How to level out a low spot in concrete?

A dip in a driveway, patio or sidewalk usually starts small: a puddle that stays after rain, or one edge that sits a little lower than the next. In Sandy Springs, with red clay soil, heavy summer storms and plenty of sloped lots, it often gets worse over a few seasons. Here is how to work out what is going on, what you can check yourself, and when to call someone in.

The short answer

Concrete rarely sags the way asphalt does. A slab is stiff, so when part of it sits low, it usually means the soil underneath has settled, washed out or shrunk, and the concrete has dropped into the gap. The surface is showing you a problem with what is underneath it.

There are two common ways to level it. The first is to lift the slab from below by filling the space under it, which is what mudjacking and other concrete leveling methods do. The second is to grind or patch the surface so the dip goes away visually. A patch only makes sense for a shallow dip in a slab that is otherwise solid and not still moving. If the soil underneath keeps going, a patch on top will crack and the low spot will come back.

Before either one, find out where the water is coming from. Lifting a slab without fixing the drainage usually means doing it again later.

What changes the answer on a real property

The same low spot can need very different work depending on why it formed. A downspout emptying right at the edge of a patio will wash soil out from under it a little with every storm. Soil that was loosely backfilled around a new house, especially next to the garage and along the foundation, can keep settling for years. Big trees cause trouble both ways: roots can push a slab up, and old roots that rot leave hollow spaces.

Around Sandy Springs the soil is mostly clay. Clay swells when it is soaked and shrinks when it dries out in late summer, so a slab can move with the seasons. Many homes here sit on sloped lots with driveways running downhill to the street. On those slopes, water runs along and under the slab edges, carrying soil with it.

These details change the job:

• How far the slab has dropped, and whether it has stopped moving or is still going
• Whether the concrete is one solid piece or has broken into several loose sections
• Whether a gutter, downspout or slope is still sending water under it
• What the slab carries: foot traffic on a walk is different from a car on a driveway apron
• Whether it tilts toward the house, which can push water against the foundation

What you can check yourself

You can learn a lot in an afternoon with a straight board, a tape measure and a look outside during the next rain.

• Lay a long straight board across the low spot and measure the gap at its deepest point. Write it down and check again in a few months to see if it is changing.
• Go outside during a steady rain. Watch where water lands, where it pools, and which downspouts empty near the slab.
• Tap the concrete with a rubber mallet or the end of a broom handle. A dull, hollow sound in one area often means there is a void underneath.
• Look at the cracks. A thin hairline crack is common. A crack where one side sits higher than the other, or one wide enough to fit a coin, means the slab has moved.
• Check the joints where the slab meets the house, the garage floor and the steps. A gap that has opened up, or a slab sloping back toward the wall, is worth noting.

Fix the water first

If a downspout empties at the slab, extend it so the water drains well away. Fill washed-out gaps at the slab edge with soil and grade it to slope away. These are small jobs, and they decide whether any repair lasts.

When it needs a professional

Some low spots can wait and some cannot. Call someone in when you see any of these:

• A raised edge on a sidewalk or step that someone could trip over
• A patio or driveway slab tilting toward the house or garage
• A hollow sound over a large area, or a low spot that gets deeper season to season
• A slab next to a retaining wall that is sinking or pulling away, since the wall and the soil behind it may be part of the cause
• Concrete that has broken into loose pieces that rock when you step on them

What a visit involves

We walk the slab with you, check where the water goes, measure how far each section has dropped and look for voids. Then we tell you plainly whether the concrete can be lifted or should be replaced. Concrete that is crumbling, broken into many small pieces or being pushed up by live roots is usually better replaced than lifted.

How mudjacking and concrete leveling fit in

Mudjacking is the method most people mean when they ask about lifting a slab. We drill a few small holes through the concrete, pump a cement-based slurry into the space underneath, and the pressure raises the slab back toward level. The slurry fills the voids that caused the drop, and the holes are patched afterwards. It is one of the concrete leveling methods we use, and the right one depends on the slab, the soil and how far it needs to rise.

Lifting keeps the concrete you already have. That means less demolition and less disturbance to the lawn and planting beds, which matters on the steep driveways and narrow side yards common in Sandy Springs. For walks with a trip edge, Sidewalk Repair may mean lifting the low section, grinding a high edge, or replacing one panel.

Be realistic about the result. A lifted slab will show the patched holes, and any cracks it already had will still be there. New patches will not match old concrete exactly in colour. And if the water that washed the soil out is still running under it, the slab can settle again. Leveling works best as the second step, after the drainage is sorted.