“The slab settled.” It is the most common sentence in a concrete-floor conversation, and it carries more assumptions than it sounds like. Settlement describes one direction of one kind of movement: the ground under part of the slab went down. Soil can also push a slab up, shrink away from it in a dry season, or creep sideways near the top of a slope. And some slab cracks involve no soil movement at all.
Those differences matter because the explanation decides the repair conversation. If a high spot is read as a low spot, or a sideways problem is treated as a vertical one, the work can end up chasing the wrong thing.
Our slab foundation repair page covers the service side: evaluation steps, slab types, and the repair families you will hear about. This post is narrower. It is a vocabulary guide for sorting what moved, and in which direction, before anyone proposes lifting, injecting, or piering.
Settlement is a kind of soil movement. Soil movement is not always settlement. And a cracked slab is not always proof that the soil moved.
First, what a slab-on-ground is supposed to sit on
A slab-on-ground floor is a concrete floor poured directly over prepared soil, usually with thickened edges or footings at the perimeter. For new construction, the California Residential Code (Section R506) sets the baseline: a minimum thickness of three and a half inches, vegetation and topsoil removed, and any fill under the slab compacted “to ensure uniform support of the slab,” with limits on how deep that fill can go unless approved. A base course of clean sand, gravel, or crushed rock usually sits on top of the prepared subgrade.
That phrase, uniform support, is the whole idea. A slab is stiff, but it is designed on the assumption that the ground beneath it behaves roughly the same from one side to the other. When a slab is in distress, the useful question is where that assumption failed, and in which direction.
Two quick caveats. Those code provisions describe how new slabs are built; they are not a pass/fail test for a slab poured decades ago. And slabs designed for expansive soil are meant to tolerate some movement; the California Building Code points designers to recognized standards for both conventionally reinforced and post-tensioned slabs (the latter use steel tendons stressed after the concrete cures). Knowing which kind you have becomes important later, and the companion post on concrete leveling vs structural repair explains why.
The four directions a slab problem can come from
It helps to sort slab complaints into four buckets. Real houses often have more than one going at once.
1. Down: settlement and loss of support
This is the bucket most people mean by “settled.” Part of the slab drops because the soil under it compressed, shrank, or washed away. Common mechanisms include:
- Fill that was never compacted well, or soft soil that slowly consolidates under load
- Water carrying fines away, for example from a leaking under-slab pipe or roof runoff concentrated at one edge
- Drying shrinkage of clay soil, where a dry spell or a removed lawn lets an expansive soil lose moisture and volume
The result tends to be a low area or a tilted section, cracks that show vertical offset (one side higher than the other), and gaps opening at walls, cabinets, or door frames on the low side. Our foundation settlement repair page covers how that problem is evaluated as a repair project.
2. Up: heave
Clay-rich, expansive soils swell when they take on water. The California Geotechnical Engineering Association’s homeowner guide on expansive soil describes forces large enough “to lift buildings and crack concrete.” Water sources are usually ordinary: irrigation, a planter against the foundation, a slow leak, poor drainage after a rain.
Heave can produce a high spot or a crown in the middle of a room, doors that bind in the wet season, and cracks that open and close as moisture changes. Here is the catch: when one part of a slab rises, the part that did not move feels “low” by comparison. A homeowner standing on the unchanged side often concludes that side sank. The geotechnical guide makes the same point about slopes, noting that a smaller amount of heave along the top of a slope “is often mistaken as settlement.”
3. Sideways: lateral movement near slopes
This bucket matters on La Jolla’s canyon-rim and hillside lots, though only a parcel review can say whether it applies to yours. The same CalGeo guide describes two mechanisms:
- Lateral fill extension: on graded slopes with expansive soil, wetting makes the soil expand toward the direction of least resistance. Away from the slope that is mostly upward; close to the slope it is largely outward toward the slope face.
- Slope creep: repeated wetting and drying lets near-surface soil on and near the slope face creep slowly downhill under its own weight.
The clues look different from vertical settlement. Patio slabs pull away from the house. Joints in side-yard walls widen. Posts that tie a patio cover to the house start to lean. Pool decks open gaps at the coping. The guide notes that telling creep from lateral fill extension “is sometimes difficult” even for professionals. A vertical lift does not address sideways movement, which is one reason this bucket deserves its own name. Our hillside foundation repair page covers slope-related foundation work, and the Mount Soledad hillside considerations post covers lot-level context.
4. Not soil at all: concrete behaving like concrete
Concrete shrinks as it cures and dries. Fine, random “map” cracking and straight hairline cracks with no offset across them are very often shrinkage, not ground movement. Slabs can also curl, with edges and corners lifting slightly relative to the center because the top and bottom of the slab dry or change temperature at different rates. Control joints are cut into slabs precisely so that shrinkage cracking happens in a line someone chose.
None of that rules out soil movement. A shrinkage crack can later become the path where movement shows up. But a hairline crack in a garage slab is not, by itself, evidence that the ground went anywhere. Our earlier post on hairline vs structural foundation cracks goes deeper into crack reading, and the foundation cracks page covers the symptom side.
Why the direction question matters
Here is a simplified example of how the vocabulary changes the conversation.
