Why Is My Patio Sinking? The Four Real Causes

Because something under it has moved, compacted or washed away. A patio is only as level as what it sits on, and the surface is the last thing to show a problem that started below.

There are four real causes, and they leave different fingerprints:

• A thin or poorly compacted base. Aggregate that was dumped and raked rather than compacted in lifts keeps consolidating under foot traffic, furniture and weather for years.
• Water washing out the fines. Concentrated water, usually from a downspout or a low corner, carries the fine particles out of the base and the soil beneath it, leaving voids that the surface drops into.
• Moving ground. Clay that softens when saturated, or shale that swells and shrinks, moves a patio that was not built to isolate itself from it.
• Failed edges. Without a working edge restraint, a paver field spreads outward, and a field that spreads also drops.

Often it is two at once. Water finds the weak spot in a thin base, and the thin base lets it.

Read the pattern. Where the patio sinks, and when, says more than how much.

Lay a long straight edge across the low area and look at the joints either side. Joints that have opened point to spreading or edge failure. Joints that are tight while the surface dips point to something happening below the base.

If the whole patio is going down, probably. The base is the part of the job nobody sees on handover day, which is exactly why it is where cheap quotes save money.

A proper base is excavated below the soft topsoil, placed in layers, and each layer compacted before the next goes on. Placing it all at once and running a plate compactor over the top compacts the top and leaves the rest loose, and the loose part settles for years. Depth depends on the ground and the use: a patio over well-drained ground and one over wet clay should not have the same base.

If you are looking at quotes for a replacement, ask each contractor how deep they excavate, what goes back in, and how it is compacted. The answers will separate them faster than the price.

If the dip is local, and there is a downspout, a valley in the roof, or a low corner near it, almost certainly. Concentrated water moving through a base carries fines with it. Every heavy rain takes a little more, and the voids grow until the surface drops into them.

The fix is the water first and the patio second. Relaying pavers over a base that is still being washed out buys a couple of years. Extend the downspout to a proper outlet, give the patio a fall that sheds water off an edge that can take it, and then repair the base.

Possibly, and where you live decides which kind. Northwest Arkansas sits on two different geological provinces twenty minutes apart.

North of Fayetteville, around Bentonville, Rogers, Springdale, Bella Vista, Centerton and Cave Springs, you are on the Springfield Plateau. The rock is the Boone Formation, limestone interbedded with chert, and where it has weathered you get red clay full of chert fragments. The rock is stable. The clay holds water and loses strength when saturated.

Around Fayetteville, Farmington, Prairie Grove and Elkins you are on the Boston Mountains, where the Fayetteville shale swells when wet and shrinks when dry. University of Arkansas geologist Walter Manger called that cycle the culprit in the area's construction problems: "Limestone is good. Shale is bad."

Soil is rarely the whole answer, though. A properly excavated, compacted and drained base is what isolates a patio from the ground's movement. Where a patio moves with the seasons, the base was not doing that job.

Because the ground there was dug out when the house was built and filled back in afterwards. Foundation backfill is often loose, and it settles for years. A patio built on it without the backfill being dug out and properly compacted follows it down.

This one matters more than the others, because a patio sinking toward the house also tilts water toward the foundation. Fix the fall, not only the level.

A localised dip in an otherwise sound paver patio is usually fixable, and it is not an expensive job. The units come up, the cause is dealt with, the base is rebuilt and compacted, and the same pavers go back down. Done properly, you cannot see where the repair was.

Widespread settlement, or a patio laid on almost no base, is not worth chasing. You fix one area and the next goes, because the cause is the whole installation. Rebuilding once on a proper base costs less over ten years than three rounds of repairs.

A settled concrete slab is different. It cannot be reset; it can only be lifted from below or replaced, and a slab that settled because of washed-out ground will keep doing it unless the water is dealt with. That is one of the real differences between a modular surface and a slab.

Most sinking is slow and gives you time. Three situations do not. A patio sinking toward the house is sending water at the foundation, and the longer it runs the worse the fix. A hole or void that keeps growing, especially after rain, needs investigating before someone steps into it. And a patio sinking beside a retaining wall can be a sign the wall or the ground behind it is moving.

Everything else, a gentle dip, a few lipped units, is worth fixing properly rather than quickly. Plan it for dry weather, when the base can be rebuilt and compacted the way it should have been.

It changes what you can do about it. Natural flagstone on a sand or gravel bed behaves like pavers: it can be lifted and reset, though irregular stones take longer to fit back. Flagstone mortared onto a concrete slab behaves like the slab, so if the slab under it has settled, the stone comes up with it and the job becomes a replacement.

"More sand will level it." Adding bedding sand under a dipped paver lifts it for a season. The sand is not a structural layer, and the thing that caused the dip is still there.

"It has settled, so it will stop." Sometimes, if the cause was a one-off like loose backfill that has now consolidated. If water is washing the base out, it will not stop until the water does.

"Sealing the pavers will stop it." Sealer protects the surface and can stabilise joint sand. It does nothing for the base below.

"Pouring concrete under the pavers fixes it for good." A rigid slab under a flexible surface changes how the whole thing drains and moves. Done as a designed detail, it has its place. Done as a patch, it creates a hard spot next to a soft one, and the joint between them is where the next problem appears.

A useful site visit looks under the problem, not just at it. That means walking the patio with a straight edge, looking at the joints, finding every downspout and where it discharges, checking the fall with a level, and looking at the edges for restraint. Often it means lifting a few pavers in the worst spot to see the base: how deep it is, what it is made of, and whether it is wet.

Ten minutes with a few pavers up tells you more than an hour of looking at the surface. If someone quotes a repair without looking underneath, they are guessing at the cause.

• What do you think caused it, and how did you tell?
• What will you do about the water, not just the pavers?
• How deep will the rebuilt base be, and how will you compact it?
• Can you reuse my existing pavers?
• What happens if you find the base is bad further than expected?

• Excavate through the topsoil and any soft spots, and judge the depth by the ground, not by habit.
• Place and compact the base in lifts.
• Give the surface a fall away from the house, and give the water somewhere to go.
• Take every downspout to a proper outlet, not the patio edge.
• Install a real edge restraint, anchored into the base.
• Dig out and recompact foundation backfill where the patio meets the house.

None of that is exotic. It is simply the part of the job that decides whether the patio is still level after three Arkansas winters.