Sources

Which Model Building Codes Do We Use?

We use the International Residential Code (2018, 2021 and 2024 editions) and the International Building Code (2021). The sections we rely on most are:

• IRC Table R301.2 and footnote b, which defines the frost line depth as a minimum footing depth set by the jurisdiction.
• IRC R403.1.4 and R403.1.4.1, footing depth and frost protection.
• IRC R403.3, frost-protected shallow foundations.
• IRC R507.3, deck footings.
• IRC P2603.5, water and sewer lines.
• IRC R105.2, work exempt from a permit, including fences up to 7 feet.
• IBC 1809.4 and 1809.5, footing depth and frost protection.

The codes are read in the ICC public-access reader. Many states and cities amend them.

Which State Building Rules Do We Use?

All 50 states and DC. For each one we recorded how the state handles frost depth. In our data, 33 states leave it to local option, and the rest fix a number, set zones or a table, or have no statewide residential code. The state page shows the rule and links its source.

Which Local Documents Do We Use?

Official city, county and state agency documents that contain a design-criteria table or a code section with a frost depth. Today 280 places in 39 states show a frost line depth from their own code, drawn from 251 documents. The full list is below, grouped by state, and each place page links to the document it came from with the date we read it. The list grows as we confirm more.

Which Climate, Soil and Population Data Do We Use?

• Climate: NOAA 1991-2020 U.S. Climate Normals, for winter context only.
• Soil: USDA NRCS soil survey, for frost-action context only.
• Population and ZIP areas: U.S. Census Bureau data, to decide which places to cover and to match ZIP codes to places.

Which Sources Do We Leave Out?

Websites that publish frost depth tables or maps without citing a code. They are often outdated or approximate, and we do not take values from them, even where they agree with ours. See how we choose sources.