Do You Need a Concrete Blinding Layer?

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When You Need a Blinding Layer

  • ✔ The hardcore surface is uneven or too rough

    If the MOT Type 1 or crushed stone base is too jagged or inconsistent, a blinding layer helps smooth it out. This ensures your slab sits flat and doesn’t sag or vary in thickness.
  • ✔ You’re installing a DPM (damp-proof membrane)

    A blinding layer is often used to protect the membrane from being punctured by sharp stones in the sub-base. It also makes laying the DPM easier and keeps it flat during the pour.
  • ✔ The slab needs a level surface for uniform thickness

    If the base isn’t level, the concrete pour may vary in depth — which can lead to weak spots or wasted material. A blinding layer helps create a consistent platform across the full area.
  • ✔ There’s a risk of puncturing the membrane

    On sites where sharp hardcore or debris might damage a damp-proof membrane, a thin layer of sand or concrete helps shield it before the slab goes down.
  • ✔ The ground is exposed to heavy rain before pouring

    If wet weather is likely between base prep and slab pour, a blinding layer can help keep the surface clean and prevent mud or water pooling. It also makes re-compaction easier if needed.

✔ How to Add a Blinding Layer Before a Slab

  1. 1

    Rake and Compact the Hardcore Surface

    Before you add a blinding layer, make sure the hardcore base is compacted properly and free of loose material. Rake the surface to remove any sharp or oversized pieces that might damage the DPM or interfere with the concrete pour. Then compact it again using a plate compactor to ensure a firm, even surface. A solid sub-base helps prevent settlement, and it’ll make your blinding layer more effective and consistent. Don’t skip this step — a poorly compacted base is one of the biggest reasons slabs crack or shift later on. You’re aiming for a level, stable surface with no dips or loose patches before moving on.

  2. 2

    Choose Your Blinding Type (Sand or Concrete)

    The two most common blinding materials are sharp sand and lean-mix concrete. Sand blinding is quicker and easier — perfect if you’re laying a damp-proof membrane (DPM) over the top. It cushions the membrane and smooths out the base. Concrete blinding is more durable and useful if you're not using a membrane or need extra strength and stability under the slab. It’s also ideal for holding rebar chairs or spacers in place. If your hardcore is coarse or uneven, a thin concrete blinding layer will fill the voids better than sand. Choose based on your slab type, weather, and what’s going above it.

  3. 3

    Spread Evenly to 25–50mm Thickness

    Whether you’re using sand or concrete, spread your blinding layer evenly across the compacted hardcore. The typical thickness is 25–50mm — just enough to cover any sharp points and smooth out high/low spots without adding too much height. Rake the material into place and use a long straight edge or screed bar to level it off. For concrete blinding, pour and spread as you go, working in manageable sections. Keep checking your depth as you go — you want an even layer that follows the fall of the area if drainage is required. Inconsistent thickness can affect your slab depth and result in wasted concrete or weak spots.

  4. 4

    Compact or Float the Surface as Needed

    Once spread, compact the sand blinding lightly with a hand tamper or plate compactor. You’re not looking to crush it — just firm it up enough to support a membrane or the slab above. If using concrete, smooth the surface with a float or trowel while it’s still workable. This gives you a level, sealed layer that’s ready to support the next step. If it’s a hot or windy day, lightly mist sand or concrete to prevent premature drying. Don’t overwork the surface — a light finish is all you need to provide a clean, even base that won't disturb during membrane laying or slab pouring.

  5. 5

    Lay Membrane or Prep for Slab Pour

    With your blinding layer in place, you’re ready for the next stage. If you’re using a DPM (damp-proof membrane), roll it out over the surface and make sure it sits flat without rips or folds. Overlap joints by at least 150mm and tape them if needed. Then install rebar or mesh (if required) using spacers or chairs that won’t punch through the membrane. If you’re pouring directly onto the blinding layer with no membrane, make sure it’s clean, dry, and not soft underfoot. Double-check your levels before pouring. This prep might seem like a small step, but it’s the foundation of a long-lasting slab.

🔹 What is a concrete blinding layer?
A concrete blinding layer is a thin layer (usually 25–50mm) of weak-mix concrete laid over compacted hardcore. Its job is to provide a smooth, clean surface for pouring a slab or laying a damp-proof membrane (DPM). It fills gaps, levels out roughness, and creates a more stable base.
🔹 Do I always need a blinding layer?
Not always. If your hardcore is already level, compacted, and free of sharp points, you might not need one. But if you're installing a DPM or the surface is rough or uneven, a blinding layer is highly recommended. It protects the membrane and helps ensure an even concrete thickness.
🔹 Can I use sand instead of concrete?
Yes — sand blinding is commonly used when laying a DPM. It’s easier to spread and quicker to install. It cushions the membrane and evens out minor surface imperfections. Concrete is better if more strength is needed or if no membrane is being used.
🔹 How thick should a blinding layer be?
Blinding layers are typically 25–50mm thick. You only need enough to smooth out the surface and create a flat bed. Any more and you’re just wasting materials. For concrete blinding, use a lean mix and don’t worry about structural strength — it’s not meant to be load-bearing.
🔹 Is blinding required by building regulations?
Not always, but it’s often recommended in specs where a DPM is present or where the slab needs to be precisely level. Some inspectors may expect to see blinding used in certain foundations — especially for habitable spaces — so it’s worth checking with your local authority or builder.