What Goes Under a Driveway: The Sub-Base Explained

For a domestic driveway carrying cars, the usual dig is 250mm to 300mm below finished level. That allows for 150mm of compacted sub-base, plus the laying course and the surface itself. Where a van, motorhome or regular deliveries are expected, the sub-base thickens to 200mm or more and the dig deepens accordingly.

Finished level matters as much as depth. The new surface must sit at least 150mm below the damp-proof course of the house and must not block airbricks. Getting this wrong causes damp problems inside the property that are far more expensive than the driveway.

Everything excavated has to leave site. On a typical Essex two-car drive that is a substantial tonnage, and it is why any quote that does not mention muck-away or skips is incomplete.

Once dug out, the exposed ground — the subgrade — should be inspected and compacted. Soft spots are dug out further and filled with stone rather than covered over. On the shrinkable London Clay that runs under much of central and western Essex, the subgrade holds water and softens badly in winter.

A geotextile separation membrane laid across the formation stops the clay and the stone mixing. Without it, over a few wet winters the fine clay pumps up into the sub-base and the stone sinks into the clay, and the effective thickness of your structural layer quietly halves.

The membrane is inexpensive. Leaving it out is one of the most common corner-cuts and one of the most consequential.

MOT Type 1 is a graded crushed stone with a range of particle sizes from around 40mm down to dust. That range is deliberate: the smaller particles fill the voids between the larger ones so that, when compacted, the material locks together into a dense, load-spreading slab.

Single-sized gravel, builders' rubble or unscreened hardcore cannot do this. Rubble in particular contains soft material, plaster and timber that break down and leave voids. If a quote specifies 'hardcore' rather than a named graded aggregate, ask exactly what is being delivered.

Recycled Type 1 is a legitimate and more sustainable option where it is properly processed and graded to specification. The important thing is the grading and the compaction, not whether the stone is virgin or recycled.

Stone must be compacted in layers. Tipping 150mm of Type 1 in one go and running a plate over the top compacts perhaps the upper 75mm and leaves the rest loose. The correct method is two layers of roughly 75mm, each thoroughly compacted with a vibrating plate or, on larger areas, a roller.

Well-compacted Type 1 sounds and feels different: it rings under a boot heel, a plate compactor stops sinking into it, and you cannot scuff a heel through the surface. Loose stone gives underfoot and shows a wheel track when a barrow crosses it.

Water plays a part too. Type 1 compacts best at the right moisture content — bone dry material will not lock together, and saturated material cannot be compacted at all. That is why good contractors reschedule groundwork rather than push on in torrential rain.

A flexible driveway surface — blocks, resin, tarmac — relies on being confined at the edges. Without restraint, the whole field of material slowly spreads outwards, joints open, and the middle loses its interlock.

Restraints can be a concrete kerb, a haunched edging course, or the house and garage walls themselves where they run along the boundary. Whatever the detail, it must be bedded and haunched in concrete on its outer face, and that concrete must have hardened before the surface is trafficked.

Resin bound driveways need particular attention here, because the resin has to be contained while it is laid. A crisp, solid perimeter is both a structural requirement and the difference between a neat finish and a ragged one.

Ask what depth they intend to excavate, what happens to the arisings, whether a separation membrane is included, what aggregate is specified and to what compacted thickness, how it will be compacted and in how many layers, what forms the edge restraint, and where surface water will go.

Any competent contractor will answer all seven without hesitation, because they are the questions we ask ourselves when pricing. A vague answer to any of them is the clearest signal you will get about the job you are about to buy.