Why patios sink,
and why it's
never the pavers

When somebody calls about a patio that's dropped, they almost always describe it as a problem with the pavers. It never is. A concrete paver is a 8,000 psi unit that will outlast the house. What moved is what's underneath it, and there are only four ways that happens.

1. Not enough base

The base is the structure. Everything above it is a wearing surface. In a freeze-thaw climate a residential patio wants eight inches of compacted crushed stone; a driveway wants twelve. We routinely open up failed patios and find three or four inches, sometimes laid straight onto topsoil.

The reason this is such a common corner to cut is that it's the largest single cost in the job and the only one that becomes invisible the moment the surface goes down. Removing four inches of base across 600 square feet saves a contractor a couple of thousand dollars in stone, machine time and disposal. It also removes the thing holding your patio up.

2. Base that was never compacted in lifts

Dumping eight inches of stone and running a plate compactor over the top does not give you eight inches of compacted base. It gives you about three inches of compacted base sitting on five inches of loose stone. Plate compactors have a limited depth of influence — for the machines used on residential work, roughly three to four inches.

So base goes down in lifts: three or four inches at a time, compacted, then the next lift. It's slower and it's the stage most likely to be skipped when a crew is trying to finish before the weekend.

3. No separation between the stone and the soil

Northern New Jersey is largely clay. Clay and crushed stone do not stay separate on their own. Under load and through wet-dry cycles the fines migrate up into the voids in the base and the stone works down into the clay, and over a few years an eight-inch base quietly becomes a five-inch base with a muddy transition zone.

A woven geotextile between the two costs a fraction of a percent of the job and stops the whole mechanism. It's the single highest-return item on the material list, and it's routinely left off.

4. Water that was never given anywhere to go

The other three problems all get worse when water is involved, and water is the reason settlement usually shows up in one specific area rather than uniformly. A patio pitched back toward the house, or a downspout discharging at the edge of the paving, will push water down through the joints and into the base repeatedly. In winter that water freezes and heaves; in spring it thaws and the void stays.

The fix is boring: pitch everything away from the house at an eighth of an inch per foot, get roof water into a pipe and carry it past the paving, and put a dry well or a daylight outlet somewhere the water is welcome.

How to tell which one you have

• A dip in one specific spot, often near a downspout or a corner — almost always water.
• General, even sag across a whole area — not enough base, or uncompacted lifts.
• Edges spreading and joints opening at the perimeter — missing or failed edge restraint, which is a different and much cheaper repair.
• Sinking that tracks exactly where a trench was dug — a utility trench that was backfilled without compaction. Common, and not the paver contractor's fault.

What a repair actually involves

The good news about pavers is that they come apart. On a settled area we lift the units, set them aside, dig out the failed base, rebuild it properly, re-screed and relay the same stone. Colour matches because it's the original material. A 100 square foot area is typically a one to two day job.

The bad news is that if the base was wrong across the whole patio, fixing one area buys you a few years before the next one goes. We'll tell you which situation you're in before you spend anything, and sometimes the honest answer is that a repair is money spent twice.