Groundwork Preparation for Driveways and Patios

Why the Ground Beneath Your Paving Matters

Most paving failures aren't the paving's fault. Ruts in a block driveway, rocking slabs, a sunken corner beside a patio step — almost always the problem sits underneath. Paving is a thin, brittle layer that only performs because load spreads through it into a compacted subbase and then into firm, undisturbed subsoil. Miss out any stage and the surface will start to move, usually after the first wet winter or the first hard frost. The good news is that good groundwork is straightforward. It just has to be thorough.

Strip the Topsoil Completely

Topsoil is organic, spongy and unpredictable. It has no place under a driveway or patio. Dig it off properly, and resist the temptation to leave a couple of inches behind "for drainage" — it will rot down, settle and drag your new surface with it.

• Expect 150–300mm of topsoil on most domestic plots, deeper where a garden has been cultivated or where old borders and lawns have built up over decades.
• Strip down to clean, undisturbed subsoil — clay, chalk, sand or gravel.
• Keep good spoil separate for levelling beds elsewhere, but never reuse it as fill beneath paving.
• Watch for old paths, rubble, tree roots and buried services. Cutting into the root plate of a mature tree can destabilise it, so take advice if one sits close to the excavation.
• Sort out muck away and skip permits before you start digging, and check whether a mini digger can actually get to the site.

Dig to Formation Level and Check the Subsoil

Formation level is the bottom of the excavation, the surface your subbase sits on. Work it out backwards from the finished paving height.

• A block paving driveway taking cars: 60–80mm pavers, 30–40mm of bedding sand and around 150mm of compacted Type 1, so roughly 250–300mm below finished level.
• A patio for foot traffic: 25–40mm of bedding over a minimum of 100mm of compacted Type 1, and more where the ground is soft.
• Add the depth of topsoil you strip, plus any extra dig needed to reach something firm.

Probe the formation with a fork or a steel rod. Any soft spot needs digging out and replacing with compacted stone. Heavy clay is workable, but it turns to slurry underfoot in the rain, so a geotextile separation layer can save a lot of grief. Don't dig the whole area and leave it open — rain, foot traffic and wheelbarrows will ruin a good formation in an afternoon. Dig, check, compact and cover in the same dry spell where you can.

Set Your Levels and Falls

Water has to go somewhere, and it needs a clear route off the paving. A fall of about 1:80 away from the building — roughly 12mm per metre — is comfortable to walk on and moves water reliably.

• Finished paving should sit at least 150mm below the damp proof course. Two brick courses is the usual rule of thumb.
• Never let the surface slope towards the house, however slight the gradient feels on paper.
• On driveways, use a crossfall towards a channel drain, a gully or a permeable edge.
• Set pegs and profiles along the edges and run taut string lines between them. A laser level earns its keep on anything larger than a small patio.
• Check levels on the formation, again on the compacted subbase, and again on the bedding. Small errors multiply fast.

Lay the Subbase in Layers and Compact It Properly

Type 1 crushed stone is the standard choice, and it's a good one: angular, well graded and it binds together when compacted. Buying cheap, rounded or single-size aggregate is a false economy.

• Spread in layers no more than 75–100mm thick before compacting. Thick layers leave loose material trapped underneath that never tightens up.
• Compact every layer with a vibrating plate for small areas, or a roller in the 1.5 to 3 tonne range for a driveway. Eight to ten overlapping passes, working from the edges inwards.
• Dampen the stone slightly if it dusts up, but never soak it.
• Allow for settlement — Type 1 loses roughly a fifth of its loose depth once compacted, so order and spread accordingly.
• Permeable paving needs open-graded stone with no fines, laid over a geotextile, so water can pass through to the ground below.

When you're finished, the subbase should be firm, level and unyielding. A heel shouldn't leave a mark, and a spade shouldn't sink in.

Edge Restraints, Drainage and Bedding

Edge restraint is not optional. Without it, blocks and slabs creep outwards, joints open and the whole surface loosens. Use concrete haunching, kerbs or a proper edging system, bedded and backed with concrete so the restraint is locked against the subbase, not just sitting on top of it.

Drainage needs thinking about before you dig, not after. Clay subsoil won't soak anything away, so a soakaway needs a percolation test to prove it will work; if it won't, you'll be looking at a connection to the surface water system or a fully permeable construction. Channel drains across a driveway threshold are often the neatest answer, and larger front gardens may need to follow sustainable drainage principles.

Finally, the bedding. For block paving, use 30–40mm of sharp sand, screeded to your falls and left undisturbed — walk on boards, not on the sand. Stone slabs generally want a damp mortar bed, typically five parts sharp sand to one part cement, or a full bed for thicker flags. And don't lay any of it on frozen ground. Frozen sand and frozen mortar expand as they thaw, and the surface above them will settle unevenly for years afterwards.