# What causes driveways to crack and sink in Phoenix?

> What causes driveways to crack and sink in Phoenix: clay soil, monsoon and sprinkler water, caliche, heat and weak base, and when to act. Start here.

URL: https://phxsundialdrivewayrepair.com/guide/why-driveways-crack-and-sink-in-phoenix/

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How a driveway is built, what the Valley's heat, soils and storms do to each layer, how to read the cracks you can see, and when a repair can wait.

Last Updated: October 26, 2026

Short answerDriveways crack and sink when the ground or base under them moves or loses support, or when new concrete shrinks as it cures. In Phoenix the usual drivers are shrink-swell clay, water from monsoon storms, sprinklers and downspouts that softens or washes out the base, and heat on fresh pours. Frost, with 0.6 freezing nights a year, rarely is.

Key takeaways

- Some cracking is built into concrete; control joints exist so it happens in a straight line.
- A crack that widens, steps or comes back after filling means the slab is still moving.
- In Phoenix, water and clay explain more cracking and sinking than cold weather does.
- Caliche rarely cracks a driveway on its own, but it changes how the base drains and how the repair is dug.
- The apron and sidewalk crossing are usually in the city right-of-way, which brings permits and local rules.

What causes driveways to crack is usually movement or lost support underneath, plus the shrinkage that every concrete slab goes through as it cures. Phoenix changes the mix of causes: cold barely figures, while heat, clay and water do most of the damage. [Sundial Driveway Repair](https://phxsundialdrivewayrepair.com/) is a service company offering and managing [driveway crack repair](https://phxsundialdrivewayrepair.com/driveway-crack-repair/) and other driveway work across Phoenix and its suburbs, fulfilled by licensed and insured contractors, and this guide explains the causes so a repair can be matched to the real one.

## How is a driveway built?

A driveway is four layers working together. At the bottom is the **subgrade**, the native soil, graded and compacted. On top sits the **aggregate base**, a layer of crushed rock that spreads the load and lets water drain away from the surface. Then comes the **pavement**: either a concrete slab or asphalt. Concrete driveways add **joints**, and sometimes **reinforcement**.

Concrete and asphalt are different systems, and they fail differently, which is why [concrete driveway repair](https://phxsundialdrivewayrepair.com/concrete-driveway-repair/) and asphalt repair use different methods. Portland cement concrete is rigid: cement paste binds sand and stone into a slab that is very strong when squeezed and weak when bent or pulled. Because it is rigid, it bridges small soft spots in the base, but when the support under it drops too far, it cracks rather than bends. It also shrinks as it dries and grows and shrinks with temperature. **Control joints**, the saw-cut or tooled lines across the slab, are deliberate weak lines so that shrinkage cracks run straight along them instead of wandering. **Expansion joints**, the flexible strips where the drive meets the garage or sidewalk, give the slab room to grow in the heat. Steel mesh or rebar doesn’t stop cracks forming; it holds the pieces together once they do.

Asphalt is flexible. Hot-mix asphalt is stone held in a binder of asphalt cement, which softens with heat and stiffens and turns brittle with age and sun. Because it flexes, asphalt follows the base down when support is lost, and the result shows up as ruts, dips and a web of fatigue cracks rather than clean breaks. The [asphalt vs concrete guide](https://phxsundialdrivewayrepair.com/guide/asphalt-vs-concrete-driveway-phoenix/) compares the two for Valley conditions.

Most driveway failures start in the bottom two layers. A slab poured on soft, poorly compacted or wet subgrade cracks and settles however well it was finished. That relationship, weak base leading to cracking and deformation, runs through every section below.

## Why do Phoenix driveways crack differently?

The Valley’s mechanisms are not the ones most driveway advice is written for. Here is how each one works here.

### Heat

Phoenix Sky Harbor averages 111 days a year at 100°F or more and 173 days at 90°F or more, with normal highs of 104.2°F in June, 106.5°F in July and 105.1°F in August (NOAA climate normals). Nights don’t give much relief: normal summer lows run from about 78°F to 85°F. Concrete grows by roughly 5 to 6 millionths of its length per degree Fahrenheit, so a 20-foot run that warms 50 degrees from dawn to afternoon lengthens by about a sixteenth of an inch, then shrinks back overnight. Joints exist to take that movement. Where a joint is packed with rock and grit, or the slab is pinned against the garage with no expansion joint, the slab pushes against itself and the edges spall or crack.

