Short answer
Settlement is the soil under a slab moving down, from load, erosion or drying clay. Heave is soil pushing up, usually when expansive clay gets wetter. Both crack slabs and bind doors, so symptoms alone rarely tell them apart. An engineer's elevation survey, ideally compared with an earlier one, shows which areas went up and which went down.
Key facts
- FPA defines heave as upward soil movement, usually from water added to dry expansive clay in the active zone, and says it normally occurs only in clay with high swell potential and a moisture source.[1]
- FPA's repair committee says common Houston-area repair piers are not tied to the grade beams and cannot prevent uplift from heaving soils; heave is not generally considered a pier failure.[2]
- NAHB guidance says most foundations are designed for up to 1 inch of soil movement, and slabs on highly expansive soil for up to about 4 inches.[3]
Three words engineers use
Most people say "settling" for any foundation movement. Engineers split it into three, because each has a different cause[1].
| Term | Direction | FPA definition (summarized)[1] | Common triggers |
|---|---|---|---|
| Settlement | Down | Soil moves down because the load above exceeds what the soil can bear. Includes compression of soil, slope movement, erosion and long-term consolidation. Can happen in any soil. | Poor or loose fill, soil washed out by water, weak soil |
| Subsidence | Down | Soil moves down because it loses moisture and shrinks. Usually clay. | Large trees, drought |
| Heave | Up | Soil moves up because water is added to dry expansive clay in the active zone. Normally only in clay with high swell potential and a moisture source. | Plumbing leaks, poor drainage, wet seasons, removed trees |
FPA calls heave "the reverse process of Subsidence"[1]. The active zone is the depth where soil moisture still changes with the seasons.
Why the difference changes the repair
Piers are built to stop a slab from going down. They are not built to stop it from going up.
FPA's repair committee says the repair piers most used on Houston-area homes "are not tied to the grade beams, and thus cannot prevent foundation uplift due to heaving soils." It adds that heave after repair "is not generally considered to be a failure of the piers"[2]. So if a slab is heaving and someone sells you piers, the heave can continue after the bill is paid.
Leveling also works differently. Most repairs lift the low areas up to match the high ones. FPA says lowering high areas is possible only "in limited circumstances"[2]. If the problem is a dome in the middle from heave, leveling means raising everything else up to that dome. That may not be the right fix at all.
When heave is the concern, FPA describes a different design: lift the whole ground floor above the soil's maximum potential rise, leave a void for the soil to swell into, and use piers deep enough with enough tension capacity to resist the uplift[2]. That is an engineered project, not a standard pier job.
The first fix for heave is usually water: find the leak, fix the drainage, or deal with the source. See slab leaks and foundation damage.
Clues that lean one way
These are clues, not a diagnosis. Use them to ask better questions.
| You notice | Leans toward | Why |
|---|---|---|
| A high spot near a bathroom or kitchen, with a big water bill | Heave from a leak | A leak adds water to clay in one area[1] |
| Edges or corners dropping after a long dry summer, near large trees | Subsidence | Trees and drought dry out clay[1] |
| A rise near where a large tree was cut down | Heave | FPA notes severed roots can let soil regain moisture and cause local heave[2] |
| A drop over an old trench, fill or a washout | Settlement | Loose or eroded soil compresses[1] |
| Cracks that open and close with the seasons | Seasonal clay movement | FPA lists seasonal moisture change as normal movement to consider[1] |
| A house in its first year or two | Early adjustment | FPA says most go through a 12 to 24 month adjustment period as landscaping and watering get established[1] |
Why one survey cannot settle it
A single set of floor readings shows where the slab is high and low today. It does not show which way anything moved. A high center could be a heaved middle, or a center that stayed put while the edges dropped.
FPA's method answers this with time-change elevations: two surveys on different dates, compared at the same points[1]. Without an earlier survey, an engineer has to assume the slab was built level, and FPA cautions that new foundations "are rarely level and often move during construction"[1]. FPA also notes the most accurate way to judge direction of movement is to measure against a fixed benchmark[1], since the reference point inside the house can move too.
If you are buying a slab house, a baseline survey now makes this question answerable later. FPA recommends it[1]. See elevation surveys and floor level readings.
Some movement is designed in
Not every bump is a failure. NAHB guidance says most foundations are designed for up to 1 inch of soil movement, and slabs on highly expansive soil can be designed for up to about 4 inches[3]. NAHB also says cracks alone are not necessarily a structural concern; they matter more with sticking doors and windows or steep floor slopes[3]. The question an engineer answers is whether movement is beyond what the slab can handle.
When to call a pro
- Floors are rising in one area, especially near plumbing: call a licensed plumber for a leak test, then a structural engineer.
- A repair company recommends piers but no one has said whether the slab is heaving or settling: ask a structural engineer before you sign.
- You have had piers installed and the slab is still moving up: ask an engineer to check for heave and water sources.
- You plan to remove a large tree near the slab: ask an engineer or arborist first.
More questions
Can foundation piers cause heave?
They can contribute. FPA notes that gaps around a drilled pier shaft can let water reach deeper clay and heave the pier, and that water jetting used to install pressed piles can wet clay and heave the pile [2].
Is heave common in northern states?
Frost heave is, but it is a different process driven by ice in frozen soil. Clay heave from moisture is the main pattern in places like Texas. See our frost heave page for the northern version.
Sources
- FPA-SC-13-1: Guidelines for the Evaluation of Foundation Movement for Residential and Other Low-Rise Buildings (Apr 2015), Foundation Performance Association, Structural Committee. Accessed 2026-10-06.
- FPA-RC-01-0: Post Foundation Repair Performance of Residential and Other Low-Rise Buildings on Expansive Soils (Jan 2004), Foundation Performance Association, Repair Committee. Accessed 2026-10-06.
- Misconceptions about the common crack (Builder Education Series), National Association of Home Builders (Walt Keaveny, PE, PG). Accessed 2026-10-06.
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