Sunken Patio Repair: Why It Sinks and How to Fix It

A patio that used to sit flat now tips toward the house, or one corner has dropped an inch below its neighbor. Maybe water pools near the back door after every storm. Sunken patio repair is one of the most common calls from homeowners in the clay-soil suburbs north and east of Austin, and the fix depends entirely on why the slab moved.

This guide walks you through the causes, the checks you can do yourself with a tape measure and a garden hose, and the repair options. It also covers how to keep the patio from sinking again, because lifting a slab without fixing the water problem only buys you a few seasons.

Why patios sink in Central Texas

A patio slab is usually 3.5 to 4 inches thick and sits on a thin layer of fill or directly on native soil. It has no deep footing like your house does. That means it follows whatever the soil underneath does.

Around Pflugerville, Hutto, Manor and Taylor, much of the ground is part of the Blackland Prairie belt. The best-known soil there is Houston Black clay, which is so well known for shrinking and swelling that it is the textbook example of a “vertisol.” The Texas state soil document notes that these soils swell and shrink markedly as moisture changes, and that this commonly limits building on them. A patio slab rides that movement like a raft.

Here are the most common reasons a patio drops.

Washed-out soil under the slab

When water runs along the edge of a slab, or gets under it through a crack or open joint, it carries fine soil away. Over time a void forms. The concrete is strong in compression but weak when it has nothing underneath, so it sags or cracks at the void.

You often see this where a downspout dumps next to the patio, or where the slab meets the house and the joint has opened up.

Poor compaction when the home was built

Builders often backfill around a new house with loose soil. A patio poured over that backfill can settle for years as the dirt compacts under its own weight and under rain. The patio usually sinks next to the house first, which is why so many patios slope toward the back door.

Clay that dries out and shrinks

In a long dry spell, clay shrinks and pulls away from the underside of concrete. The slab sits on a thinner support, then drops when the soil lets go. When rain returns, the clay swells again, but unevenly. The result is a patio that moves in different directions at different corners.

If you want the full explanation, read our post on why concrete sinks in Texas clay.

Tree roots on one side

Large trees pull a lot of water from clay. A live oak or a cedar elm near the patio can dry one side of the soil far more than the other. That uneven shrinking tilts the slab. Roots can also lift part of a patio, which looks like a sunken area on the opposite side.

Is it a drainage problem or a soil problem?

The two overlap, but you can usually tell which one is in charge. Spend 20 minutes on these checks after the next rain.

Watch where the water goes. Stand on the patio during or right after a storm. Does water run toward the house? Does it pool at the low corner? Water that sits on a slab for more than a few hours is a sign the slope is wrong or the slab has dropped.
Check the gutters and downspouts. If a downspout ends within a few feet of the patio, that water is soaking the same soil the slab sits on. Many downspouts in newer subdivisions simply end on the lawn.
Look at the joints and cracks. Open joints between the patio and the house, or wide cracks, let water travel under the slab.
Measure the drop. Lay a straight 4-foot level or a long board on the slab and measure the gap with a tape measure. A drop of half an inch across a joint is a trip hazard and a good candidate for leveling. A slab that has dropped several inches may have a bigger void under it.
Tap and listen. Tap the slab with a hammer or the end of a pipe. A dull, hollow sound suggests a void beneath that spot.
Check the house too. Look for cracks in brick above doors and windows, doors that stick, or gaps between the patio and the wall that keep widening. If you see these signs, the movement may be coming from the foundation, not just the patio.

That last point matters. We do not do structural foundation repair, and we are not engineers. If your house shows signs of movement, have a structural engineer or a foundation repair company evaluate it before you spend money on a patio. We can help with the drainage side, which often protects the foundation too. See our post on signs of poor drainage around a foundation for the warning signs to look for.

Your repair options

Once you know the cause, you can pick a repair. There are three realistic paths for a sunken patio.

Option 1: Lift the slab with foam leveling

Polyurethane foam leveling involves drilling small holes, about 5/8 inch across, in the slab. A technician injects expanding foam through those holes. The foam fills voids and lifts the slab back toward level, usually in one visit. The patio can often be used within hours.

