At an inspection in Woodbury, I saw that the deck sloped toward the house. Was it built that way? Nope. The footings had been pushed up by frost heave.
Every spring, farmers head into the fields and pick up rocks that appeared after the snow melts. They weren’t there in the fall. Where did they come from?
Frost heave. When the ground freezes, the moisture in the dirt expands. Soil that contains more moisture expands more than soil that does not. Water does not expand a whole lot, like 9%. but it exerts a whole lot of force, in the area of 50,000 pounds per square inch. The water in the soil forms a disc, called an “ice lens”. The solution is to install footings lower than the frost level. In Minnesota, that’s about 42″. But that’s not the whole story.
But first, let’s back up a bit. So what if the deck get’s pushed up a bit, and is not quite level. Folks don’t spend a lot of time on the deck in the winter, and besides, it goes back down in the spring, right?
Almost right. The space beneath the raised footings sometimes gets filled in with soil, so the posts can’t return all the way.
More important than that is the strain on the connections at the house. The deck was not designed to move. If the deck breaks free from the house, the folks on the deck are going to have a very bad day.
So. Back to the footings, Check with your local building code folks for post depth requirements, because they change depending on where you are.
I got great advice from FamilyHandyman.com ( love those guys!). It seems the frozen soil actually grabs the post on it’s way up, even when the post is very deep. So builders use wax-coated tubes to reduce traction. The tube doesn’t go all the way to the bottom of the hole created for it, so that the concrete at the bottom creates a bulb. (Think of an upside-down tootsy roll.) And to keep the bulb from breaking off, rebar is inserted, with a bend at the bottom.