Why does concrete sink? The 6 usual causes

The short version

Concrete is stiff. It does not sink on its own. It sinks because the ground under it gave way somewhere, and the slab bridged the gap until it cracked or tipped. Leveling refills that space. Unless you also stop whatever emptied it, the space can open again. HomeGuide puts it simply: if poor drainage or soil erosion caused the sinking, the same issues will affect the repaired slab unless corrected^[3].

Cause 1: Loose or poorly compacted fill

This is the most common reason a fairly new slab drops. If the ground was dug out and refilled before the pour, or if topsoil and soft spots were left in place, that soil keeps compressing under the slab's weight for years. NRMCA's guidance for slabs on ground is direct: remove all topsoil and soft spots, compact the soil or granular fill by rolling, vibrating or tamping, and slope it for drainage. It also says never to place concrete on a frozen subgrade^[1].

Clues: sinking along a line where a trench was dug (a sewer or utility line), or along the house wall where soil was backfilled after the foundation was built.

Cause 2: Water washing soil out

Water that gets under a slab carries fine soil away and leaves a void. Common sources:

• Downspouts that discharge at the slab edge.
• A gap between the slab and the house that lets roof water run straight down.
• Slabs that slope toward the house, so water pools against the edge.
• Sprinkler heads aimed at the slab edge.

Clues: a hollow sound when you tap the slab, soil missing at the edge, or a slab that keeps dropping after every heavy rain. FPA names poor drainage as a failure cause for both mud and foam lifts^[2].

Cause 3: Clay soil shrinking

Some clay soils swell when wet and shrink when dry. In a drought, a slab on that clay can drop. When the rains return it may rise again, though often not evenly. FPA lists "extreme seasonal moisture changes" and "soil moisture content changes" as reasons slab lifts fail^[2].

How common is this soil? Using USDA soil survey data, we calculated the share of rated soil with high or very high shrink-swell potential (linear extensibility of 6% or more) by county. Dallas County, Texas came out at about 70%, Tarrant County (Fort Worth) about 60% and Harris County (Houston) about 42%. Franklin County (Columbus) and Cuyahoga County (Cleveland), Ohio, came out near 1%^[5]. These are our numbers, not an official USDA rating; the method is on our soil shrink-swell data page. More on this soil at expansive clay soil.

Cause 4: Frost heave and spring thaw

In cold climates, water in fine-grained soil freezes into layers called ice lenses. They grow as they pull in more water, and they push the slab up. Frost heave needs three things: frost-susceptible soil, freezing temperatures that reach it, and a water supply^[4]. In spring the ice melts faster than the water can drain, so the soil turns soft and loses strength. That is called thaw weakening^[4]. A slab pushed up in January can settle lower than where it started by April.

This is the Ohio and Illinois pattern. See frost heave.

Cause 5: Trees and roots

Large trees pull a lot of water from the soil, which can shrink clay under a nearby slab. Roots can also lift slab edges as they grow. FPA lists trees growing close to the slab among the causes of lift failure^[2]. A root lifting a walk is heave, not sinking, and leveling cannot fix it.

Cause 6: Leaking pipes

A leaking water line, sewer line or irrigation pipe under a slab can wash soil out or soften it. FPA lists broken sewer or water pipes under the slab as a reason both mudjacking and foam injection fail^[2]. If a slab sinks fast in one spot with no obvious outside water source, ask about a leak test.

Sinking, heaving or curling?

Not every uneven slab sank. Before you price a lift, check which way it moved.

What you see	Likely cause	Can leveling fix it?
One slab lower than its neighbor	Settlement or washout	Yes, if the slab is sound
One slab higher, with roots nearby	Root heave	No; grind, cut roots with care, or replace
Slab higher in winter, lower after thaw	Frost heave	Partly; fix drainage first
Edges and corners lifted at joints	Curling from drying shrinkage	Usually no; curling lifts edges as the top dries faster than the bottom[6]
Crack where a slab meets a wall or step	Missing isolation joint, restraint	Not a leveling problem[7]

NRMCA explains that curling comes from the top of a slab drying and shrinking faster than the bottom, which lifts edges and corners^[6]. It also notes that isolation joints are needed where driveways and patios meet walks, garage slabs, stairs and other fixed points, or the slabs crack when they cannot move independently^[7].

How to stop it before you lift

1. Watch the slab during a hard rain. See where the water goes.
2. Extend downspouts several feet past the slab and away from it.
3. Seal the joint between the slab and the house with a flexible sealant.
4. Fill low soil at slab edges and regrade so water runs away.
5. Ask a plumber about a leak test if the cause is not obvious.
6. Then get leveling quotes. See mudjacking vs polyjacking.

When to call a pro

• The slab is next to the house and is sinking or tilting toward it.
• A slab has a hollow sound over a large area or soil is visibly missing at the edge.
• The slab sank quickly in one spot with no outside water source: ask about a plumbing leak.
• Cracks or sticking doors inside the house appeared at the same time: see signs of slab foundation problems.