The Layer Nobody Photographs: Specifying a Patio Sub-base That Won't Let You Down

Nobody photographs the sub-base. There's no camera roll full of compacted crushed limestone, and no client has ever asked for a close-up of the separation geotextile. Fair enough — it's grey, it's muddy, and by Friday there's a very nice patio sitting on top of it.

But if you hang around enough failed patios, a pattern emerges fast. The slabs that rock, the joints that crumble, the corner that's dropped 15mm since the summer of 2022 — the culprit is almost never the paving. It's what's underneath, or rather what isn't.

This is the layer that decides whether you get ten years out of a patio or thirty, and it's the layer most homeowners never think to question.

Why the invisible bit does all the work

A patio is a load-spreading exercise. Every chair leg, every barbecue, every person standing on one foot at a garden party sends a point load down through the slab and into whatever is below. Done properly, that load fans out through a compacted granular layer and dissipates harmlessly into the ground. Done badly, the load punches a small hollow, the slab loses support at one corner, and the whole thing starts behaving like a see-saw.

UK ground makes this harder than it sounds. Clay subgrades shrink and swell with the seasons, frost gets into anything that holds water, and plenty of gardens sit on 150mm of topsoil over a builder's rubble tip from the 1960s. A sub-base has to bridge all of that.

Type 1: the default, and usually the right call

For a domestic patio, MOT Type 1 is the sensible starting point. It's a well-graded crushed aggregate — typically limestone or granite, 0-40mm with the fines left in — specified under the Specification for Highway Works Clause 803. Because it contains fines, it binds up under compaction into a tight, stable layer. It is not free-draining, and it isn't meant to be.

Depth for pedestrian use:

• 75mm is the bare minimum for a lightly used patio on firm ground
• 100mm is where I'd want to be for anything in normal family use
• 150mm is the sensible answer on soft ground, made-up ground, or a former vegetable bed

Compact it in layers no deeper than 75-100mm with a vibrating plate, not in one 200mm dump that only gets the top 40mm consolidated. Shove a spade into it afterwards. If it moves easily, it isn't compacted.

Type 3: permeable, often mis-sold

Type 3 (Clause 805) is an open-graded, no-fines sub-base. Water passes through it, which makes it the right material for permeable paving systems — but only when the whole assembly is designed as one, with open joints, a drainage outlet, and geotextile above and below.

Where it goes wrong is when someone is told "Type 3 is the modern one, it drains" and it gets buried under a conventional patio of solid slabs on mortar joints. Now you've built a reservoir. Water that gets in through the joints can't get out sideways fast enough on a clay subgrade, the layer stays saturated, and frost does the rest. It's also easily contaminated by fines and easy to over-compact into something that no longer drains at all.

Ask directly: is this a permeable system, or a standard patio? The sub-base should match the answer.

The substitutes that should stay in the skip

Crusher run or scalpings are cheaper and wildly variable. Some are fine; plenty are too fine-heavy and migrate under load. If it's coming off a grab lorry with no paperwork, you don't know what you're getting.

Hardcore and brick rubble are not a sub-base. They're a future settlement hollow with sharp edges.

Sharp sand on its own — no. That's a bedding layer, roughly 30-40mm, and it only works when it's sitting on something solid.

Geotextile: cheap insurance, not a substitute for depth

A woven or non-woven separation membrane over the subgrade costs very little and stops fine particles pumping up into the Type 1, which keeps the sub-base working as designed. On clay it's genuinely worth having. What it won't do is make up for 60mm of badly compacted material, so don't let it be offered as an upgrade in place of proper depth.

Fall is built below, not on top

This is the bit that catches people out. A patio needs a fall of roughly 1:60 to 1:80 away from the house — about 15-25mm across a three-metre run. That fall belongs in the sub-base.

If the sub-base comes out level and the fall is created by varying the bedding sand, you end up with 20mm of grit at the house end and 70mm at the far edge. Thick bedding is unstable bedding. That's precisely how slabs end up rocking two winters later. Get the granular layer to the right levels, then lay slabs on a consistent 30-40mm of compacted sharp grit.

Questions worth asking before work starts

• What sub-base are you using, and to which specification?
• How deep is it, and how many compacted layers is that?
• Is there geotextile going down, and over what?
• How are you achieving the fall — in the sub-base or the bedding?
• Does the sub-base extend past the paving edge, or is it being held by proper restraint?
• What's the subgrade like under the topsoil?

What failure actually looks like

It rarely arrives dramatically. First a slab sounds hollow when you tap a corner. Then a joint opens a couple of millimetres. Then water starts sitting in one spot after rain, and by year six you've got a rocking slab, a hairline crack and a repair bill that would have covered the geotextile and an extra 50mm of stone several times over.

Get the layer nobody photographs right, and the patio on top quietly does its job for decades.