# Why is my driveway sinking after heavy rain? > Why a driveway starts sinking after heavy rain or a monsoon storm: water in the base, washed-out edges, downspouts and clay. Check these signs first. URL: https://phxsundialdrivewayrepair.com/guide/driveway-sinking-after-heavy-rain/ Learn 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. Last Updated: October 26, 2026 Short answerA driveway sinks after heavy rain when water gets under the slab and softens or carries away the base it sits on. Storm runoff along an edge, a downspout aimed at the drive, open cracks and joints, and clay soil that swells and then shrinks are the usual routes. The repair holds only once that water path is fixed. Key takeaways - Sinking after a storm points to the base and subgrade, not the surface. - Phoenix gets a few heavy bursts a year rather than steady rain, so one storm can move a lot of water across a drive. - Mud or fine sand washing out from under an edge is a sign a void is forming. - A slab that is still in one piece can often be lifted; a broken one is replaced on a rebuilt base. - Fixing where the water comes from is part of the repair, not an extra. A driveway sinking after heavy rain is almost always a support problem underneath, not a surface one: water got into the base and either softened it or carried part of it away. [Sundial Driveway Repair](https://phxsundialdrivewayrepair.com/) offers and manages [sunken driveway repair](https://phxsundialdrivewayrepair.com/sunken-driveway-repair/) and the drainage work that goes with it across Phoenix and the surrounding Valley, fulfilled by licensed and insured contractors. This guide explains what storm water does under a slab and how to tell which repair fits. ## What happens under a driveway when it rains hard? A slab rests on an aggregate base of crushed rock, which rests on compacted native soil, the subgrade. Both layers carry load only while they stay firm and in place. Water attacks them in three ways. The first is **softening**. Saturated soil loses strength, and a slab that was fully supported on dry ground now rests on a layer that gives under each car. The second is **erosion**. Water moving along an edge or through an open joint picks up the fine particles in the base and carries them out, which leaves a void, a pocket of nothing under the concrete. Every car that crosses the slab after that pushes it down a little and squeezes more wet fines out, a process called pumping. The third is **swelling and shrinking clay**. Where the subgrade contains expansive clay, it swells when soaked and shrinks again as it dries, so the slab rises unevenly and then drops lower than before. All three end the same way: the concrete is left spanning soft or missing support, and because it is rigid, it cracks or tilts instead of bending. Asphalt fails the same way but shows it differently, with dips, ruts and a web of [alligator cracking](https://phxsundialdrivewayrepair.com/guide/alligator-cracking/) where the base has gone soft. ## Why do Phoenix storms do this much damage? Because the rain comes in bursts. Phoenix Sky Harbor averages only 7.22 inches of rain a year, but about 4.1 days a year bring half an inch or more and about one day brings an inch or more (NOAA climate normals). The National Weather Service defines monsoon season as June 15 to September 30, and July and August each average just over nine-tenths of an inch. Maricopa County’s flood control district notes that floods come especially during summer monsoon and winter rainstorms. Hard desert ground, roofs, block walls and the driveway itself shed that water quickly, so it arrives at the slab edge as moving sheet flow rather than a gentle soak. Clay soils add to it. USDA soil survey samples show moderate or higher shrink-swell at 78.4% of sampled points in Gilbert and 76.9% in Chandler, for example, so a storm that wets one side of a drive can lift and then drop that side. Soil maps describe an area, not your lot; they raise or lower the odds without proving the cause. Where caliche lies close under the base, water that gets in drains down slowly and sits in the base longer, which the [caliche guide](https://phxsundialdrivewayrepair.com/guide/caliche-and-driveways/) covers. ## Where does the water get in? A sinking spot is the end of a water path, and finding the start of it is what makes a repair last. ### Downspouts aimed at the drive A downspout turns a whole roof section into one concentrated stream. Where it empties onto the driveway, or at the edge beside it, that stream runs along the slab edge, scours the base out from under the corner and puts water into the joint between the drive and the house. A corner that has dropped below a downspout, or a gap opening along the garage edge, is the classic pattern. Moving or extending the downspout is gutter work and sits outside driveway repair, but the repair should be planned around where that water will go afterward. ### Edges and low spots Water that runs along an unsupported edge undercuts it. Where the drive sits lower than the yard, or a flat spot holds a puddle after the storm, the water has time to soak through the joints. Grading rules point the same way: Maricopa County requires lots to be graded so drainage doesn’t harm neighboring property, and Scottsdale’s homeowner drainage guide says driveways must not block or divert flow and need a dip or a culvert at least 18 inches across where they cross a roadside ditch in the right-of-way. ### Open cracks and joints A crack or a joint that has lost its sealant is a funnel straight into the base. That is why [expansion joint repair](https://phxsundialdrivewayrepair.com/driveway-expansion-joint-repair/) and crack sealing matter more before monsoon season than after it. ### Irrigation that stacks on the storm A sprinkler head spraying the slab edge or a drip line beside it keeps the soil there wet between storms, so the rain lands on ground that is already soft. Salt River Project delivers flood irrigation to roughly 22,000 homeowners, and where a flood-irrigated yard meets a driveway, the edge sees standing water on a schedule as well as in storms. ## What are the signs that the base is washing out? Look for these after a heavy rain: - **Mud or fine sand at the edge of the slab,** or a fan of silt on the