers of houses sit on expansive clay and never develop a structural problem, because the moisture around them stays reasonably even. The soil is the precondition; the water management is usually what decides the outcome.
A mature tree that has been drying clay for forty years has established an equilibrium. Removing it lets the soil rehydrate and swell, and heave after tree removal is a well-recognized failure mode. The British standard has a whole clause on precautions for it.
National damage totals, even good ones, say nothing about a particular house. A 53-year-old one says less.
Concrete shrinks as it cures. Fine cracking in the first year or two is expected and generally unrelated to soil movement.
Soaking one side of a house through a drought while the other bakes manufactures precisely the differential movement that damages a structure. Consistency is the objective, not saturation.
Small-scale geological mapping is for orientation. What is under one corner of one lot is frequently not what is under the other corner, which is why engineers measure rather than look things up.
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Three short steps: the job, your home, and how to reach you. Nothing goes to a foundation company unless you agree.
A straight answer first. Matching is still being built, so this is not an instant quote. If a wall is moving or a door frame has racked out of square this week, call a structural engineer first.
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Questions
How far should a tree be from a house foundation?
Is there an American standard for trees near foundations?
Should I cut down the tree near my foundation?
What is the difference between settlement and heave?
Do expansive soils cause $2.3 billion of damage a year?
Which states have the worst expansive soil?
Can a plumbing leak cause foundation movement?
Written and audited by
HyreFoundation Research Desk
Primary-source research and fact checking
We read the model building code, the federal soil and moisture guidance and the insurer position ourselves, and we publish each figure with the document it came from and the date we retrieved it.
Where a number cannot be traced to a primary source, we say so instead of printing it. HyreFoundation does not inspect, engineer or repair foundations. Authorship is organizational: this desk, not a named persona.
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How this desk works
- Primary sources only. Code statements come from the International Residential Code, Chapter 4, as the model code. Soil statements come from USDA and USGS. Insurance statements come from the policy language and the Insurance Information Institute. We open the source rather than cite a blog that cites it.
- The code is a model, and adoption is local. Your jurisdiction adopts an edition and amends it, and foundation chapters are amended more than most because frost depth and soil are local facts. Every code figure says so.
- We hold no job data. We have no dataset of bids, pier counts or invoices, so no page claims an average from one. Calculators show the arithmetic and the inputs they assumed, and label the result as an estimate.
- We do not diagnose your house. A crack guide can tell you which pattern to measure and when to call a structural engineer. It cannot tell you whether your foundation is failing. An engineer on site does that work.
- We do not repair foundations, and we take no payment for placement, ranking or a favorable mention. Nobody buys a position on this site.
Authorship on this site is organizational: the analysis belongs to the desk rather than to a named individual, and we do not publish credentials we do not hold. Our editorial policy sets out how we source, date and correct what we publish.
Sources & retrieval dates
Information on this page is general and varies with soil, climate, foundation type, building code, insurance policy and circumstance.
It is not engineering, insurance or legal advice, and no page can tell you whether your house has a structural problem: that needs a licensed professional on site.
HyreFoundation is an independent foundation resource, not a repair contractor or an engineering firm, and does not perform, supervise or warrant foundation work.