Temperature Limits for Salt on Driveways

How Salt Works

• Salt works as a deicer by lowering the freezing point of water, returning frozen water to its liquid state. For instance, a 23 percent saltwater solution freezes at minus 60 degrees Fahrenheit, where plain water freezes at 32 degrees Fahrenheit. The salt lowered the freezing point of the water by 90 degrees. To be effective, salt must dissolve into water to form a brine solution. On driveways and roads, salt gets the water from the snow and ice surrounding the salt crystals. It also needs heat from sun, air, pavement and traffic friction. But the need for heat to help melting means salt loses its effectiveness when temperatures drop far below the freezing point.

Temperature Limits

• Salt melts five times as much ice at 30 degrees Fahrenheit as it does at 20 degrees Fahrenheit. According to a 1996 Wisconsin Transportation Department study of deicing chemicals, 1 lb. of salt will melt 40 lbs. of ice at 30 degrees, but you’d need 5 lbs. of salt to melt 40 lbs. of ice at 20 degrees. Salt becomes ineffective at 15 degrees and below. Also, the time needed to melt ice with salt increases as temperature drops. The Wisconsin study showed salt took 3 minutes to melt 1/8 inch glare ice at 30 degrees, but almost 20 minutes to melt the same ice at 20 degrees and an hour at 15 degrees. Highway departments generally stop using salt when the temperature drops below 15 degrees Fahrenheit.

Application

• A spreader with a spinner is an effective way to spread salt over smaller areas such as a driveway and sidewalks. Spinner spreaders are available as manual or powered push types, or powered units that mount on pickup trucks. Spread salt at a rate of 8 oz. per square yard of driveway. Store your salt and your spreader inside a covered, waterproof structure until you are ready to use it. Clean your spreader after use to prevent rust damage to its working parts.

Environmental Impact

• Salt solutions follow water flows and will adversely affect grass, shrubs and other vegetation close to the roadside. Salt is toxic to plants in soil concentrations above 90 parts per million. Repeated doses of salt spray can kill buds, and over a period of years salt exposure can stunt plant growth on the side facing the driveway. Salt can speed deterioration of pavement, particularly in the first year, and cause corrosion of your car’s underbody.