Heavy sub-base under paving up to 50 cm thick

For ordinary block paving under a family car on good sandy or gravelly ground, a sub-base of roughly 25 to 30 cm is usually enough. A designer or experienced paving contractor will suggest a build-up close to half a metre when a thinner layer would not spread the load into the soil well enough.

Typical situations

• Clay or loamy soil that turns soft when wet and is prone to frost heave in winter
• An area used by vans, lorries delivering building materials, skip lorries or farm machinery
• A driveway or yard entrance that a heavy vehicle crosses now and then, for example a fuel delivery or a concrete mixer
• A high water table, or a plot where water stands for a long time after rain
• Made ground, an old backfilled trench or soil with organic material that has to be dug out and replaced
• A sloping plot where part of the area must be built up and levelled

Why does thickness matter so much? Crushed stone spreads the pressure of a wheel outwards, roughly in the shape of a cone. The thicker the layer, the larger the area of soil that carries the same load, and the less the soil underneath compresses. A thick layer also protects the soil from frost. Soil that does not freeze does not heave, so the paving does not ripple in winter.

Both items in the price list describe the same activity: delivering, spreading and compacting aggregate. The difference is not simply more stone. The method, the equipment and the risks all change.

• Number of layers: a sub-base up to 30 cm is usually compacted in one or two layers, while 50 cm normally means three, sometimes four, each compacted and checked separately
• Build-up: a thick sub-base often has a coarser grade or quarry rock at the bottom and a finer crushed stone on top, whereas a thin one is usually a single material throughout
• Excavation: with the bedding and paving, the total depth reaches roughly 60 to 70 cm, which means more soil to dig and remove and a higher chance of hitting buried services
• Equipment: 50 cm calls for a heavier reversible plate compactor or a vibrating roller, while a light hire-shop plate suits thin sub-bases
• Deliveries: at this thickness one square metre usually takes more than a tonne of aggregate, so the same area needs noticeably more lorry loads
• Time: every extra layer means another round of spreading, watering, compacting and measuring
• Edges: a thicker sub-base should extend beyond the kerb or edging, otherwise the edge breaks away under wheels, so the dig is wider

If you are not sure which variant to choose, the soil and the traffic decide. A bearing test on the formation, the compacted bottom of the dig, often shows whether 30 cm is enough or the thicker build-up is a must. Saving on thickness over weak soil rarely pays off, because repairing sunken paving means lifting everything back down to the soil.

Permission and buried services

A paved area, especially a parking space or a driveway joining a public road, may need permission under local building regulations. Some places also have rules on how surface water from front driveways must drain. Do not assume nothing is needed: check with the local building authority before work starts, and for a new entrance from a public road also with the council or highway authority that looks after the road. In a block of flats or on shared land, the building management or owners' association has the final say.

Before digging to around 70 cm, have the buried services located and marked: gas, electricity, water, drainage and telecoms. Service pipes and cables to houses are often shallower than they should be. If a gas pipe is damaged during the dig, leave the area straight away, do not use naked flames or switch electrical equipment on or off, and call the local emergency number or 112. Gas pipes may only be repaired by a qualified, registered gas engineer, and damaged cables only by a qualified electrician or the network operator.

A prepared formation

The sub-base goes onto the formation, the compacted bottom of the dig. It should already follow the fall, with no mud, roots or soft spots. Excavation, removal of spoil, kerbs and the sand bed are usually separate items in the price list. Check in the quote exactly what is included.

Logistics

• Access for the aggregate lorry and a place to tip the load
• Firm ground for the lorry to reverse over without damaging the lawn or a neighbour's kerbs
• A water supply for wetting the aggregate in dry weather
• Agreement with the neighbours if compacting close to their fence or wall
• A fixed level point from which the finished paving height is set, such as the garage threshold or front door

Well-graded crushed stone

The most common material is crushed stone with a continuous grading, for example 0/32, 0/45 or 0/63. In the UK the familiar equivalent is Type 1 sub-base, often called MOT. It contains both coarse stones and fines that fill the gaps, so it compacts well and locks together. A coarser 0/63 suits the lower layers, and a finer 0/32 or 0/22 goes on top, where a true surface for the bedding is easier to achieve.

Quarry rock and open coarse grades

On very soft or wet soil a layer of large quarry rock or an open grade such as 32/63 is sometimes placed at the bottom. The stone presses into the soft soil and forms a firmer base, and it also lets water drain away. On top of it you need a layer with fines, otherwise the crushed stone above gradually washes into the voids and the sub-base settles over time.

Recycled aggregate

Crushed concrete or recycled construction aggregate can be a cheaper and greener option. It works well if it comes from a certified recycling plant, has a known grading and contains no wood, plastic, plaster or large amounts of brick. Unknown demolition rubble does not belong in a sub-base: it softens in water and cannot be compacted reliably.

Geotextile and drainage

On clay or loamy formation a separation geotextile is laid under the stone, usually a non-woven type of roughly 200 to 300 g/m² according to the manufacturer. It stops fine soil working up into the stone and the stone being pushed down into the soil. Without it, a thick sub-base on clay mixes with the soil over the years and loses its strength. Where water stands, a perforated land drain belongs at the bottom of the dig, laid to fall towards a soakaway or a drain, if the drainage owner allows it.

