Frost heaving occurs when soil expands and contracts due to freezing and thawing. The severity of this effect is determined by the ability of the soil to retain moisture and the climate. In certain conditions this force is strong enough to lift the foundation of a deck causing the deck to separate from the house.
Even if the results are not this catastrophic, the prolonged effect of this rise and fall of the deck will take its toll by weakening the structure and creating an uneven surface. Because of this you need to make sure you install the foundation for your deck at a suitable depth below the frost line.
This is a diagram map that assigns a minimum frost footing depth for each region of the country. The colder the temperatures in your region, the deeper you will have to dig. You can also find out by asking your local building inspections department. Frost jacking can also act to lift frost footings. Frost jacking occurs when the upper part of the buried pier is grabbed by frozen soil and pushed upwards. Using a flared base to the footing will help hold it in place. A good insurance against frost jacking is to wrap the pier in polyethylene sheeting to prevent the soil from bonding to the concrete.
