What to Put Under Paving Slabs to Stop Them Sinking

Why a Proper Base Matters Under Paving Slabs

A proper base under paving slabs is what keeps them from moving sinking or rocking about every time you walk over them. It gives the slabs a firm even surface to sit on and spreads the weight out so the ground underneath does not shift or settle unevenly. Without a solid base even the best laid slabs will start dipping cracking or wobbling before long especially after a bit of rain or frost.

The base does all the hidden work. It supports the slabs holds the levels steady and helps with drainage too so water does not pool underneath and soften the ground. Skipping the base or doing it half hearted is one of the quickest ways to ruin a paving job. Get the base right from the start and the rest of the job will go down smoother and stay looking good for years.

Should I have MOT Type 1 Under Every Paving Job

You do not need MOT Type 1 under every single paving job but it is the safest choice for most of them especially if the area is going to take any weight or regular use. For patios paths driveways and anything that needs to last it gives you a solid well compacted base that holds everything steady and stops the slabs from sinking over time.

If you are laying a small path or a decorative area with no weight on it you might get away with compacted sharp sand and a bit of type 2 or even old hardcore. But if the ground is soft or you want the slabs to stay put for the long haul MOT Type 1 is the better option. It holds its shape drains well and gives you peace of mind that the job is not going to shift or dip after a few winters.

Can Sharp Sand Alone Support Paving Slabs?

• ✔ Sharp sand alone isn’t stable enough long-term
  
  While it might seem fine at first, sharp sand without a sub-base can shift, wash out, or settle unevenly — especially after heavy rain or frost. This leads to rocking, dipping, or cracked slabs over time.
• ✔ The sand is for bedding, not structural support
  
  Sharp sand creates a level, workable laying bed for the slabs. It helps you adjust heights and keep everything aligned — but it’s not designed to bear load or prevent movement on its own.
• ✔ A compacted sub-base is essential underneath
  
  For paving to last, the sharp sand needs to sit on a solid foundation like compacted MOT Type 1. This prevents shifting, absorbs weight, and helps the slabs stay flat and firm for years.
• ✔ Skipping the sub-base usually means redoing the job
  
  Without proper support, the slabs will eventually move out of place. It’s a common cause of failed patios and relaying them later is far more work than doing it right from the start.

How Drainage Affects the Stability of Paving Slabs

• ✔ Water trapped under slabs softens the base
  
  If water sits below the surface or soaks into the sub-base, it slowly weakens the ground. Over time this causes dips, loose joints, or full slab movement.
• ✔ Poor drainage leads to frost heave in winter
  
  When water stays trapped and freezes, it expands — pushing slabs up and causing uneven spots. This is a common issue on driveways and patios during cold months.
• ✔ A proper fall prevents water pooling
  
  Slabs should be laid with a slight slope (fall) so water runs off instead of collecting. Even a 1:60 fall is enough to keep the surface dry and stable.
• ✔ Sub-base choice affects drainage performance
  
  MOT Type 1 is ideal because it compacts well but still allows water to drain. Avoid materials that hold moisture or turn to mush when wet.
• ✔ Use geotextile on wet or clay-heavy ground
  
  On problem soils, adding a geotextile layer keeps the sub-base separate and prevents it from sinking into soft clay. It also helps manage water flow under the slabs.