street below the drive. That is base material leaving. - **A panel that sounds hollow** when tapped with a hammer handle, or rocks slightly when a car drives over it. - **A new crack across the middle of a panel or at a corner,** or a joint that has opened or stepped since the last season. - **Water standing against the garage** or running toward the house where it used to drain away. - **On asphalt,** a soft spot, a new dip, or a patch of fine interconnected cracks. The first visit looks at the whole drive: the surface, the joints, the edges, where water runs, and the transitions to the garage, sidewalk and street. Hollow spots are found by tapping, and a section is sometimes cut out to see the base. Photos can’t show whether a void is there or how far it runs. ## Which repair fits a driveway that sank in the rain? The condition of the slab and the base decides it. | Condition | Usual repair | Why | | --- | --- | --- | | Slab in one piece, settled, small void | [Driveway leveling](https://phxsundialdrivewayrepair.com/driveway-leveling/) with cement slurry or foam | Fills the void and raises the panel without new concrete | | Slab cracked through or broken into pieces | Remove the section and rebuild the base ([concrete driveway repair](https://phxsundialdrivewayrepair.com/concrete-driveway-repair/)) | A lift can’t rebuild broken concrete | | Base soft or wet across a wide area | Section or full [driveway replacement](https://phxsundialdrivewayrepair.com/driveway-replacement/) with base correction | The support has failed, not just one spot | | Asphalt with a soft dip or alligator cracking | Cut out and rebuild the failed area ([asphalt driveway repair](https://phxsundialdrivewayrepair.com/asphalt-driveway-repair/)) | Surface patching over a wet base won’t hold | | Any of the above with an active water source | Add [driveway drainage](https://phxsundialdrivewayrepair.com/driveway-drainage/) corrections | Without them the new support washes out too | The [mudjacking vs polyjacking guide](https://phxsundialdrivewayrepair.com/guide/mudjacking-vs-polyjacking/) compares the two lifting methods, and the [driveway leveling cost guide](https://phxsundialdrivewayrepair.com/guide/driveway-leveling-cost/) gives typical ranges. Findings that change the scope, such as a soft base or caliche, are explained before the work goes ahead. ## What can wait, and what can’t? Act soon if a panel rocks or sounds hollow under a car, an edge or corner is breaking away, water now flows toward the house, or a trip edge has opened where the drive meets the sidewalk. Those get worse with each storm. A crack that stayed the same width through the whole season, or a slight dip that drains, can wait for a planned repair, though sealing it before the next monsoon is cheap protection. The [pre-monsoon driveway checklist](https://phxsundialdrivewayrepair.com/blog/pre-monsoon-driveway-checklist/) lists what to look at before the season starts. If your driveway dropped in the last storm, take photos of the low spot and of the water’s path the next time it rains, then call the number at the top of this page. If you are weighing a claim, the guide to [whether homeowners insurance covers driveway damage](https://phxsundialdrivewayrepair.com/guide/does-homeowners-insurance-cover-driveway-damage/) explains what decides it. In this guide - What happens under a driveway when it rains hard? - Why do Phoenix storms do this much damage? - Where does the water get in? - What are the signs that the base is washing out? - Which repair fits a driveway that sank in the rain? - What can wait, and what can’t? Get an Estimate For sunken driveway repair anywhere in the Valley. ## More questions on this ### Why does my driveway feel hollow after a monsoon storm? A hollow sound when you tap the slab usually means soil under it has washed out or compacted, leaving an empty pocket. Concrete can bridge a small void for a while, but cars flex the slab over it until it cracks. Mark the area and keep heavy vehicles off it until it has been checked. ### Can a driveway that sank in a storm be lifted back into place? Often, if the slab is still in one piece and the soil underneath is stable once the water is controlled. Lifting with cement slurry or polyurethane foam fills the void and raises the panel. If the slab has broken into pieces, or the base keeps getting wet, cutting out the section and rebuilding the base lasts longer. ### Should I fill the cracks right after a storm? Not before you know whether the slab is still moving. Sealing keeps water out, which helps, but filling a crack over a fresh void hides the symptom without restoring support. Photograph the crack, mark its ends, check where the water came from during the next rain, and decide on lifting or replacement first. ## Where to go next ### The service behind this guide - [Sunken Driveway Repair](https://phxsundialdrivewayrepair.com/sunken-driveway-repair/) Sunken driveway repair in Phoenix: why slabs drop at the garage and edges, how voids form, and when to lift, rebuild the base or replace. ### Related guides - Learn [What causes driveways to crack and sink in Phoenix?](https://phxsundialdrivewayrepair.com/guide/why-driveways-crack-and-sink-in-phoenix/) 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. - Decision Guide [Mudjacking vs polyjacking: which is better for lifting a sunken driveway?](https://phxsundialdrivewayrepair.com/guide/mudjacking-vs-polyjacking/) How each lifting method works, where they really differ, how Phoenix clay and storm water affect the choice, and the cases where lifting isn't the answer at all. - 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. - Decision Guide [Does homeowners insurance cover driveway damage?](https://phxsundialdrivewayrepair.com/guide/does-homeowners-insurance-cover-driveway-damage/) Coverage turns on the cause of the damage and the wording of your policy. Here is how to read the two together, and what a repair estimate can and can't do for a claim. - Cost Guide [How much does driveway leveling cost?](https://phxsundialdrivewayrepair.com/guide/driveway-leveling-cost/) What mudjacking and polyurethane foam lifting cost per square foot, why small jobs price as a minimum, what drainage and joint work add, and where lifting stops being the cheaper choice.