How We Find and Verify Every Frost Line Depth

Where Do Our Frost Line Depths Come From?

They come from official building codes and local government documents, in that order. We start with each state’s adopted residential code and any statewide rule, then look for the design-criteria table that a city or county fills in under IRC Table R301.2. That table is where a jurisdiction states the minimum footing depth below finish grade. We checked all 50 states and DC.

In 33 states the state leaves frost depth to local option, so the number lives in a city or county document. The other states fix a number, set zones or a table, or have no statewide residential code.

How Do We Check Each Value?

A value is shown only after the quote that contains it has been read in the official document and logged. Our review log has 310 entries, and a good share of them were rejections or corrections. Real examples of what we reject:

• A document from the wrong state or the wrong jurisdiction.
• A table that was never filled in.
• A plumbing rule mistaken for a frost depth, such as pipes “6 inches below the frost line”.
• A misread number, such as 96 where the document said 5, or a rainfall figure read as a depth.

The reading is done by AI research assistants against the document text, with the project team resolving the uncertain ones. We do not yet have a named licensed reviewer. The About page says so.

What Do the Labels on Each Page Mean?

Why Do Some Places Show Nearby Values Instead of Their Own?

Because many jurisdictions never publish a frost depth, or publish it in a scanned file we cannot read with confidence. We would rather show nothing than an estimate presented as code. Nearby values are always marked as someone else’s, and they are not a legal depth for your lot.

How Do We Use NOAA and USDA Data?

As context only. We use NOAA climate normals and the USDA soil survey to describe the winter and the soil in an area. Neither is used to calculate or replace a code depth.

How Often Do We Update?

Each value carries the date its document was read, and pages carry a “checked” month. Codes change when a jurisdiction adopts a new edition, so values can go out of date. If you find a newer document, send it to us.

How Do You Report a Wrong Value?

Use the template on the contact page. Include the place, the value you see, the value you think is right, and a link to the official document.

Who Reviews This Work?

At the moment, the project team and AI research assistants. No named licensed engineer or inspector has signed off on the numbers. See About for what that means for how you should use the site: as a research guide, with the building department as the final word.