diyApr 3, 20268 min read

Deck Footing Frost Line Depth by City: How Deep Do Your Footings Need to Be?

PermitDeck Team

Data-Driven Home Improvement Intelligence

Frost line depth determines how deep your deck footings must go. We compiled real frost line data for 50+ cities from 0 inches (Louisiana) to 48 inches (Boston, Madison).

How Deep Do Deck Footings Need to Be?

Your deck footings must extend below the frost line — the depth at which the ground freezes in winter. If footings are too shallow, the freeze-thaw cycle will heave them upward, cracking your deck and creating a safety hazard.

This is one of the primary reasons cities require deck permits and inspections: to verify your footings reach adequate depth.

Frost Line Depths by City

After researching 100 US cities for deck permit requirements, we compiled frost line data from official sources. Here's what we found:

No Frost Concern (0-6 inches)

Southern cities where ground freezing is rare or nonexistent:

CityFrost LineSource Baton Rouge, LA0 inchesNo frost concern Miami, FL0 inchesNo frost concern Orlando, FL0 inchesNo frost concern Houston, TX0-6 inchesMinimal frost

Shallow Frost Line (12 inches)

Mild winter cities:

CityFrost LineNotes Knoxville, TN12 inchesMinimum per city Birmingham, AL12 inches Charleston, SC12 inches Savannah, GA12 inches Greensboro, NC~12 inchesPiedmont NC

Moderate Frost Line (18-24 inches)

Mid-range cities:

CityFrost LineNotes Tulsa, OK18 inchesPer Title 51 Oklahoma City, OK18 inches Richmond, VA~18 inches Virginia Beach, VA18 inchesCoastal VA Boise, ID24 inches Spokane, WA24 inchesPer Joint Frost Policy Wichita, KS24 inches Reno, NV~24 inchesHigh desert

Deep Frost Line (30-36 inches)

Cities with serious winters:

CityFrost LineNotes Colorado Springs, CO30 inchesPer PPRBC Section RBC302.4.63 Salt Lake City, UT30 inches Philadelphia, PA36 inches Kansas City, MO36 inches St. Louis, MO36 inches

Very Deep Frost Line (40-48 inches)

The deepest frost lines in major US cities:

CityFrost LineNotes Providence, RI40 inchesStatewide (RI) Anchorage, AK42 inchesMin for exterior footings Des Moines, IA42 inchesAll footings Detroit, MI42 inches Minneapolis, MN42 inches St. Paul, MN42 inches Madison, WI48 inchesDeepest major city Boston, MA48 inches

Why Frost Line Depth Matters for Your Deck

The freeze-thaw cycle

Water in soil expands when it freezes, pushing anything above it upward. When soil thaws, it settles unevenly. This cycle repeats all winter long, and footings above the frost line will gradually shift, tilt, and crack.

The real-world consequences

Structural damage: Posts shift, beams separate, boards crack

Safety hazard: Uneven surfaces, wobbly railings, collapsed sections

Failed inspections: Your deck won't pass final inspection with shallow footings

Insurance issues: Damage from improperly installed footings may not be covered

Exceptions: When Shallow Footings Are Allowed

Some cities allow exceptions for specific deck types:

Anchorage, AK: Exterior decks not greater than 72 inches above grade may use near-surface pier blocks on adequate soils. Decks under 30 inches and uncovered don't need footings below frost line.

Boise, ID: Freestanding decks not supported by the dwelling don't require footings below the frost line.

St. Paul, MN: Decks not attached to a structure with frost footings may be exempt from the 42-inch requirement.

These exceptions generally apply to small, freestanding, low decks. Attached decks almost always require full-depth footings.

How to Find Your Exact Frost Line

1. Check our city guides — We include frost line data where available

2. Call your local building department — They know the exact requirement for your area

3. Check your building code — Most cities specify frost depth in their adopted IRC amendments

The Cost Impact of Deep Footings

Deeper frost lines mean higher construction costs:

12-inch frost line: Standard post holes, minimal extra cost

36-inch frost line: Requires deeper holes, more concrete, potentially rental of a power auger

48-inch frost line: Significant excavation. Budget an extra $50-$100 per footing compared to a 12-inch frost line city

For a typical deck with 6-8 footings, the frost line depth difference between Savannah, GA (12 inches) and Boston, MA (48 inches) could add $300-$800 to your project.

Use our deck cost calculator to estimate your total project cost including foundation work.

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*Frost line depths shown were compiled from official city building codes and may vary within metro areas. Always confirm the exact frost depth requirement with your local building department before construction.*