From Q. My deck rose out of the ground last winter! What is causing this, and what can I do to stop it?
A. You had a frost heave. It occurs when water freezes below the concrete footings and/or attaches to the rough sidewalls of the footings. As the freezing water expands, approximately 9 percent in volume, it exerts about 50,000 lbs. of pressure per square inch— which can easily lift a deck.
While decks can withstand some movement, if it moved up more than 1 in., check for structural damage. If the deck is attached to your house, the connection may be damaged as well.
Unfortunately, there’s no easy remedy. First, dig out the footing that heaved. Next, make sure the hole for your new footing extends below the average frost depth for your region. And if you have a lot of clay in your soil, make your hole a foot or two deeper. Call your local building inspector to get the proper footing depth, as well as to ask if you need a new building permit.
Make sure the bottom 12 in. of your hole flares out wider than the vertical walls. Insert a smooth-sided cardboard form tube, raised 12 in. from the bottom of the hole. This form keeps frost from gripping the sides of the concrete pier.