Where sinking shows itself first
A sunk slab rarely shows up as one dramatic drop. More often it is a step you notice at a joint that was not there before, a corner that sits a little lower than the panel next to it, or water standing in a spot that used to drain fine after every rain.
A garage door that used to close flush against the slab and now catches on one side is often the first thing people actually notice, well before they look closely enough to see the step underneath it. Anything that was fitted to the slab back when it was flat tells on it the same way. A gate that drags. A downspout extension that no longer sits tight to the ground. A bottom step that suddenly feels taller than the rest of them.
What is actually moving underneath
The concrete itself is almost never the reason a slab sinks. What moved is the subgrade, the ground the slab was set on. Soft or organic soil that was never properly compacted settles under the weight of the slab over time. Red clay, common under a lot of ground here, takes on water and swells, then shrinks back down as it dries out, which shifts whatever is sitting on top of it season after season.
Water is the other common cause. Runoff from a downspout, or a grade that pitches the wrong way, can run under an edge and wash the fine material out from underneath, leaving a void the slab eventually settles into. A void does not have to be large to matter. Once the ground stops touching the underside of a corner, that corner is holding its own weight on nothing, and down it comes.
- Soil that was never fully compacted before the pour
- Clay that swells and shrinks with moisture through the seasons
- A void washed out by water running under an edge
Telling a slab that dropped from one that was never flat
Not every low spot is settlement. Some slabs were finished slightly off from the day they went in, and nobody notices for years, until a puddle finally gets big enough to be annoying. Standing over it, the two look about the same. They are not the same problem, and they do not lead to the same conversation, so it is worth ten minutes to work out which one you have.
The simplest check is a string. Pull a mason line tight across the panel, corner to corner, an inch or so above the surface, and watch the gap under it as you go. A slab that was flat and then settled tends to dish toward one corner or one edge, with the gap opening up the further you move that way. A slab that was screeded off a little out of true usually runs a fairly even plane, just tilted, with no sag in the middle of it.
Then read the joints. Everything poured that day was flat across every joint at the time, so a joint where one side now sits higher than the other is movement, and there is not much arguing with it. Check the joint on the far side of the low panel as well. If the surface steps down onto that panel from one side and steps back up on the other, the panel itself dropped and its neighbours stayed put.
The last check costs nothing but patience. Watch where the water goes after the next real rain. Water heading somewhere it never used to go, or sitting where it always used to run off, says the shape of the slab has changed since you last paid attention to it. Water that has run the wrong way every year since the pour is a different problem, and often a smaller one. It is worth knowing which of the two you have before anybody starts talking about taking concrete out.
Fixing it means fixing the ground, not just the slab
A real fix means taking the affected section out, dealing with whatever caused the ground to move, water, compaction, or grade, and rebuilding the base before anything gets repoured. A fresh layer poured over the top leaves that same ground sitting there untouched, which is why it is not the shortcut it looks like. That whole process is what our tear out and replacement page walks through in full.
It is also usually possible to replace only the section that sank rather than the whole driveway or patio, once the base underneath that section has been dealt with properly.