A brief science of gritting and ploughing

By on 09/02/2012 in Factsheets, Winter Alerts
  • Although the process of road salting is referred to as gritting there is in fact little or no grit involved. What is actually spread on the roads is mined rock salt (Sodium Chloride) of a size and composition for road use.
  • Rock salt was first used for the purposes of winter road maintenance in the 1940’s and is an effective de-icer for areas that receive road traffic.
  • It is a hydroscopic material in that draws moisture to it and also draws heat from the environment rather than releasing it.
  • The rock salt is spread on to the road in varying rates of spread dependent on the severity of the conditions.
  • The effectiveness of rock salt on roads is enhanced by vehicles passing over larger granules and grinding it into smaller particles spread across the road surface area. This then forms a solution with a higher de-icing capability.
  • Whilst water freezes at 0°C, the presence of the salt prevents water from freezing until -6°C to -8°C.
  • However, salt starts to become less effective at -5°C and almost ineffective at lower temperatures.
  • In extremely low temperatures, or heavy snowfall, a mix of salt and grit may be used to aid traction.


  • In relation to snow ploughing this provides the ability to remove large accumulations of snow from the centre of roads.
  • However it should be recognised that ploughing has limitations in that it effectively skims the road surface leaving around 30-50mm of snow on the surface.
  • It can also result in large amounts of snow accumulating at the sides of roads and on footways and following prolonged periods of gritting can reduce the overall effectiveness of de-icing operations by removing slush which has a high level of salt solution.

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