A sand blinding layer is used to smooth over rough or uneven hardcore before installing a damp-proof membrane (DPM) or concrete slab. It helps protect membranes from puncture, fills small gaps, and makes laying insulation or concrete easier. But here’s the catch: it’s not always needed — and when installed wrong, it can actually cause problems. This guide explains exactly when to use a sand blinding layer, what type to use, how thick it should be, and how to install it the right way for a clean slab build-up.
Before reaching for a tonne of sand, ask whether you really need blinding. If your compacted hardcore layer is smooth, fine-grained, and you’re using a heavy-duty DPM, you may not need it. But if the surface is rough, sharp, or uneven — particularly with recycled fill — sand is essential to protect the membrane and ensure full contact. If insulation is being laid above, sand blinding helps prevent it rocking or bridging over hollows. It’s a practical fix, not a decorative layer — and skipping it can result in punctured membranes and uneven slab finishes.
Use clean, dry building sand or soft sand for blinding — not sharp sand or washed concrete sand. The particles should be fine enough to fill gaps between stones, but not so dusty that they blow around or cause slip. Avoid using wet or clumped sand, as it’s hard to spread and doesn’t level cleanly. For large areas, dry bulk bags are ideal. If you’re unsure, ask your supplier for blinding-grade soft sand. Don’t use kiln-dried or block paving sand — it’s too fine and can move under pressure.
Before laying the sand, make sure the hardcore is compacted, levelled, and free from sharp protrusions. Use a rake or straight edge to knock down any high spots. Compact again if you’ve made adjustments. The better your hardcore layer, the thinner and more consistent your blinding will be. Sand is not a substitute for proper compaction — it’s just a smoothing layer. If there are deep hollows or patches, top them up with more hardcore and re-compact before adding sand.
Tip or shovel sand into small piles across the slab area, then use a rake or straight edge to spread it evenly. Aim for a consistent thickness of 10–20mm max. You don’t want it deep — just enough to fill gaps and cushion the DPM. Work in sections, checking levels as you go. Avoid leaving ridges or lumpy areas. If the sand is too deep, it can compress unevenly under the slab, causing soft spots or shifting insulation boards. Keep it tight, level, and just enough to do the job.
Once spread, give the sand a light compaction pass — but don’t overdo it. Use a hand tamper or a low-power wacker plate on vibration-only mode if available. The goal is to settle the sand and flatten it — not force it into the hardcore or push it aside. If you over-compact, you’ll just churn it up or create hollows. A single, light pass is usually enough. Walk the area and test by foot — it should feel firm and even with no movement under pressure.
Once the sand is in place and compacted, don’t leave it exposed for long. Rain will wash it out or clump it, and wind will shift it. Lay your damp-proof membrane or insulation straight away to lock the blinding in place. Avoid walking on it too much, especially with boots — it’ll leave uneven spots. If you need to leave the job overnight, cover the sand with polythene sheets or boards to protect it. From here, you’re ready for the next layer of your slab build-up.
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