Salt (regular road salt = sodium chloride) starts to struggle around 20°F (about −6°C) and is basically ineffective once you get near 10–15°F (about −9 to −12°C), depending on conditions.

The short, practical answer

  • Above 20°F (−6°C): Road salt works well and melts ice reliably.
  • 20–15°F (−6 to −9°C): It still works, but more slowly and you need more of it.
  • Around 15°F (−9°C): Many highway agencies treat this as the “practical limit” where plain rock salt stops being a good deicer.
  • Below about 10–12°F (−12 to −11°C): Salt is mostly useless on its own; the brine can’t stay liquid and refreezes.

So if you’re wondering “at what temp does salt not work on roads,” the real- world answer most road crews use is: below about 15°F, don’t count on it.

Why salt stops working in the cold

Salt works by lowering the freezing point of water, creating a salty brine that stays liquid below 32°F and melts ice from the surface down. As the temperature drops:

  • There’s less liquid water for the salt to dissolve into, so it can’t form an effective brine.
  • Even when brine forms, at very low temperatures it freezes again, so it no longer flows or breaks up the ice.

That’s why, in deep cold snaps, roads can stay icy no matter how much salt is spread.

What’s used when it’s too cold?

When it’s colder than about 15°F, crews often:

  • Switch to other chemicals like calcium chloride or magnesium chloride, which work down to much lower temperatures (as low as −20°F for calcium chloride).
  • Use treated or pre-wetted salt to squeeze out a bit more performance in marginal temps.
  • Add sand for traction when melting isn’t realistic.

Information gathered from public forums or data available on the internet and portrayed here.