What to Know About Frost Lines

Frost lines are critical to construction in cold climates, but they’re important in mild regions, too. Learn what frost lines are and why they matter.

When a home is built according to code in its region, a homeowner may never need to worry about frost lines. But when you’re building a new home, addition or deck, knowing how deep frost can reach in an average winter is a crucial part of the process.

The frost depth and building codes differ from region to region. Here’s what you need to know about frost lines before undertaking a home build.

What Is the Frost Line?

The frost line is the depth at which moisture will freeze in the ground. Due to variations in winter temperatures and soil moisture content, the frost line isn’t the same everywhere. The frost line is much deeper in Minnesota than it is in Florida, for example.

Why Do Frost Lines Matter?

Think of what happens when something freezes and thaws. It expands and contacts. A can of soda explodes when left in the freezer or the trunk of a car in winter. Frozen roads and sidewalks result in cracks, pits and potholes. The same things happen below ground when the surrounding soil and water freeze.

Frost lines matter because “anything above them can move when it freezes,” says Josh Risberg, Family Handyman’s set builder and one of the editors working on magazine’s The Getaway project. This affects building foundations, deck footings and underground utility lines. “The farther north you go, the farther down you need to go for footings,” says Risberg.

If footings are not placed below the frost line, they can heave in the winter when the ground freezes. The shifting can cause lots of problems, including “cracking in the foundation, drywall or other things, like doors not shutting right,” Risberg says. He adds you’ll also have issues if you “attach something that is below the frost to something that is above the frost.”

This is one of the reasons why building permits and inspections are necessary. Building a deck is a popular DIY project, but things quickly go wrong when the frost depth isn’t accounted for.

How to Know the Frost Line Depth

If you live in a region with bitterly cold winters, the National Weather Service says your frost line can be as deep as six feet — or more! It is much shallower in warm regions like Arizona or South Florida. Take a look at our frost depth map above to see the frost line in your area.

Don’t rely on the map alone, though. “Each zone is different,” says Risberg. “Check with your local building inspectors on what depths are needed.” This is another argument for building permits.

Local codes and regulations are designed to fit local needs. Because the frost line changes depending on your zip code, your local building office or government will give you the right requirements for your specific area. It’s important to comply with local codes, not only to avoid fines, but to build structures that are safe and long-lasting.