1. Check the formation: depth from the level point, fall, evenness and soft spots. Dig out soft spots and replace them with stone.
2. Compact the formation with a plate compactor or roller. On wet clay, wait for it to dry, or choose a ground treatment such as lime stabilisation by a specialist firm.
3. Lay the geotextile with overlaps of roughly 30 to 50 cm and turn it up the sides of the dig.
4. Install the drain if one is designed and surround it with gravel.
5. Tip and spread the first layer, usually 15 to 20 cm after compaction. Keep heavy machinery wheels off the bare geotextile.
6. Adjust the moisture: wet dry stone, let saturated stone drain.
7. Compact the layer with several passes, working from the edges to the middle and overlapping each pass, until the stone no longer moves under the plate and leaves no marks.
8. Spread and compact the following layers the same way, finishing with the finer grade.
9. Shape the surface to the same fall the paving will have, usually around 2 % away from the house.
10. Check levels, evenness with a straightedge and bearing capacity, if a test has been agreed.

Why in layers

A plate compactor only reaches a certain depth. A heavy reversible plate handles a layer of roughly 20 to 30 cm, a light one far less. If 50 cm were tipped in one go, the surface would look firm, but the bottom would stay loose. It would then compact later under the cars, and the paving would sink in wheel tracks.

How long it takes

On a smaller drive, a crew with a mini digger and a heavy plate usually finishes the sub-base in a few days. A larger area or poor access takes longer. Stone does not set or dry like concrete, so once it is compacted and checked the bedding and paving can follow straight away. Rain is what holds things up: waterlogged clay formation cannot be compacted, and you have to wait for it to dry. In winter, stone should not be laid on frozen formation, and frozen aggregate should not be used.

Easier

• Open access, so the lorry can tip straight into the dig or right beside it
• A simple rectangular shape without many corners and manholes
• Good, dry soil and settled weather
• A formation that is already dug to the right depth and compacted

Harder

• Narrow access, where stone has to be moved by wheelbarrow or a small dumper
• Manholes, gullies and service connections that need compacting by hand around them
• Work close to old foundations, where strong vibration can cause cracks, so lighter equipment and thinner layers are used
• Wet or weak formation, soil replacement, drainage
• A slope where the edges have to be secured against slipping

With a deep dig right next to the house, take care not to undermine the foundations. If the bottom of the dig would go below foundation level, agree the method with a structural engineer.

Trades before and after

Everything that runs under the area must be finished before the sub-base: surface water drainage, ducts for cables to the gate and lighting, the water supply pipe and any irrigation. Digging through a compacted sub-base later is costly and usually leaves a weak spot. Kerbs are set in concrete either on the lower part of the sub-base or before the final layer, depending on how the firm works. What matters is that the sub-base extends beyond the kerb. After the sub-base comes the sand or grit bedding and the paving, and the paving layer should accept the sub-base and confirm the levels.

• Tipping the full depth at once: after a year, ruts appear where the car drives
• No geotextile on clay: the sub-base gradually mixes with the soil and the paving ripples and sinks
• A stone "bathtub" in clay with no fall on the formation and no drainage: water sits in the sub-base, freezes in winter and lifts the paving
• Dry, dusty stone that was not wetted: it looks compacted but breaks up under load
• Unknown recycled material with wood and plaster: it softens and settles after a few years
• A sub-base that stops exactly at the kerb: the kerb tilts under wheels and the paving edge spreads
• Fall created in the sand bed instead of the sub-base: the bed is thin in one place and thick in another, and the paving settles unevenly
• Hand compaction around manholes done only roughly: dips form around the covers
• Loaded lorries driving over the finished sub-base during the build: the ruts must be levelled and compacted again

In practice the problem often does not show at first. The paving is level in its first year, and faults in the sub-base appear only after the second or third winter. That is why it pays to check each layer while the work is going on, not just the finished surface.

You can build the sub-base yourself on a small area, for example a parking space for one car or a base for a garden shed, if access is good and the sides of the dig do not collapse. Do not climb into a dig with steep sides until they are battered back, and do not dig yourself where services run. For a larger area or for heavy vehicles, leave the job to a firm that can carry out a bearing test.

1. Have the buried services located and check with the local building authority whether the area needs permission.
2. Mark out the area with pegs and string line, 20 to 30 cm wider than the paving will be.
3. Dig out to the depth of the sub-base, bedding and paving, and remove roots and topsoil.
4. Shape the bottom to the fall and compact it with a hired reversible plate compactor.
5. Lay the geotextile with overlaps if the soil is clay.
6. Spread the first layer of coarser crushed stone so that it is roughly 15 cm after compaction.
7. Wet the stone if it is dry and compact it until the plate stops bouncing and leaves no marks.
8. Continue with the next layers in the same way, using the finer grade for the top layer.
9. Set string lines at the right height and fall and check the surface with a long straightedge.
10. Top up and recompact any spots where the straightedge shows a dip.

Wear ear protection and sturdy boots while compacting. The plate is heavy, so on a slope never pull it towards yourself.

Accept the sub-base before the paving crew arrives. Once the paving is down, you can no longer see it. If a bearing test with a static or light dynamic plate was agreed, ask for the test report. The required value on the formation and on the sub-base is set by the design or by the paving manufacturer according to the expected load.

• The sub-base thickness matches the agreement, checked with a trial hole or by measuring from the level point during the work
• The sub-base was compacted in layers, not in one go
• There is geotextile underneath where the soil required it
• The surface follows the fall the paving will have, and water runs off rather than standing
• A long straightedge shows no significant dips or humps, and evenness meets the paving layer's requirement
• Driving a car over it or stamping on it leaves no marks and the surface does not spring
• The sub-base around manholes and gullies is as firm as everywhere else
• The sub-base extends beyond the planned kerbs
• The material matches the agreed grading, and recycled aggregate comes with proof of origin
• Ducts and service pipes under the area are laid and recorded on a sketch
• The bearing test report has been handed over, if a test was agreed