Heat does the most harm while concrete is new. Fresh concrete in hot, dry, windy weather loses water from its surface faster than it can bleed up, which causes plastic shrinkage cracks and a weak, dusty top. That is why hot-weather practice from the American Concrete Institute (ACI 305.1) caps concrete at 95°F at discharge for general work, and why summer pours in the Valley start early. Asphalt has the opposite problem in summer: its binder softens, so a fresh or thin drive scuffs where wheels turn in place and ruts under parked weight. The City of Phoenix measured its cool-pavement coating at about 10.5°F to 12°F cooler at the surface than traditional asphalt around midday, a reminder of how much heat dark pavement holds. The guide to [how heat affects driveways in Phoenix](https://phxsundialdrivewayrepair.com/guide/how-heat-affects-driveways-in-phoenix/) goes further into both.

### Freeze-thaw (almost none)

In colder climates, water freezing in cracks and in the soil causes heaving and surface scaling, and de-icing salt makes it worse. Sky Harbor averages 0.6 nights a year at or below freezing, and no month has a normal high at or below freezing (NOAA). Outlying stations see more: about 9.7 nights at Tempe ASU, 10.5 at Chandler Heights and 11.1 at East Mesa. Those are light overnight freezes that thaw by midday, not the long freezes that drive frost heave, and Valley streets aren’t salted. So when a Phoenix driveway heaves, the cause is almost always the soil, not frost.

### Shrink-swell clay

Some Valley soils contain clay that takes on water and swells, then shrinks as it dries. USDA soil survey samples show how common that is in places: moderate or higher shrink-swell at 78.4% of sampled points in Gilbert (high at 47.1%), 76.9% in Chandler, 61.4% in Tempe and 46.2% in Mesa, against 11.4% in the town of Paradise Valley. A slab on that soil rises when the ground is wet and settles when it dries. If it gets wet unevenly, from a sprinkler on one side or a downspout at one corner, it moves unevenly too, and cracks where the support changes. This is the Phoenix version of driveway heaving: clay swelling, not ice. Soil maps describe an area, not a lot, so they raise or lower the odds on a given property without proving the cause.

### Caliche

Caliche is a layer of soil cemented by calcium carbonate, hard enough in places to need a breaker. USDA surveys flag cemented layers mostly along foothill and desert edges: 29.2% of sampled points in Phoenix’s Rio Vista village, 20.5% in Peoria and 18.2% in the town of Paradise Valley, and none in the Tempe, Chandler, Gilbert or Glendale samples. Caliche doesn’t crack a driveway by itself. What it does is slow water draining down through the soil, so water that gets under a slab can sit in the base above it, and it makes digging out a failed base or regrading for drainage slower and costlier. The [caliche guide](https://phxsundialdrivewayrepair.com/guide/caliche-and-driveways/) explains how it changes a repair.

### Monsoon washouts

The National Weather Service defines monsoon season as June 15 to September 30. Sky Harbor averages only 7.22 inches of rain a year, but it arrives unevenly: about 4.1 days a year bring half an inch or more, and about one day an inch or more. Water from those storms runs off roofs and hard desert ground fast, and when it runs along a driveway edge or collects at a low spot, it carries the fines out of the base. The edge loses support first, then the panel. Maricopa County’s drainage rules require lots to be graded so drainage doesn’t harm neighboring property, and in Scottsdale a driveway that crosses a roadside ditch needs a dip or a culvert at least 18 inches across in the right-of-way. Good subsurface drainage keeps the soil under a slab stable; water that sits there softens it. The guide to [driveway sinking after heavy rain](https://phxsundialdrivewayrepair.com/guide/driveway-sinking-after-heavy-rain/) covers the warning signs, and [driveway drainage](https://phxsundialdrivewayrepair.com/driveway-drainage/) covers the fixes.

