Craft
What Is Under a Patio, and Why It Decides Everything
You will never see any of it again after the day it is covered, and it decides everything that happens to the surface for the next twenty years.
A terrace is a surface laid on a structure. Almost every conversation about one concerns the surface. Almost every failure concerns the structure.
What is actually down there
From the bottom up: undisturbed subgrade, excavated to depth and compacted. Then a base layer of graded aggregate, placed in lifts and compacted each time rather than dumped and rolled once. Then a thin bedding layer, screeded flat. Then the units. Then something at the perimeter holding it all from spreading outward.
Each of those has a way of being done properly and a faster way, and the faster way is undetectable on handover day.
Depth is not a preference
Excavation depth is set by what the ground is and what the surface will carry, not by habit. A terrace that only ever holds furniture and people is a different structure from a driveway. But in a climate that freezes and thaws repeatedly, even a light-duty surface needs enough well-drained material beneath it that moisture is not sitting in the frost zone.
Cutting depth is the easiest saving to make in a quote and the most expensive to correct, since correcting it means removing everything above it.
Compaction in lifts
Base material compacts properly only in layers. Fill the whole excavation and run a plate over the top and the upper few inches consolidate while everything below stays loose - which then settles over the first two or three years, unevenly, wherever loading happens to concentrate.
This is the difference that shows up as a dip near the doors, or a low spot that holds water after rain, on a terrace that was flat when it was finished, and it is the reason resetting a moved terrace starts with lifting a small area to look at the base.
Drainage happens in the base, not just on the surface
Surface falls send rain off the top. That leaves the water that gets through the joints, which on any permeable-jointed surface is a lot of it. If the base cannot pass that water somewhere, it holds it - and held water in a freeze-thaw climate is the mechanism behind most heaving.
So the base has to be free-draining material, and there has to be somewhere for the water to go. That second half is the one that gets left out. There is more in drainage.
Edge restraint, the cheap thing that fails loudly
Unrestrained edges migrate. Units at the perimeter shift outward under load, joints open behind them, sand escapes, and the movement works inward from there. It is inexpensive to do correctly and it is one of the most common causes of a terrace looking tired after a few seasons.
How to ask about this without being a specialist
Four questions do most of the work, and any competent contractor will welcome them:
- What depth are you excavating to, and why that depth here?
- What base material, and to what compacted depth?
- Is it compacted in lifts, and how many?
- Where does water in the base go?
Compare those answers across quotes. They will explain most of the price differences you are looking at (the rest is usually access and spoil), and they matter far more than which stone is on the sample board.