Almost everything that goes wrong with a paver installation goes wrong underneath it, during a stage that lasts a day and then disappears under several tons of stone and brick. By the time the symptoms show, the evidence is buried. That is the whole problem with this trade: the part that decides whether your patio is still flat in fifteen years is the part you are least able to inspect.
What follows is what we look at when we are called out to somebody else's work. Some of it you can check while the job is running, which is far more useful than checking afterwards. Some of it you can check on a surface that has been down for years, to work out whether you are looking at a maintenance problem or a build problem, because those have very different price tags.
1. Nobody asked about your soil
This is the earliest warning and the most reliable one, and it happens before anyone picks up a shovel.
Base depth is not a fixed number. It depends on what the surface will carry and what it is sitting on. Expansive clay, which shows up across parts of the San Fernando Valley and in Simi Valley, swells and shrinks with moisture and needs more base than the sandy, well-draining ground in Oxnard. A driveway needs more than a patio because a two-ton car turning its wheels while stationary grinds at the surface in a way that foot traffic never does.
So when a contractor quotes the same base depth for every job in Southern California. They are quoting from a template rather than from your property. Ask any bidder two questions: what base depth are you using on my lot, and why that depth. You do not need to be able to evaluate the answer technically. You need to find out whether they have an answer at all.
2. The base went in as one lift
Base rock is compacted in layers, lifts, because a plate compactor can only densify so much material below it. Dumping six inches of rock and running a compactor over the top gives you a firm crust with loose material underneath, which is worse than useless because it looks correct.
If you are around while the work happens, this one is genuinely easy to observe. You should see material go in, get compacted, then more material, then compacted again. If the base appeared in a single pour and the compactor came out once. That is a real problem and it is worth raising immediately rather than after the pavers are down.
3. There is no edge restraint
A paver field is held together by the fact that every unit is pressed against its neighbours. At the outside edge there is no neighbour, so something has to take that load, a plastic or metal edge restraint spiked into the base, or a concrete haunch.
Without it, the perimeter units migrate outward under traffic. It happens slowly, over a season or two, and the first thing you notice is that the joints near the edge have opened up and sand is disappearing from them. Then the whole field loosens progressively inward, because each row that loses support stops supporting the next.
On a finished surface you can usually see whether restraint exists by looking at where the pavers meet soil or planting. If the edge units simply stop at dirt with nothing holding them, that job has a countdown running on it.
4. The cuts are bad, and they are in the wrong places
Cut quality is the most visible sign of care, which is exactly why it is worth reading. Look along any edge where the pattern meets a wall, a step or a curve. The cuts should be clean and consistent, and. This is the part people miss, they should be roughly the same size as each other.
A run of cut pieces that get progressively narrower means the field was not laid out from a proper centreline. The installer started at one edge and let the error accumulate until the last row was a sliver. That is a planning failure, and while it is only cosmetic, it tells you something about how the rest of the job was approached.
Very small slivers are also a structural weak point: a piece under about a third of a full unit has little to interlock with and works loose faster than anything else on the surface.
5. Water goes the wrong way, or nowhere at all
Every hard surface needs fall. A paver patio is more permeable than concrete because the joints let some water through, but it is not a drain, and on a compacted base most of the water still runs across the top.
The test is trivial and you can do it with a hose. Run water onto the surface and watch. It should move away from the house, consistently, without pooling. Water that sits in a low spot after the flow stops means the grade is wrong there, and standing water on a paver surface is not just a puddle, it works joint sand out and softens the base beneath it over time.
On the sloped cities in our service area this matters far more. In Glendale, Monrovia and the foothill lots around La Cañada Flintridge, a surface that channels winter runoff toward a foundation is a genuinely expensive mistake, and it is one that a beautiful-looking patio can hide until the first wet year.
6. The joints are empty, or were never properly filled
Joint sand is structural. It is what transfers load from one paver to the next, and a surface with depleted joints is a surface where every unit is working alone.
On a new installation the sand should sit close to the bottom of the paver chamfer, the small bevel around the top edge of each unit, and it should have been compacted in and activated, not just swept across and left. On an older surface, sand sitting visibly low, or joints with growth in them, means it is time to re-sand and reseal.
The important distinction: low joint sand on an otherwise flat, solid surface is maintenance, and it is inexpensive relative to the alternative. Low joint sand on a surface that also rocks underfoot is a symptom of something structural, and re-sanding it will not fix anything.
7. Units move when you stand on them
This is the one test that separates a maintenance problem from a build problem, and it costs nothing.
Walk the surface deliberately, putting weight on individual pavers, particularly near edges, near the middle of large open areas, and anywhere a vehicle turns. Nothing should rock. A unit that tips slightly under your weight has lost support beneath it, which means either the base was inadequate there or water has been washing fines out of it.
One rocking paver in one spot is usually a localised repair, lift it, correct the base, reset it, and it is invisible afterwards. Several separate areas moving is a different conversation, because it suggests the base is thin across the whole installation. Patching that repeatedly becomes a treadmill, and after a few laps you will have spent more than a proper rebuild would have cost.
What to do with what you find
If the job is still running, raise it immediately. Contractors are far more willing to correct a base while the base is still visible than to relift a finished surface, and a reasonable one will not object to being asked what depth they are working to.
If the job is finished and you are finding these signs, get someone to look at it who is not the person who built it. What you want to establish is which of the two categories you are in: joints and sealing, which is routine and cheap, or base, which is not. We do this assessment on installations we did not build, and we will tell you when the honest answer is that sealing it would be taking your money for something that will not solve your problem.
And if you are still choosing a contractor, the single most useful question you can ask is the base one. Not because you will grade the answer, but because the people who cut that corner do not have an answer ready.
Related reading: how pavers and concrete really compare in this climate, and what actually moves the price of a paver project.