Frost Line Depth by State: How Deep Your Footings Actually Need to Go

The frost line is the deepest point in the ground that freezes during a normal winter. Anything sitting in soil that freezes is going to move, concrete, brick, fence posts, deck piers, all of it. That's the whole reason footings exist.

If you put a footing above the frost line, the soil freezes underneath it, expands, lifts the footing an inch or two, and then drops it back down a different inch or two when it thaws. Do that for a few winters and your fence leans, your deck pulls away from the house, and your slab cracks.

Below the frost line, soil temperature stays roughly constant year-round. Nothing moves. That's where footings belong.

What goes wrong when you ignore frost depth

We've watched a lot of weekend projects fail in the same way. Someone pours a 12-inch deep concrete pad in central Pennsylvania for a shed. The first winter is fine. By year three the slab has a half-inch crack running diagonal across it and the shed door won't latch in February.

That's frost heave. The fix is always the same, break it out and start over deeper, and it always costs more than digging the extra 18 inches the first time.

The other failure mode is fence posts. A 6-foot wood fence with posts buried 18 inches deep in Wisconsin is going to tilt within 4 years. We've seen it dozens of times. The posts didn't rot. The fence didn't blow over. The ground just kept lifting them.

Frost line depth by state

This table is built from adopted local building code values across the US. These are the numbers building departments require for footings, not the National Weather Service historical-average frost depth. The local code number is what you actually need.

Where states have a wide range (Texas, California, North Carolina), the range reflects real differences between coastal and mountain or southern and northern parts of the state. Look up your county.

Why the local code number is deeper than the actual frost

Two reasons.

First, codes are written for the worst winter in roughly 100 years, not an average winter. A bad winter in central Pennsylvania can push frost down to 38 inches even though most years it stops at 28. The code uses the 38.

Second, soil isn't uniform. A footing under a driveway sits in compacted, exposed dirt that loses heat faster than a footing under shaded grass. The code assumes the worst-case soil exposure for your area.

So the 42 inches your township requires isn't paranoia. It's the number that survives the bad year, in the bad spot, with the bad soil.

How to find your exact local frost depth

Three options, in order of how much we trust them:

1. Call your county or municipal building department. They will give you the adopted frost depth for footings, plus any special rules for your soil type. This is the only number that matters when you pull a permit. Takes 5 minutes.
2. Check your state's adopted residential code. Most states use the IRC (International Residential Code) with state amendments. The state amendments file lists frost depth by county. Search "[your state] residential code frost depth."
3. Use the table above as a starting point. Fine for back-of-envelope project planning. Don't pour concrete based on it.

We've had readers email us asking why the number their building department gave them is different from a frost map they found online. The answer is always the same: the building department is right. The map is averaged data from a weather station 40 miles away.

What this means for common projects

Fence posts: Bury one-third of the post plus the frost depth, whichever is greater. A 6-foot fence in central Ohio with 36 inches of frost depth needs a 36-inch hole, not a 24-inch one. Yes, that's a lot of digging.

Deck footings: Below frost line, no exceptions. Sonotube footings are typically 4 to 6 inches taller than the frost depth so the top of the concrete is above grade.

Shed slabs: A floating slab can sit above frost line if the soil is well-drained and the slab is thick enough to handle some movement. Most building departments still want a thickened edge that goes below frost. Check before you pour.

Concrete patios: Most patios are unconnected to the house and ride the frost movement as a slab. They will heave a little, especially the corners, but properly drained gravel base under the slab handles most of it. Patios are not footings.

Fence posts in Florida: No frost depth requirement. Bury one-third of the post (24 inches for a 6-foot fence) and you're done.

What can go wrong

We've collected the most common frost-line mistakes from reader emails over two seasons:

• Measuring from the wrong reference. Frost depth is measured from final grade, not from the existing ground. If you're going to backfill 8 inches, your hole needs to be 8 inches deeper than the frost number.
• Forgetting the gravel base. A 6-inch gravel pad at the bottom of the hole counts as part of the depth. Dig 6 inches deeper than the frost line so the top of the gravel is at frost depth.
• Pouring on a warm day in winter. If the ground is already frozen above where you're pouring, the concrete will set on top of an ice plug. The plug thaws in spring and your footing sinks.
• Trusting an old contractor's number. Codes get updated. A neighbor who poured a footing 30 years ago might have used a 24-inch depth that is no longer code in your county.
• Skipping the permit. Frost depth is the single most-checked dimension on a footing permit. Inspectors measure it. If it's short, the inspector makes you dig it out and start over.

If you spot an error in the state table or have a county-specific number that contradicts what we have, email us. We update this guide quarterly and credit the catch.

Last updated May 2026