Foam is light. Sources in the industry put it at roughly 2 to 4 pounds per cubic foot, versus more than 100 pounds per cubic foot for the cement slurry used in mudjacking. That matters on a patio where the soil underneath is already soft, because you are not adding weight to ground that has already failed once. Foam also resists washout, so water moving under the slab will not carry it away.

Our concrete slab leveling service uses this approach. For a full comparison with the older method, read foam leveling vs mudjacking.

Foam works well when the slab is still in good shape. The concrete itself needs to be mostly intact. A slab that has crumbled or shattered into many pieces is usually cheaper to replace.

Option 2: Mudjacking

Mudjacking pumps a cement and soil slurry under the slab. It has been used for decades and can work on a patio, but the holes are larger (often cited around 1.5 to 2 inches), the slurry is heavy, and the area may need a day or two to cure before heavy use. It is a reasonable choice in some cases, and the price is lower than foam. The tradeoff is the weight and the larger holes.

Option 3: Replace the slab

If the slab is badly cracked, spalled, or has dropped many inches, replacement may be the better choice. A new patio needs proper base preparation. Without it the new slab will fail the same way.

Before you pour new concrete, fix the water problem. A common pattern in this part of Texas goes like this: a new slab goes in, the downspout still dumps beside it, and the patio starts sinking again within a few seasons.

General price ranges

Costs vary a lot by site, slab size, access and how deep the voids go. As a general market range only, not a quote, lifting a small residential slab with foam commonly costs less than replacing it, and replacement tends to be several times more expensive when you add demolition and haul-off. A free on-site look is the only reliable way to price your patio.

Fix the water so the patio stays up

Leveling treats the symptom. Drainage treats the cause. For most sunken patios, the water fix is the part that decides how long the repair lasts.

Move the roof water away

Roof water is the biggest single source of water near a patio. A 1,500-square-foot roof sheds roughly 900 gallons in a one-inch rain. If your downspouts end on the lawn near the slab, that water soaks the soil right where you do not want it.

Underground downspout drains carry that water in a buried pipe to a safe discharge point, such as a pop-up emitter in a lower part of the yard, or the street where your city allows it. Extending the discharge point a long way from the patio keeps the soil beneath it steadier.

Regrade so the ground slopes away

The ground next to the patio should fall away from the slab and the house. The standard building code language says lots should be graded so the surface falls at least 6 inches in the first 10 feet away from the foundation walls. A patio is not the house, but the same logic holds. Water should leave the area, not collect at the edge.

Our yard drainage and regrading service rebuilds that slope. If your yard is flat or tilted toward the patio, a French drain or a surface swale can capture water before it reaches the slab.

Seal the joints

Open joints and cracks let water run straight under the patio. Sealing them keeps surface water out. It does not stop all moisture, but it removes the easy path. Our concrete crack and joint sealing work uses flexible sealants made for outdoor slabs.

Manage trees and watering

If a big tree sits within a few feet of the patio, the roots may be the main driver. Cutting roots can hurt the tree, so talk with a certified arborist first. Also check your sprinkler heads. A head that sprays the slab edge every morning keeps that soil wet and can wash it out.

What to do before a repair

Follow this order and you will avoid wasted money.

Photograph the patio and mark the low spots with chalk or tape.
Walk the yard after a rain and note where the water goes.
Check for house movement and call an engineer or foundation company if you see it.
Call 811 before any digging. In Texas, you or your contractor must contact Texas811 at least two business days before you dig, even for small jobs. The call is free, and it keeps you from hitting a gas, electric or communications line.
Ask your HOA or city if you need approval. Rules about drainage, grading and work in easements vary, so check with them instead of guessing.
Get the drainage plan and the leveling plan together, so one does not undo the other.

When to call a pro

Some patios are fine for a weekend project. If the drop is under a quarter inch and the slab is solid, you may only need to seal the joint and move a downspout. Call a professional when:

The patio has dropped more than half an inch at a joint.
You hear hollow sounds across a large part of the slab.
Water pools against the house.
The sinking keeps coming back after earlier repairs.
You are not sure whether the problem started with the patio or the house.

In the Pflugerville area, a sunken patio is often the first visible sign that water is behaving badly on the property. Fixing the water and lifting the slab together is usually the only approach that lasts. If you want a straight answer on what your patio needs, request a free estimate and we will look at the slab, the downspouts and the grading in one visit. You can also read about how we work in Pflugerville and the surrounding towns.