Formation of a Sub-Base up to 30 cm Thick

The thickness of a sub-base is not a guess. It depends on what will stand or drive on the surface and on the soil underneath. The same garden path might manage with a thinner build-up on sandy ground and need more on soft clay.

• Up to 30 cm is the usual build-up for footpaths, patios, garden areas and a driveway for ordinary cars on firm, well-draining ground. It is normally laid in two layers, each compacted on its own.
• Thicker build-ups are needed under heavier loads or on weak ground: a garage slab, a drive used by vans or delivery lorries, a car park, or where soft, wet soil has to be dug out and replaced with stone.

Under the foundation slab of a house, the thickness and make-up of the sub-base are set by the designer or structural engineer, often based on a ground survey. That is not a place to improvise. For a new building or a larger hard-standing area, check with your local building authority whether you need permission or approval, and consider drainage rules too, since many areas expect rainwater from new paving to soak away on your own land.

A contractor usually follows this order:

1. Measure the area, mark out the edges and set the finished height and fall.
2. Strip the topsoil and any soft, organic or waterlogged soil down to firm ground.
3. Dig out to the required depth and remove the spoil or store it on site.
4. Compact the bottom of the excavation and check for spots that sink or squeeze up mud.
5. On clay or damp ground, lay a separating geotextile so the stone is not pushed into the soil.
6. Spread the first layer of coarser aggregate to an even depth.
7. Compact it with a vibrating plate, working lengthways and then crossways with overlapping passes.
8. Add and compact the next layer until the full thickness is reached.
9. Trim the top layer to the set levels and fall, checking with a straightedge and a level as the work goes on.

Each layer is usually around 10 to 20 cm thick, depending on how heavy the compaction equipment is. A light plate only compacts a thin layer properly, while a heavier plate or roller copes with more. That is why 30 cm of stone is never tipped in one go: it would be firm on top and loose underneath.

A sub-base works like a sandwich: coarse, load-bearing stone at the bottom and finer material on top that can be levelled accurately. A typical build-up looks like this:

• Lower load-bearing layer of coarse crushed stone, for example 32/63 grading, which spreads the load and drains well
• Upper layer of well-graded crushed aggregate such as 0/32 (in the UK often sold as Type 1 sub-base), which fills the gaps and locks everything together
• Finishing surface that is trimmed to level, with a thin bedding layer added later for block paving or slabs

Why crushed stone rather than round river gravel? The sharp edges interlock when compacted, so the layer locks solid. Rounded stones roll over each other and never quite settle. The material should be clean, without clay or organic matter, otherwise it turns to mud after rain and loses its strength.

How demanding the work is, and so what it ends up costing, depends mostly on the site. The job is simpler with:

• level ground and firm, dry soil underneath
• good access for a digger and a tipper lorry right up to the area
• the option to keep the dug soil on your own land

It takes longer and costs more with:

• a slope that has to be levelled, or clay and waterlogged ground that needs geotextile or drainage
• tight access behind the house, where stone goes in by wheelbarrow and digging is done by hand
• spoil that has to be taken away a long distance
• wet weather, which slows everything down

• Layers too thick. The surface feels solid, but deeper down the stone stays loose and settles over time.
• Too few passes with the plate. In practice an area is often run over once and called finished. Compaction should continue until the plate no longer leaves marks.
• Bone-dry material. Completely dry aggregate compacts poorly. Light dampening helps, soaking it does harm.
• Topsoil or mud left at the bottom. Organic soil rots away slowly and the sub-base above it sinks.
• No fall and no drainage. Water sitting in the sub-base weakens it, and in winter frost can lift the paving.
• Skimping on depth under a drive. A thin sub-base under car wheels soon pushes into ruts.

For a small garden path or patio on firm ground you can do this yourself with a hired vibrating plate. Leave drives on soft ground and anything under a building to a professional.

1. Check for buried pipes and cables before you dig.
2. Mark out the area with pegs and string, and set the finished height with a gentle fall away from the house.
3. Dig out the topsoil and soft soil until you reach firm ground.
4. Compact the base of the excavation with the plate.
5. Lay geotextile if the ground is clay or damp.
6. Spread the first layer of stone no thicker than the plate can handle, usually around 10 to 15 cm for a hire plate.
7. Dampen the stone lightly and compact it in overlapping passes, first lengthways, then crossways.
8. Add the second layer and compact it the same way.
9. Check the levels with a straightedge and fill any dips before compacting again.

Look over the sub-base before any paving or concrete goes on it. After that, the only way to fix a fault is to dig everything up again. Check that:

• The total thickness matches what was agreed
• Topsoil and soft soil were removed down to firm ground
• Each layer was compacted separately, not the full depth at once
• Walking on it or running the plate over it leaves no marks and nothing sinks
• The levels match the plan and the fall runs away from the house or towards a drain
• A 2 m straightedge shows no obvious dips or humps
• The edges are compacted as thoroughly as the middle
• Under a building slab, compaction has been tested the way the designer specified, for example with a plate load test