A living room slab has a noticeable low corner near the back door. One reading: the corner settled because a downspout has been dumping water there for years and the soil lost support. Another reading: the center of the house heaved after a planter bed was installed against the opposite wall, and the “low” corner is closer to where the slab started. A third: the house is near a slope, the patio outside the back door is moving outward, and the corner shows distress from lateral pull at the connection rather than a drop.
All three can produce the same complaint underfoot. Each points to a different investigation and a different set of possible responses. Lifting the low corner makes sense in only one of them, and even then only after the water source is dealt with.
This is why a good evaluation tends to start with elevations taken across the whole floor, not just at the spot that bothers you. A relative elevation survey shows the shape of the slab: a dish, a dome, a one-way tilt, a step. It does not prove mechanism by itself, but it keeps everyone from arguing about one corner.
Clues that lean toward each bucket
None of these is a diagnosis. They are prompts for better questions.
| What you notice | Leans toward | Still check |
|---|---|---|
| Low area near a leak, downspout, or wet edge | Settlement / loss of support | Whether the water source is still active |
| High spot or crown mid-room; doors bind in wet months | Heave | Irrigation, planters, plumbing history |
| Cracks that open and close with the seasons | Moisture-driven volume change (heave or shrink) | Pattern over at least a full wet/dry cycle |
| Patio or flatwork pulling away from the house on a slope-side lot | Lateral movement or creep | Slope distance, retaining walls, parcel hazard maps |
| Fine random cracks, no offset, no change over years | Concrete shrinkage | Whether new cracks with offset appear later |
| Edges slightly higher than center; joints with small lip | Curling or edge heave | Age of slab, exposure, moisture |
Dated photos with a ruler in frame, notes on when doors bind, and a record of irrigation or plumbing changes are more useful than any one measurement. For whole-house slope patterns, our sloping floors causes checklist walks through cause classes across both slab and raised homes, and sinking foundation early signs covers how progressive signs tend to cluster.
What the City of San Diego looks at
La Jolla is part of the City of San Diego, so City Development Services rules apply on City parcels. Two documents explain why soils come up early in slab work:
- Information Bulletin 141 requires a preliminary or as-graded geotechnical report from a registered design professional for buildings supported on fill or on expansive soil as defined in the California Residential Code. Where a presumptive soil bearing value is used instead, the plan note acknowledges that if the building inspector observes or suspects fill or expansive soil, a geotechnical report may still be required.
- The City Seismic Safety Study maps geologic hazard categories that can trigger geotechnical requirements for specific parcels (San Diego Municipal Code Table 145.1803). A mapped category on your lot is a reason to look, not a diagnosis of your slab, and ordinary slab distress should not be blamed on the Rose Canyon Fault without evidence.
The CalGeo guide also suggests checking home buyer disclosures, the building department, or a geotechnical professional to learn whether a property has expansive soil. Disclosures and old soils reports from the original construction are worth digging out before an evaluation.
Structural repair to a slab or its footings generally needs a City Building Permit (SDMC §129.0202). Replacing under-slab plumbing generally needs a separate plumbing permit, with narrow exceptions such as stopping a leak without replacing pipe. Structures 45 years or older often add historic review when exterior work is permitted, and Coastal Overlay or steep-hillside rules can add layers on some parcels.
Who sorts this out
- You can document pattern, timing, water sources, and seasonal behavior.
- A home inspector can report visible indications of possible movement. That is a visual screen, not an engineering opinion. A foundation inspection focused on the structure goes further.
- A geotechnical engineer or engineering geologist is the right professional when the question is what the soil is doing and why: expansive soil, fill, slope movement, or a hazard category.
- A California-licensed civil or structural engineer enters when the load path, slab system, or stamped repair plans are involved.
- A licensed contractor with the right classification builds permitted work once the scope is clear.
La Jolla Structural evaluates slab movement questions for homeowners across La Jolla and we coordinate with licensed engineers as needed. We do not publish invented license numbers, warranties, or stamped-plan claims on this site.
Cost factors, not prices
We do not publish repair prices. The factors that shape cost on a slab question include whether the slab is conventional or post-tensioned, how much flooring and cabinetry must be removed for access, what investigation is needed (elevation survey, plumbing testing, soils work), which direction the movement is coming from, whether drainage or plumbing must be fixed first, and which permits or parcel overlays apply. Our post on what drives foundation repair cost covers those factors in more detail.
A practical next step
If your slab has a low spot, a crown, cracks with offset, or flatwork pulling away from the house, write down what changed and when, and note any leaks, irrigation changes, or nearby grading. Then get the floor measured as a whole before anyone names the cause.
Request a Free Quote for an on-site evaluation, or call (650) 739-6537 to talk through what you are seeing. Once the direction of movement is clearer, the companion post on concrete leveling vs structural repair explains what lifting can and cannot do.
Sources (accessed October 5, 2026)
- California Residential Code (2022), Section R506 Concrete Floors (on Ground), ICC Digital Codes
- California Geotechnical Engineering Association, Coexisting with Expansive Soil: An Informational Guide for Homeowners
- City of San Diego Information Bulletin 141, Soil Bearing Capacity
- City of San Diego Seismic Safety Study; San Diego Municipal Code §129.0202, Table 145.1803
- Post-Tensioning Institute, Slab-on-Ground (PTI DC10.5 design standard)