United States Frost Depths for Residential Structures

What is Frost Depth?

Frost depth (or the “frost line”) is the depth in the soil above which water may freeze during the winter. Colder areas with longer winters have deeper frost depths, and areas with milder winters have shallow or no frost depth. Water expands when it freezes. Placing a foundation above the frost depth could allow water to freeze below the foundation and lift it, which is called “frost heave.” This can lead to all sorts of issues, from cracked foundations to leaning structures.

How Does Frost Depth Impact My Pergola Installation?

An important step in the construction of your pergola is to place the pier foundations below the local frost depth. Doing so will avoid issues with frost heave, keeping your pergola stable and level throughout its life.

How Do I Find My Local Frost Depth?

Finding your local frost depth can be challenging if you don’t know where to start. To assist you, we’ve done the research already and have summarized it by State in the sections below and interactive map above. Sources of information are given so you can independently verify.

If information for your local frost depth is not available below, you can find this yourself. There are several places you can check to do so:

1. Contact your city or county Building Department. They should be able to tell you very easily.

2. Perform an online search. We recommend the following search terms (in order of likelihood of helpful result):
   - [city name] municipal code, then search for the term “frost”
   - [city name] frost depth
   - [city name] building code
   - [city name] residential code
   - [city name] design criteria
   - [city name] minimum footing depth
   - [city name] Table R301.2

If these search terms do not immediately yield an answer, you may have to search through different documents on the city/county website to find what you’re looking for. We’ve found that this information can sometimes be found in a city’s Deck Guidelines document.

Minimum Foundation Depth

Regardless of the defined frost depth for your area, you should install your pergola piers no shallower than 12″, unless you have bedrock that projects shallower than that. This is the minimum foundation depth permitted by the IBC (Section 1809.4) and the IRC (Section R403.1.4). Most municipalities have either adopted the IBC/IRC or use it as a model code.

Nomenclature

IBC = International Building Code
IRC = International Residential Code

Disclaimer

Note that the information below is provided for reference purposes only, may be out of date, and only applies to residential construction. You should always check with your local Authority Having Jurisdiction (likely the local Building Inspector) to confirm that you are using the correct frost depth. If you find that any of our information is in error, please let us know.