### Irrigation water

Year-round watering keeps soil next to a driveway wet in a way rainfall never would. Where a sprinkler head sprays onto the slab edge or a drip line runs alongside it, the soil there stays wetter than the rest, which is exactly the uneven wetting that moves clay. Some Valley yards still take flood irrigation: Salt River Project delivers it by gravity to roughly 22,000 homeowners, and private ditches and pipes on the owner’s property are the owner’s responsibility. Where a flood-irrigated yard meets a driveway, the edge of the slab and the base under it see standing water on a schedule.

### Tree roots, only where there is a tree

Roots can lift and crack a slab where a large tree grows close to the drive, and a thirsty tree can shrink clay soil beside it. Where there is no tree near the drive, look elsewhere; where there is one, a raised panel next to the trunk is the giveaway, and the [tree roots guide](https://phxsundialdrivewayrepair.com/guide/driveway-damaged-by-tree-roots/) covers what can be done.

## What do different driveway cracks mean?

A crack’s shape and behavior are clues, not a diagnosis. The same-looking crack can have more than one cause, and the visit checks what the surface can’t show.

| What you see | What it often means | What usually decides the repair |
| --- | --- | --- |
| Hairline cracks in a new slab, shallow and random | Plastic or drying shrinkage | Usually cosmetic; seal if water gets in |
| A crack along a control joint | The joint doing its job | Nothing, or [reseal the joint](https://phxsundialdrivewayrepair.com/driveway-expansion-joint-repair/) |
| A straight crack between joints, edges level | Shrinkage with joints spaced too far apart | Clean, fill and seal if it stays dormant |
| A crack that has widened or one side has stepped up or down | The slab is moving: settlement, swelling clay or lost base | Find the movement before filling; [lifting](https://phxsundialdrivewayrepair.com/driveway-leveling/) or section replacement |
| A broken corner or crumbling edge | Lost edge support, often from water or heavy loads | Fix the water source; patch or replace the section |
| Cracks radiating from one spot, or a web of cracks in one panel | Concentrated load or a void under the slab | Check for a void; replace the panel on a rebuilt base |
| Rust stains along a crack | Water reaching and corroding the reinforcement | Remove unsound concrete, deal with the steel, seal the crack |
| Fine surface crazing or flaking | A weak top layer from finishing or fast drying | Resurface if the slab below is sound ([spalling guide](https://phxsundialdrivewayrepair.com/guide/what-causes-driveway-spalling/)) |
| Asphalt: interconnected “alligator” cracks | Fatigue from a weak or wet base | [Remove and rebuild the failed area](https://phxsundialdrivewayrepair.com/asphalt-driveway-repair/); crack filler won’t hold |
| Asphalt: large rectangular blocks | Age and oxidation of the binder | Seal the cracks; consider an overlay on a sound base |
| Asphalt: cracking and breaking along the edge | Edge without support, often water or traffic at the edge | Rebuild the edge and correct drainage |

The single most useful test is time. Mark the ends of a crack with a pencil and photograph it with a coin for scale. If it is the same in three months, it is probably dormant and sealing is enough. If it has grown, the slab is moving and filling it treats the symptom. The [alligator cracking guide](https://phxsundialdrivewayrepair.com/guide/alligator-cracking/) covers the asphalt patterns in more depth.

## Why do driveway slabs sink?

A slab sinks when the ground under it compresses, washes out or shrinks. There are three main mechanisms. The first is poor compaction: subgrade or base placed loose, for example in backfilled utility trenches or around the foundation of a new house, keeps settling for years under the weight of a slab and cars. The second is erosion: water running along an edge, from a downspout or a monsoon sheet flow, carries fine soil out from under the slab and leaves a **void**, an empty pocket that the concrete bridges until it can’t. The third is shrinking clay, which drops a slab when the ground dries out after being wet.

Voids are the hidden part. A panel over a void sounds hollow when tapped, may rock slightly under a car, and often cracks across the middle or at a corner. Where the slab is still in one piece, [sunken driveway repair](https://phxsundialdrivewayrepair.com/sunken-driveway-repair/) can lift it and fill the void with cement slurry or foam. Where it has broken, the panel comes out and the base is rebuilt. Either way, the water that made the void has to be dealt with, or the new support washes out too.

