What to Use Under Patio Slabs (Best Underlay for Long-Term Stability)

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A patio is only as good as the ground it sits on — and if you skip the right underlay, you’ll end up with rocking slabs, puddles, and weed invasions in no time. The base under your patio is what gives it strength and structure, while the underlay layers control drainage and movement.

In this guide, we’ll break down exactly what materials you need under patio slabs, when to use geotextile fabric, and how to build a base that won’t sink, shift or crack over time.

What Should Go Under Your Patio Slabs?

  • ✔ MOT Type 1 Hardcore

    This is your main sub-base layer — crushed stone that compacts down to provide strength and support. Typically laid 100–150mm deep and compacted with a wacker plate.
  • ✔ Geotextile Fabric

    Laid between soil and hardcore, this stops the MOT Type 1 from sinking into soft ground. It also helps prevent weeds from pushing through over time.
  • ✔ Sharp Sand or Bedding Mortar

    Sharp sand can be used as a laying course (25–50mm) for paving slabs. Mortar gives a more permanent fix but takes more prep. Both need to sit on a solid base.
  • ✔ Jointing Sand or Pointing

    After laying, you’ll need to fill the gaps between slabs with kiln-dried sand or a weatherproof pointing mix to lock everything in and stop movement.

With Geotextile Fabric

  • Prevents hardcore from sinking into soft ground
  • Improves drainage while blocking soil mixing
  • Reduces weed growth under paving
  • Extends lifespan of the patio foundation

With Geotextile Fabric

  • Extra cost and installation time
  • May not be needed on firm, well-drained soil
  • Wrong fabric type can affect drainage if not breathable

Without Geotextile Fabric

  • Cheaper and faster installation
  • Fine for firm, compact clay or stony ground
  • Less material handling on-site

Without Geotextile Fabric

  • Higher risk of sub-base sinking on soft ground
  • Weeds more likely to push through joints
  • Reduced long-term base stability

✔ How to Build the Right Base Under Your Patio

  1. 1

    Excavate the Area to the Correct Depth

    Start by digging down far enough to allow for all layers — sub-base, laying course, and slab thickness. For most patios, that means at least 150–250mm depth. Remove all topsoil, roots, and soft patches.

  2. 2

    Lay Geotextile Fabric (if needed)

    If the soil underneath is soft or prone to holding water, lay a roll of geotextile membrane before adding your sub-base. Overlap edges by 150mm and secure in place. This helps separate layers and improve drainage.

  3. 3

    Add and Compact MOT Type 1

    Add MOT Type 1 in layers of no more than 75mm at a time. Use a vibrating plate compactor after each layer. Build up to your required depth — usually around 100mm for light use patios, 150mm for heavier traffic or vehicles.

  4. 4

    Add Your Bedding Layer

    Use sharp sand or a full mortar bed depending on how permanent you want the patio. Screed or level the surface so slabs can be laid evenly with your chosen fall for drainage (typically 1:60 gradient away from buildings).

🔹 Can I lay patio slabs directly on soil?
It’s not recommended. Soil shifts with moisture and temperature, which causes slabs to sink, rock, or crack. Always use at least a compacted sub-base of MOT Type 1 beneath the slabs for long-term stability.
🔹 Do I need geotextile fabric under a patio?
Not always, but it’s a smart upgrade — especially on clay or wet ground. It stops the hardcore from mixing with soil over time and prevents settlement or dips from forming under the slabs.
🔹 What’s the best sand to use under paving slabs?
Use sharp sand (not building sand) if dry laying. For a more permanent base, mix sharp sand with cement in a full mortar bed — this stops slabs from shifting and is more durable long-term.
🔹 Can I use gravel instead of MOT Type 1?
No — gravel doesn’t compact the same way. MOT Type 1 includes dust and fines that bind together when compacted, creating a solid base. Loose gravel will shift under load and cause uneven paving.

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