Sinking Slab: Lift It or Replace It? An Honest Answer

Concrete sidewalk lifting in Chicago means raising a sunken slab back to level instead of tearing it out. Whether that is the right call depends on why the slab sank and what shape it is in. We can't tell you from a photo, and we won't say one method always wins. What we can do is lay out the options and the questions that separate a fair assessment from a sales pitch.

For the broader repair decision, see fix it, replace it, or wait. For a stoop that has dropped, see why a concrete stoop sinks.

What does "lifting" a slab mean?

Lifting is a family of methods that aim to raise a settled slab without removing it, usually by filling the void underneath and pushing the slab up. You will hear names like mudjacking, slabjacking and foam lifting. Contractors differ on the materials and the equipment.

We are not going to rank them here or quote how long any of them lasts. Our own fact base does not include a verified engineering source on how these methods perform, so claims about durability, speed or cost would be guesses. Treat any contractor who promises a specific result before looking at your slab with caution. Ask what they are filling, what they expect the slab to do afterward, and what happens if it settles again.

Why did the slab sink in the first place?

This is the question that decides everything. A slab that dropped lost support, and concrete guidance lists settlement among the typical causes of cracking and movement, alongside drying shrinkage, thermal movement, lack of control joints, overload and restraint (American Concrete Institute).

Common reasons a slab loses support, without us claiming which applies to yours:

• Soil that was never well compacted. Guidance says the subgrade should be well compacted with topsoil and soft spots removed (NRMCA).
• Water running under or beside the slab. Soil should slope away from a building, and downspouts should discharge well away from the wall (University of Minnesota Extension).
• A void that formed over time as the ground under the edge shifted.

If the cause is still active, such as a downspout dumping beside the walk, raising the slab without fixing the water may just be a delay. That is true whichever method you choose.

When is lifting worth discussing?

It is worth a conversation when the slab is in one piece, or cracked but still sound, when the settlement is the main problem, and when you want to avoid tearing out concrete that is otherwise in good shape. A flat, structurally sound slab that dropped an inch at one end is the kind of case a contractor may reasonably propose lifting for.

Keep in mind how concrete behaves. Most random early cracks rarely affect structural integrity or service life (NRMCA). A hairline crack is not a reason to replace a slab. A slab with a step between two halves is a movement problem, and lifting or replacing is on the table.

When does replacement make more sense?

Replacement is usually the honest answer when:

• The slab is broken into several pieces, or crumbling at the surface.
• The slab has moved a lot, and the base under it is clearly bad.
• The slab was poorly built to begin with, for example without proper joints. Most cracking comes from construction practices such as missing joints, poor subgrade preparation, watered-down concrete or inadequate curing (NRMCA).
• The slab is a panel in a sidewalk and the City or a notice is asking for a full panel replaced.

Replacement lets you fix the base, add proper joints and resolve drainage at the same time. That is the main thing lifting cannot do, because lifting leaves the slab as it was built.

What about permits and the public sidewalk?

Work in the public way, including the sidewalk, curb, alley and parkway, needs a CDOT permit separate from any building permit (City of Chicago). That applies to any method, whether you lift or replace. A concrete walkway on your own lot that is not in the public way, no more than 30 inches above adjacent grade and not over a basement does not need a building permit under the City's list (City of Chicago). Rules change, so confirm with the Department of Buildings or CDOT before work starts.

What if you wait?

A small settled slab that is stable is not an emergency, and monitoring it with dated photos is a fair choice. A step between slabs is a trip hazard, though, and it can grow if water keeps getting under the edge. Waiting makes sense when the lift or drop is small and not changing. It stops making sense when someone has already stumbled, when the gap under the edge is opening, or when a notice has arrived. If you are unsure, an assessment costs you a visit, not a commitment.

What to ask before you decide

1. Why did this slab settle, in your opinion, and how can you tell?
2. What are you doing about the water or the base so it does not settle again?
3. If you lift it, what is your plan if it settles again, and what is in writing?
4. If you replace it, what goes under the new slab and where do the joints go? See why the lines are there.
5. Is the slab in the public way, and who pulls the permit?
6. Why this method over the other one?

If someone gives you a price or a method before they have looked at the slab and asked about the water, that is your answer about the assessment.

So, lift it or replace it?

Honest answer: it depends on the slab and the reason it sank, and the choice is yours to make with a contractor who looked at it. Akats does concrete flatwork in Chicago, and our concrete team will give you a straight read on whether yours is a lift candidate, a replacement, or a downspout and a wait. The concrete guide has the rest of the picture.

Common questions

This article explains general rules and common situations in Chicago. It is not legal advice, and City rules change. For a specific property, check with the Chicago Department of Buildings or a licensed professional.