How deep should a block paving sub-base be?

Sub-base depth is the single biggest factor in whether a driveway lasts or ruts. The right figure depends on what the paving carries and what it sits on.

Typical depths for driveways and patios

The right sub-base depth follows the load. A patio, path or pedestrian area only carries people and garden furniture, so a compacted MOT Type 1 layer of roughly 75–100mm over firm ground is usually sufficient. A driveway carries the concentrated weight of car tyres, plus braking and turning forces, so the standard rises to roughly 100–150mm of well-compacted Type 1. These are general ranges, not absolutes: the figure is driven by the ground beneath and the traffic above, and a competent installer will assess the formation before settling on a depth rather than applying a fixed number to every job.

Why clay and heavy traffic need more

Two things push the depth above the typical range. The first is poor ground. Heavy clay shrinks and swells as it wets and dries, and drains slowly, so it gives less reliable support; made ground (old fill, demolition rubble, soft garden soil) is inconsistent. On such formations the sub-base may need to be deeper, and a capping layer of coarser stone beneath the Type 1 may be added to bridge the soft ground. The second is load: a drive used by a large van, a motorhome, a caravan or regular heavy deliveries asks far more of the base than one parking a single family car, and the sub-base depth and compaction should reflect that.

• Firm, free-draining formation, family car: toward the lower end of the driveway range can be adequate.
• Clay or made ground: toward the upper end or beyond, often with a capping layer.
• Heavy or commercial vehicles: a thicker, carefully compacted sub-base designed for the load.

When the ground is uncertain, the cautious choice is to dig deeper and compact more thoroughly — under-building the base is far more expensive to fix later than a little extra stone now.

Working out the total dig depth

The sub-base depth is only part of the calculation. To get the finished surface at the right level, the total excavation has to allow for every layer stacked on the formation: the sub-base, the laying course and the blocks themselves. A useful way to think about it is to add them up. With a driveway sub-base of, say, 150mm, a sharp sand laying course of around 30–50mm, and blocks commonly 50–60mm thick, the total dig from finished level down to formation is in the region of 230–260mm — and more on soft ground that needs a deeper base or capping. The blocks are bedded slightly proud of the screeded sand before compaction, so allowance is made for them to settle down to level when vibrated in.

Two constraints govern the finished level. First, the surface must sit at least 150mm below the damp-proof course (DPC) of the house, and clear of air bricks, so it cannot bridge damp into the wall — this is a frequent and avoidable mistake. Second, the whole surface must carry a gentle fall (often around 1:40 to 1:60) so water runs to a drainage point and away from the building, which means the formation and sub-base are graded to that slope, not laid dead flat. Because the sub-base depth, the layers above it, and the falls all interact, the right approach is to set the finished levels and falls first, then work down through the layer thicknesses to fix the dig depth — rather than excavating to a guessed depth and discovering the levels do not work. Getting this arithmetic right at the start is what keeps the drive draining, clear of the DPC, and structurally sound for its full life. A useful sense-check before digging is to lay out the layers on paper for your specific drive — formation level, sub-base depth, laying course and block thickness — and confirm the resulting finished level lands where you want it against the house, the garage threshold and the street, adjusting the dig depth rather than discovering a clash once the excavation is already done.

Depth, compaction and permeable builds

Depth and compaction work together, and it is worth being clear that a deeper sub-base does not help if it is not compacted properly. Type 1 has to be laid and densified in layers, because a single thick lift cannot be compacted through its full depth — the bottom stays loose and consolidates later under traffic, producing exactly the rutting that extra depth was meant to prevent. So the right specification is always a depth and a method: build the stone up in lifts, compact each one with a plate compactor or roller, and check the level and falls as you go. A shallower sub-base that is thoroughly compacted can outperform a deeper one that was tipped in and rolled once.

Depth also changes if you build a permeable driveway rather than a conventional one. Permeable block paving is increasingly used on front drives because directing rainwater into the ground (rather than to the road) generally keeps the work within permitted development. But permeable construction does not just swap the jointing — it uses a different, open-graded (clean, no-fines) crushed stone sub-base that can both carry load and store and pass water, and that layer is often deeper than a standard Type 1 sub-base because it doubles as a water-storage reservoir sized to the rainfall it must hold. The exact depth then depends on the area drained, the soil's ability to soak, and whether the system discharges slowly to a drain. The practical takeaway is that there is no single universal number: a standard family-car drive on firm ground sits around 100–150mm of compacted Type 1; soft ground, heavy vehicles, and permeable designs all push that figure up, and in every case the depth only delivers if the layer is compacted in lifts to a firm, even, correctly-falling surface.

Frequently asked questions

Sources & further reading

• Pavingexpert — sub-base depth and design
• Marshalls — block paving installation advice
• MyJobQuote — block paving cost and installation guide

Figures on this page are typical UK ranges drawn from published sources and depend on your specific site. They are guidance, not a quotation.