Heave is the reverse: a panel rising, usually at a joint or against the garage, as clay swells or roots grow. In Phoenix it follows the soil’s moisture, so it tends to track watering and storms, and it can partly reverse as the soil dries.

## When does a cracked or sinking driveway become urgent?

Most driveway damage is slow, and not every crack is a problem. These signs mean it is worth acting soon:

- **A [trip edge](https://phxsundialdrivewayrepair.com/concrete-trip-hazard-grinding/) where the drive meets the sidewalk.** In Phoenix, City Code makes the owner of a lot next to a sidewalk responsible for keeping it in repair and liable for injury from a defect left unrepaired, and after notice the City can make the repair and add the cost to the tax roll.
- **Water running toward the house or pooling at the garage.** A slab that has settled toward the house sends water where it can do the most harm.
- **An edge or corner breaking away,** especially where it carries a car or a truck.
- **A crack that is visibly wider than last season,** or one side of it rising or dropping.
- **A panel that rocks or sounds hollow,** which points to a void that can collapse under load.

These can usually wait for a planned repair: hairline cracks that haven’t changed, cracks along control joints, light surface wear or flaking on a slab that is otherwise level, and asphalt that has turned gray but is still smooth. Waiting does have a cost on open cracks, because every storm puts water into the base, so sealing early is cheap insurance.

## What can and can’t be told from photos?

Photos show the pattern of the damage, where the water goes and whether the joints are working. They can’t show the base. Whether a crack is dormant, whether a panel sits over a void, how deep caliche lies, and whether the subgrade is wet all take a look in person: tapping for hollow spots, checking the joints for movement, and sometimes cutting out a section to see what is underneath. Some causes only show once the old concrete is out. A good estimate says which conclusions are confirmed and which are the likely explanation.

## Should you repair, resurface or replace?

The cause decides it, not how bad the surface looks. If the base is sound and the damage is local, repair it: seal the cracks, lift a settled panel, patch a spall, or replace one section. If the surface is worn but the slab is level and solid, [driveway resurfacing](https://phxsundialdrivewayrepair.com/driveway-resurfacing/) can renew it, though cracks and settlement underneath come back through any overlay. If cracking runs across most panels, the base has failed widely, or several sections are broken, [driveway replacement](https://phxsundialdrivewayrepair.com/driveway-replacement/) rebuilds from the subgrade up. The [repair vs replacement guide](https://phxsundialdrivewayrepair.com/guide/driveway-repair-vs-replacement/) works through the decision, and the [driveway repair cost guide](https://phxsundialdrivewayrepair.com/guide/driveway-repair-cost-phoenix/) puts typical Phoenix-area ranges on each option.

## Who owns the apron, and when is a permit needed?

The apron, the strip of driveway between the sidewalk and the street, and the curb beside it are usually in the public right-of-way, not your lot, but you are often responsible for them. Under the City of Phoenix Driveway Policy for Single Family Development, residential driveway concrete in the right-of-way is 6-inch Class A; on streets with roll or ribbon curb, up to 10.5 feet of concrete drive there counts as on-site work with no separate driveway permit, while a longer run, or any drive on a street without curb, needs one. Other cities run their own systems. Mesa, for example, issues right-of-way permits only to contractors licensed by the Arizona Registrar of Contractors, and the [Mesa page](https://phxsundialdrivewayrepair.com/service-areas/mesa/) covers its details. Where a rule for a given suburb isn’t published here, it is confirmed per address with that city. The [apron responsibility guide](https://phxsundialdrivewayrepair.com/guide/who-is-responsible-for-the-driveway-apron/) and the [driveway apron repair](https://phxsundialdrivewayrepair.com/driveway-apron-repair/) page go further.

