Engineering • Sub-Base

How Thick Should a Patio Sub-Base Be? (Real-World Depth Rules)

There is no single “correct” sub-base thickness. The right depth depends on soil type, drainage, slab weight, and loading. This guide explains the real-world rules that stop patios sinking, cracking, and moving.

Quick Answer

  • 100mm → absolute minimum for firm, free-draining ground.
  • 150mm → recommended standard for most domestic patios.
  • 200mm+ → required for clay soils, heavy slabs, or high-load areas.
  • Thin sub-bases compress over time and cause uneven settlement.
  • Thickness is useless without proper compaction.
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Why Sub-Base Thickness Matters

The sub-base is the load-spreading foundation of your patio. Its job is to distribute weight across the ground and resist movement.

If the sub-base is too thin, loads concentrate in small zones. This causes local settlement, slab rocking, and joint cracking.

Thickness also controls how well the patio:

  • Resists freeze–thaw expansion
  • Handles water saturation
  • Survives heavy furniture or hot tubs

*(Related: What Is a Patio Sub-Base? • Why Patios Sink at Edges)*

Standard Patio Sub-Base Depth Rules

These are real-world engineering guidelines, not marketing numbers:

  • 100mm — absolute minimum for firm, dry, free-draining soils.
  • 150mm — recommended for most domestic patios.
  • 200mm+ — required for clay soils, slopes, or heavy loading.

These depths refer to compacted thickness — not loose material tipped into the hole.

*(See also: Sub-Base Compaction Explained • Patio Ground Preparation)*

Sub-Base Thickness by Soil Type

Soil strength is the single biggest variable in sub-base design.

  • Free-draining sand/gravel: 100–150mm
  • Firm loam: 150mm minimum
  • Clay or made ground: 200mm+
  • Waterlogged ground: 200mm+ with drainage layer

Clay soils expand when wet and shrink when dry. Thin sub-bases cannot resist this cyclic movement.

*(Deep dive: Clay Heave Explained • Drainage Layers Explained)*

Sub-Base Thickness by Slab Weight

Heavier slabs impose higher bearing stress on the ground.

  • Concrete flags (40–50mm): 100–150mm
  • Sandstone (20–30mm): 150mm
  • Porcelain (20mm): 150–200mm
  • Large-format slabs (900×600+): 200mm+

Thin sub-bases under heavy slabs almost always lead to rocking or cracking.

*(Related: Why Porcelain Paving Cracks • Why Sandstone Paving Fails)*

Compaction Rules (Where Most Installations Go Wrong)

A thick sub-base that isn’t properly compacted behaves like a thin one.

  • Lay in layers no deeper than 50mm.
  • Compact each layer fully before adding the next.
  • Use a vibrating plate or roller compactor.
  • Moisten dusty material to improve binding.

Most patios fail not because the wrong depth was chosen, but because everything was dumped in one lift and barely compacted.

*(See: Sub-Base Compaction Explained • Why Patios Fail After 2 Years)*

What Happens If the Sub-Base Is Too Thin

Thin sub-bases fail slowly — which makes the cause hard to diagnose.

  • Local settlement creates rocking slabs.
  • Edges sink faster than the centre.
  • Joints crack repeatedly.
  • Slabs start sounding hollow.

These symptoms usually appear 6–36 months after installation.

*(Failure modes: Why Patios Fail • Why Patios Move in Winter)*

What This Means For You

  • If your patio is sinking → the sub-base is too thin.
  • If joints keep cracking → the sub-base is compressing.
  • If slabs rock → load is not being spread evenly.
  • If you're rebuilding → increase depth, don’t just replace slabs.
  • If you're on clay → 200mm+ is not optional.

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