Should I Use a Wacker Plate on Sand Before Laying Slabs?

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Using a Wacker Plate on Sand

  • Creates a firmer, more stable surface under slabs.
  • Reduces the chance of dips, rocking, or long-term settlement.
  • Helpful if sand was tipped loosely and needs levelling.

Using a Wacker Plate on Sand

  • Can flatten or over-compact your screeded levels if done too late.
  • Not always needed — especially if laying slabs on mortar.
  • Vibrating dry sand too much may cause uneven compaction across the surface.

Leaving Sand Uncompacted

  • Ideal for a screeded sand bed — keeps levels accurate and ready for slabs.
  • Common method when laying slabs on a mortar bed or full mortar mix.
  • Slabs can be adjusted individually without disrupting the whole base.

Leaving Sand Uncompacted

  • Higher risk of movement or sinking if the sub-base underneath wasn’t solid.
  • Loose sand can settle over time, especially under heavy furniture or foot traffic.
  • Not suitable for block paving or traffic-bearing areas without some compaction.

What a Wacker Plate Actually Does (and When It’s Used)

  • ✔ Compacts loose materials down tight

    A wacker plate vibrates and presses loose material like MOT Type 1 or sharp sand to remove air gaps and make the surface firm and stable.
  • ✔ Always used on sub-bases

    You should always compact your sub-base (like MOT Type 1) in layers. This gives your slabs a solid foundation that won’t sink or move later.
  • ✔ Sometimes used on sand beds

    If you're dry-laying slabs or using sand as a screeded bed (like block paving), light compaction is often used to firm it up before laying.
  • ✔ Not used on mortar

    If you're laying slabs on a full mortar bed, there’s no need to compact the sand — the mortar does the work, and slabs are tapped into place.
  • ✔ Helps prevent future movement

    Compacting the right layers helps avoid slab wobble, dips, or shifting down the line. If it’s going to bear weight — compact it properly.
🔹 Should I compact sand before laying slabs?
It depends on your laying method. If you're dry-laying on sharp sand or using it as a screeded bed (like for block paving), a light compaction can help firm it up. If you’re laying on mortar, there’s no need to compact the sand underneath — just make sure your sub-base was compacted properly.
🔹 Can I use a wacker plate after laying the slabs?
No — wacker plates should never be used directly on top of slabs unless you're doing block paving with a special plate guard. For paving slabs, you risk cracking them. All compaction should be done before laying the slabs, not after.
🔹 What happens if I don’t compact the sand?
If your sub-base is compacted properly, skipping sand compaction may be fine for light-use areas. But for dry-lay jobs or thicker sand layers, it may settle over time — leading to rocking slabs, dips, or cracks. Compacting helps lock the layer in place.
🔹 How thick should sharp sand be under slabs?
A typical sharp sand bed is 25–40mm thick. Any deeper than that and you’ll want to compact in layers to prevent soft spots. Keep the layer consistent to avoid dips and to make laying easier and more stable.

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