Frost line depth in Maryland

Maryland’s Building Performance Standards require every jurisdiction to adopt the model residential code, then leave the climate-specific entries to be completed locally. Montgomery County publishes 30 inches in its residential deck guidance, and Baltimore County publishes the same figure in its deck construction details.

Thirty inches is a reasonable working assumption for the Washington and Baltimore suburbs, and a poor one for Garrett County in the far west, where the Appalachian plateau produces a genuinely different winter. The state’s own freeze record shows the gap: Oakland in Garrett County reaches its median first freeze in early October, seven weeks ahead of the Chesapeake shore.

Maryland has not amended the water service burial rule, so the model-code language governs: at least six inches below the local frost line and never less than twelve inches down. In the 30-inch counties that puts the service line three feet under.

Across all 43 Maryland stations with published normals, the median first 32°F night falls around November 2 and the median station records 21 nights a year below 20°F, which is the coldest threshold NOAA counts and a decent proxy for a hard freeze rather than a light frost. The state’s spread runs from OAKLAND 1 SE, where the median first freeze is October 9, to SOLOMONS at November 26.

The median Maryland station reaches freezing around 2 November and records about 21 nights a year below 20°F, one of the milder profiles among the states with a real winter. That median hides a large split. Oakland, at elevation in Garrett County, has its median first freeze on 9 October; Solomons, on the Chesapeake, not until 26 November. Maryland’s 43 reporting stations are the fewest of any state page here, so the medians are thinner than most.

Where these numbers come from

The Maryland frost figure was researched against the body that publishes it, and it’s recorded with a confidence note of “local jurisdiction, varies” in our source ledger. Freeze dates are NOAA normals for stations in the state, picked by a fixed rule rather than by hand: first-order stations, spread west to east. The full method is on our methodology page.

• Freeze dates and cold-night counts: NOAA NCEI U.S. Climate Normals, 1991 to 2020, annual and seasonal by station. Retrieved 2026-09-09. Field definitions are in the by-variable readme.
• ZIP code locations: U.S. Census Bureau 2023 ZIP Code Tabulation Area gazetteer, matched to the nearest station by great-circle distance.
• Montgomery County MD, guidelines for building residential decks (2021 IRC) The Maryland frost depth figure and basis.
• Baltimore County MD, deck construction guidelines The Maryland frost depth figure and basis.
• 2021 International Residential Code, Section P2603.5 (Freezing), ICC free code viewer The water service line burial rule.

The full method, including how we pick a station and what we do when there is not one close enough, is on our methodology page.