Deck Footing Size Troubleshooting

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Signs of a Settling or Undersized Footing

Deck pulling away from the house. A growing gap at the ledger, flashing that has torn, or lag screws that have elongated their holes often means the outer posts dropped relative to the band joist. Joist hangers and a ledger that was installed correctly do not create a 3/4-inch air gap on their own. Measure the gap at several bays. If it is worse at midspan posts than at the corners, those interior footings are the suspects.

Uneven deck surface. Stand a 4-foot level on the decking in both directions. A dish over one post, a ridge over another, or a consistent tilt away from the house is settlement, not “the boards dried.” Compare that map to the post grid. The low post is the one whose pad moved.

Cracking. Cracked concrete at the top of a tube, a split in a post base, stucco cracking on the house under the ledger, or decking fasteners that keep backing out are movement. Hairline shrinkage in a pad is not the same as a pad that has tilted. Photograph and date the cracks; seasonal growth is the story.

Rule out the other two tables before you blame the dirt. Midspan bounce with a level deck can be an undersized beam or joist. A deck that is both bouncy and out of level is often both.

Undersized Plan vs Shallow Frost — Two Different Failures

Plan size (diameter / square side / thickness) is Table R507.3.1. Depth is R507.3.2 and R507.3.3. You can pass one and fail the other.

• Undersized plan: the pad is too small for the tributary area and the real soil. The soil compresses. Settlement is often slow and worse on the most heavily loaded interior posts. Fix is a larger pad, more posts (which cuts tributary area), or engineered piers — not a longer 4x4.
• Shallow frost: the pad may be plenty wide and still sit above the frost line. Water in the soil freezes, expands, and lifts the post. Spring thaw drops it somewhere else. The deck racks, the ledger gap opens and closes with the seasons, and the surface is never quite the same twice. Fix is depth to the local frost line (or solid rock, or another approved method), not a fatter cookie at the same elevation.

Frost Depth Considerations by Region

Frost depth is a local number in Table R301.2, not a national average you can memorize. The ranges below are order-of-magnitude context so you know whether a 24-inch tube is even in the conversation. Always verify the published depth with the building department.

• Little to no frost (roughly 0–12 in): much of Florida, the Gulf Coast, south Texas, low-desert Southwest, coastal southern California, Hawaii. You still need undisturbed bearing soil, often 12–24 inches below grade. Some coastal AHJs still want deeper tubes for wind uplift, not frost.
• Shallow frost (roughly 12–24 in): interior South, parts of the Carolinas and Tennessee, mid-elevation Southwest. 24–36 inch holes are common once you add the bearing layer.
• Moderate frost (roughly 24–42 in): Mid-Atlantic, Kentucky, Ohio Valley, Missouri, lower Midwest. 36–48 inch holes are normal. Southington, CT publishes 42 inches on its deck handout — that is a real AHJ number, not a blog guess.
• Deep frost (roughly 42–60+ in): New England, Great Lakes, Upper Midwest, northern Plains, mountain West. 48–66 inch holes, sometimes deeper. A correctly sized 16-inch-diameter pad that stops at 30 inches will still heave here.

Detached low decks sometimes get a frost-depth exception in a given code cycle; attached decks generally do not. If the house is frost-protected, the deck that is attached to it usually must be too. Confirm the exception in the adopted code before you pour a shallow pad because a forum said it was fine.

What to Do Next

1. Map the post grid and measure tributary area for the low post. How: guide.
2. Look up the cell in the calculator. If the existing pad is smaller than the tabulated square or round, plan size is a problem. If the pad is large enough but the hole is above frost depth, depth is the problem. Both can be true.
3. Check the beam and joists so you are not blaming dirt for a span miss: beam troubleshooting, joist troubleshooting.
4. Sistering a post onto a cookie that already moved does not reset the soil. New bearing, new depth, or an engineer.

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