For the most part, we build decks just as we did 50 years ago, and footing placement is a good example of that. Now, as ever, footings must be set below the frost line if you want to maximize deck lifetime.
If you’re from a warm region that doesn’t experience freezing temperatures, you might not know what a frost line is and why it’s important to your deck construction. Frost line is the depth to which water can freeze in the soil. If your footings are above the frost line, you risk the structure’s stability because as water freezes and expands, it can create damaging upward pressure on your structure. Here’s everything you need to know about determining the frost line in your specific location.
What is the Frost Line?
The frost line, also known as frost depth, refers to the depth at which groundwater in the soil is expected to freeze during the coldest part of the year. In deck construction, knowing the frost line is crucial because it directly impacts the stability and integrity of the deck’s foundation.
Frost depth varies significantly across different climates and geographic locations. In colder regions—like the northern United States, Canada, and mountainous areas—frost depth can reach several feet. In contrast, warmer regions in the southern U.S. might have frost depths of only a few inches, or none at all. Soil type and moisture content also affect frost depths.
Why is Frost Line Depth Important?
Frost depth is important because it determines how deep structures must be anchored to avoid frost heave, the upward shifting of the ground that occurs when it freezes. This affects more than decks. Frost heave can also cause problems with fences, foundations, and retaining walls.
- Decks. If the footings aren’t placed below the frost line, the deck could shift or become unstable due to seasonal ground movement.
- Fences. Fence posts that aren’t set below frost depth might lean or even pop out of the ground after freeze-thaw cycles.
- Foundations. Shed foundations that don’t extend below the frost line could crack, settle, or suffer structural failure.
- Retaining walls. If the footing of a retaining wall is too shallow, the wall may not remain anchored, instead being pushed outward or upward.
How to Determine Frost Line Depth
The local building code specifies the frost line depth for footings based on field experience and historical data from frost tube instruments and climate observations. The National Weather Service monitors frost depths because they are a key factor in hydrologic forecasting. Frost depth lines can change from year to year depending on snow cover, moisture amounts, and air temperature. Even so, building code requirements for frost line depths rarely change. Some jurisdictions allow frost-protected shallow foundations.
Frost Line Depth by Zip Code
Several websites show frost lines by state, but these depths aren’t to be used to determine deck footing depths. Only your local building officials have that authority.
Some websites offer a zip code search, but the search is limited to fewer than 100 zip codes. I’m unaware of any database that lists the frost depth for every US zip code. In any case, the depths listed may or may not agree with the building code. So, I repeat: check with local building authorities first. And before you dig, get the all-clear from your utility providers so you don’t accidentally strike a cable or pipeline.
Building Considerations & Best Practices
Follow these basic steps to get the footing depth right the first time:
- Extend the footings below the frost line, not to it. That is, if the frost line is 24 inches, dig the footing hole a few inches deeper than that. Dig it even deeper if you plan to add gravel or crushed rock to the bottom for better stability and drainage.
- Use durable materials that withstand freeze-thaw cycles. Most decks use concrete footings, but helical piles, which are made of steel, are another option.
- Consult the building code before construction. Visit your municipality’s website to check local building codes. Better yet, pay a visit to the building inspector’s office. Experienced local building contractors will also have the necessary expertise.
Conclusion
Failing to account for frost depth is one of the most common and costly mistakes that DIY builders make. It can lead to structural failure, safety hazards, and costly repairs in the future.
If you want your deck to stand strong and stable for years to come, install the right size footings at the right depth. That means below—not at—the frost line. Once the footings are at that depth, the soil under them will not freeze or expand. That provides your deck with a solid, unmoving base
Sources
TrexSeal. How Long Does a Deck Last? Retrieved April 23, 2025, from https://trexseal.com/blog/how-long-does-a-deck-last
Minnesota Department of Natural Resources. Frost Depth in Minnesota for Winter 2025. Retrieved May 20, 2025, from https://www.dnr.state.mn.us/climate/journal/frost-depth-minnesota-winter-2025.html
Minnesota Department of Labor and Industry. Frost Depth. Retrieved May 20, 2025, from https://www.dli.mn.gov/sites/default/files/pdf/bc_map_frost_depth.pdf
NWS North Central River Forecast Center. Frost Depth. Retrieved May 20, 2025, from https://www.weather.gov/ncrfc/LMI_FrostDepthMap
National Centers for Environmental Information. Frost Protected Shallow Foundations. Retrieved May 20, 2025, from https://www.weather.gov/ncrfc/LMI_FrostDepthMap
Protect. Creating a Floating Deck Over a Flat Roof. Retrieved April 23, 2025, from https://trexprotect.com/uk/blog/creating-a-floating-deck-over-a-flat-roof/
TrexProtect. Cost of Deck Installation: How Much Does It Actually Cost? Retrieved April 23, 2025, from https://trexprotect.com/blog/cost-of-deck-installation-how-much-does-it-actually-cost/
Plasticine House. 8 Ground Level (Floating) Deck Footings Options. Retrieved April 23, 2025, from https://plasticinehouse.com/floating-deck-footing-options/
House by the Bay. How to Build a Floating Deck. Retrieved April 23, 2025, from https://www.housebythebaydesign.com/how-to-build-a-floating-deck/
Seamster. Build a Floating Deck. Retrieved April 23, 2025, from https://www.instructables.com/Build-a-Floating-Deck/