Slabs are the easy part of a patio to judge. You can see them, feel them, pick a colour, compare a price per square metre. So that's where most quotes get compared, and where most corners get cut on the parts you can't see. But a patio doesn't fail at the surface. It fails from the bottom up — the ground moves, the bed gives, the joints let go — and by the time the slab itself is the problem, something under it went wrong a long time ago. This is the case for spending your attention, and your money, on the three things beneath the stone.
A patio is a building. The sub-base is the foundation
Think of hard landscaping the way you'd think of a house. Nobody judges a house by its render. They ask what the foundations are on. Paving is no different: the slab is the render, and the sub-base is the footing that decides whether the whole thing holds its line.
The sub-base is a compacted layer of MOT Type 1 — crushed stone graded from around 40mm right down to dust, so it locks together into a stable, free-draining mat rather than a loose pile of gravel. For a domestic patio in normal ground we're laying and compacting it to at least 100mm, and 150mm where it takes more load or the ground's less certain. On clay, which moves as it wets and dries, it goes deeper still — 200mm is not unusual, because clay is the ground most likely to lift a patio it wasn't built to cope with. Much of Hampshire sits on clay, so this is a real decision here, not a textbook one. Which ground you're actually building on varies a good deal across the area we cover. See soils across the Solent for the detail.
The number that matters more than the depth is how it's compacted. Loose stone tipped in and tamped once will settle unevenly for the first year, and every slab above a soft spot starts to rock. It has to go in and be whacked down in layers — no more than about 75mm at a time under a plate compactor — so the whole depth is dense the whole way through, not just crusted on top. It's slow, it's the least glamorous day on any job, and it's the day that decides everything. That's the part of the paving and driveway work that never appears in a photograph and matters most.
A full bed, not five dabs
Here's the one that separates a proper job from a fast one. The slab should sit on a full, continuous bed of mortar — the whole underside supported, edge to edge. The British Standard for this work, BS 7533, calls for exactly that: full contact bedding.
What happens instead, far too often, is "spot" or "dot-and-dab" bedding — five blobs of mortar under each slab, one in each corner and one in the middle, and the slab dropped on top. It's quicker, it uses less material, and on the day it looks identical. But it leaves the underside of the slab mostly hollow. Those voids are where the trouble lives. A slab bridging air over dabs of mortar cracks under a point load — the leg of a table, a heel, a wheelbarrow. And the voids fill with water, which brings us to the part of the British winter that does the real damage.
Water sitting in a void under a slab freezes. Frozen water expands. It lifts the slab, cracks the bed, and works the joint loose — and then it thaws, drains a little, and does it again on the next cold night. That freeze-thaw cycle is the single most common way a patio comes apart in this country, and a full mortar bed with nowhere for water to pool is the thing that defeats it. Five dabs practically invites it in.
The mix itself has to be right
A full bed still has to be the right mortar, mixed right. This is where "more cement must mean stronger" quietly does harm. A bed that's too rich in cement goes rigid and brittle, and a brittle bed cracks as the ground beneath it moves — because ground always moves a little. Too lean, and it never gains the strength to hold, so it crumbles and the slab settles. The bed wants to be strong and slightly forgiving at once. That's a matter of the right proportion, the right consistency, and — the bit that gets skipped when it's cold or it's late — actually letting it cure before the patio takes weight.
None of that is exotic knowledge. It's just decisions that each cost a little time and a little discipline, made correctly and repeatedly, by someone who'll still be on the job at four o'clock on a wet Friday. That's the whole argument for one set of hands on every site: the spec only holds if the person laying it cares whether it does.
Joints are the last line, and the first to go
The joint is what people notice first when a patio ages — the dark line between slabs opening up, crumbling, sprouting weeds. It's also the seal that keeps water out of everything we've just talked about, so a failing joint isn't only cosmetic. It's the leak that lets the frost back in.
Loose kiln-dried sand brushed into the gaps is fine for a block driveway that's tightly interlocked; it's the wrong answer for patio slabs, where it washes out in the first heavy rain and takes the weeds' side thereafter. We point the joints properly — a mortar or a purpose-made jointing compound, worked in and struck, so the surface reads as one sealed plane. Done well it's the detail that still looks crisp when the patio's a decade old. It's also, not by coincidence, one of the "three winters" tests we hold our own work to: the joints shouldn't have opened up by the third one.
Falls and drainage: give the water somewhere to go
Everything above comes back to water, so the last decision is the most basic one: the surface has to shed it. A patio is laid to a slight, deliberate fall — a gentle slope away from the house — so rain runs off to where you want it, not back toward the wall or into a low spot where it stands. Get the fall wrong and even a perfect bed spends its life sitting in water; get it right and most of the freeze-thaw problem never gets the chance to start. On a bigger scheme that means thinking about drainage properly — channels, soakaways, where the run-off actually ends up — before a single slab is bedded.
What this costs, and why it's the cheaper number in the end
All of this is why the lowest quote is so rarely the lowest finished cost. The sub-base depth, the full bed, the right mix, the pointed joints — every one of them is time and material you could leave out to shave the price, and every one of them is why the studio isn't the cheapest and doesn't pretend to be. The price reflects the spec. What you're buying isn't a nicer slab; it's a patio that's still flat and tight in twenty-five years instead of being relaid in five. We break down where that money actually goes in our piece on what's in a hard-landscaping quote, and the material choice sitting on top of it all is covered in porcelain versus sandstone.
You can see the difference in the ground on the work we've kept records of — the full hardscape in Warsash and the two-tier porcelain terrace in Fareham were both built base-up to this spec, which is the reason they'll still be sitting flat long after the photos look dated.
If you're weighing up quotes and one is a good deal cheaper than the rest, it's worth asking exactly what's going in under the slab — the sub-base depth, and whether it's a full bed or dabs. If you'd rather just have it built properly the first time, send a brief through here and I'll come and take a look.