## What should you check before hiring?

Check the license first. In Arizona, a contractor generally needs a Registrar of Contractors license when a job’s labor and materials top $1,000 or when the job needs a permit. The small-job exemption doesn’t apply when a permit is required, when a larger job is split into small contracts to stay under the limit, or when the person advertises without stating “not a licensed contractor”. The classification should fit the work: R-9 covers residential concrete, R-13 residential asphalt paving, and R-21 covers pavers and gravel driveways but excludes concrete driveways. You can look up any license on the Registrar’s site; the [license lookup guide](https://phxsundialdrivewayrepair.com/guide/how-to-check-a-contractor-license-in-arizona/) shows how.

Then check that the quote names a cause and a method that fits it, says whether base repair and drainage corrections are included, and covers permits for any work in the right-of-way. A quote that fills cracks on a slab that is still moving, or resurfaces over a sunken panel, is pricing the symptom.

If you have read this far and still aren’t sure what your driveway is telling you, the [condition assessment](https://phxsundialdrivewayrepair.com/tools/driveway-condition-assessment/) sorts the signs, or call the number at the top of this page and describe what you see.

In this guide

- How is a driveway built?
- Why do Phoenix driveways crack differently?
- What do different driveway cracks mean?
- Why do driveway slabs sink?
- When does a cracked or sinking driveway become urgent?
- What can and can’t be told from photos?
- Should you repair, resurface or replace?
- Who owns the apron, and when is a permit needed?
- What should you check before hiring?

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## More questions on this

### Is it normal for a new concrete driveway to crack?

A few fine cracks in the first months are normal, because concrete shrinks as it dries and the control joints can't catch every one. Hot, dry, windy pours can also show shallow plastic shrinkage cracks within hours. Cracks that are wide, offset, or running across several panels in a new drive point to base, joint spacing or placement problems worth raising with the installer.

### Will a driveway that has sunk keep sinking?

It depends on why it sank. If the base was poorly compacted and has since settled, movement often slows once the soil consolidates. If water is still washing soil out from under an edge, or clay is still wetting and drying around a leaking sprinkler, it keeps going. Fixing the water source is what turns a lift or a new section into a lasting repair.

### Can tree roots crack a driveway in Phoenix?

They can where a large tree grows close to the drive, because roots that thicken under a slab lift and crack it, and a tree that pulls water from clay soil can shrink the ground beside it. Roots are a cause to check for where there is a tree, not one to assume, and a raised, cracked panel next to a trunk is the usual sign.

## Where to go next

### The service behind this guide

- [Crack Repair](https://phxsundialdrivewayrepair.com/driveway-crack-repair/) Driveway crack repair in Phoenix for concrete and asphalt: dormant vs working cracks, filling vs sealing, and when a crack points to the base.

### Related guides

- Learn [Why is my driveway sinking after heavy rain?](https://phxsundialdrivewayrepair.com/guide/driveway-sinking-after-heavy-rain/) What a monsoon storm does to the layers under a driveway, how to tell a washed-out base from a slab that is only cracked, and which repairs hold once the water is dealt with.
- Learn [What is caliche soil in Arizona, and how does it affect a driveway?](https://phxsundialdrivewayrepair.com/guide/caliche-and-driveways/) How caliche forms, where soil surveys flag it around Phoenix, why a compacted subgrade matters more than the caliche itself, and how a cemented layer changes drainage, excavation and the price of a repair.
- Learn [Is your concrete cracking due to heat, and what else does Phoenix summer do to a driveway?](https://phxsundialdrivewayrepair.com/guide/how-heat-affects-driveways-in-phoenix/) The physics of a slab that grows and shrinks every day, why heat does its worst damage to concrete in the first hours, how asphalt binder softens and ages under Valley sun, and what owners can do through a long summer.
- Decision Guide [Driveway repair vs replacement: which one does your driveway need?](https://phxsundialdrivewayrepair.com/guide/driveway-repair-vs-replacement/) The signs that point to a repair, the ones that point to a new driveway, the middle option of replacing sections, and the Phoenix soil, water and right-of-way rules that tip the choice.
- Learn [Who is responsible for the driveway apron?](https://phxsundialdrivewayrepair.com/guide/who-is-responsible-for-the-driveway-apron/) The apron usually sits on public land, yet the repair often falls to the homeowner. Here is how ownership, upkeep and permits split between you, the city and anyone who dug it up.