Blinding sand is a layer of sand placed above a hardcore fill, typically made from building aggregates, to create a level working surface. It also prevents sharp stones and gravel in the aggregate mixture from coming into contact with the damp proof course and tearing the membrane.
Hardcore infills provide a stable and robust base for foundations, roads, driveways, and other heavy-load bearing areas. However, without blinding sand, they would not be as effective.
In addition to creating a level and smooth surface, the top layer of sand stops moisture from rising and finding its way into the foundation, which at this stage of the construction process, is yet to be poured.
It is called blinding sand as it covers the hardcore layer and prevents it from being seen.
In instances where a foundation needs to be particularly strong, concrete made using portland cement may be used as a blinding material instead of sand. This method of binding also means that there is no risk that fine sand will fall into the gaps of the hardcore aggregate and become uneven.
What Type of Sand is Used For Blinding Sand?
Building sand and sharp sand are the most commonly used materials for blinding a hardcore layer. If there is a damp proof course, then building sand should be used as it is soft, unlike sharp sand, which could penetrate the PVC membrane.
Sharp sand is preferred in situations where the blinding layer does not come into contact with the damp proof course. This is because it will compact easier and is better at allowing water to drain away.
How Much Blinding Sand Do You Need?
Architects tend to specify a 50mm deep layer of blinding sand. This must be compacted using a wacker plate or hand roller to ensure the gaps at the top of the hardcore are filled and to ensure there is no movement.
The below table shows how much sand you will need to blind an area using a 50mm thick layer.
|Area to be blinded by sand||Amount of sand required|
Note: these are estimates and the actual amount will vary depending on the extent of compacting and the frequency of the gaps within the hardcore fill.
How Much Does Blinding Sand Cost?
The cost of blinding sand will depend on which type of sand you use and how much you need. Bulk bags of sharp sand, which contain around 800kg, cost £66 on average. The same amount of building sand would cost £65.
These are retail prices that include VAT but not the cost of delivery, which varies from merchant to merchant.