Deck footing failure
A footing either moves up or down, and the two have different causes. Heaving is frost; settling is soil. The repair depends on which one you have, and they look similar from above.
Heaving or settling?
| Symptom | Heaving (frost) | Settling (soil) |
|---|---|---|
| When it appears | After the first hard freeze | Gradually, over seasons |
| Direction | Deck rises, often unevenly | Deck drops at one or more posts |
| Seasonal pattern | Returns partway in spring | Never comes back |
| Typical cause | Footing above frost depth | Undisturbed soil not reached, or poor drainage |
A deck that is level in autumn, out of level in February and partly back in May is heaving. A deck that has been slowly dropping at one corner for three years is settling.
Heaving: the footing is too shallow
There is no surface fix. The footing has to go below the frost line, which means digging it out and re-pouring — or, on a deck low enough to allow it, replacing the footing with a properly embedded one.
What people try instead, and why it does not work:
- Shimming the post. It corrects the height for one season and the footing heaves again next winter, now with a shim that has compressed.
- Adding a second footing beside it. The new one heaves too, unless it is deeper.
- Cutting the post. Same problem in reverse.
Two details reduce heave without extra depth: a flared base, which resists uplift, and a slip sleeve or tapered form so the frost grips the soil rather than the concrete. Both are cheaper than digging deeper and neither is a substitute for depth.
Settling: the soil is the problem
- Footing bearing on fill. The hole was dug through disturbed soil and the concrete sits on it. Fill consolidates and the footing drops.
- Water. A downspout discharging beside a post washes soil away or softens it. This is the single most common cause of one corner dropping.
- Undersized footing. A footing whose bearing area is too small for the load will settle into the soil over time.
Fix: first redirect the water. Then, if the movement has stopped, the post can be shimmed with a solid, non-compressible shim (steel, or a treated block) and re-fastened. If the movement continues, the footing needs to be underpinned or replaced.
Deck blocks are a special case
A precast block sitting on the surface has no depth at all, so it is entirely in the frost zone and it bears on whatever the surface soil is. In a cold climate it will heave; on soft ground it will settle. They work in warm climates for low decks on drained, firm grade — and that is a narrow set of conditions.
When to stop and get it looked at
- The ledger has separated from the house.
- More than one post has moved in different directions.
- The beam has pulled away from a post.
- The deck is more than 30″ above grade and moving.
These are not shimming situations. A moving deck at height is a fall hazard, and the repair is a structural one.
What this page will not size
- It does not size a replacement footing. Bearing area depends on load and soil capacity.
- It does not give your frost depth — see frost depth.
- It does not cover helical piles or underpinning systems, which are specialist repairs.
- It does not calculate concrete — that is the footing calculator.
Questions people actually ask
Why is my deck sinking on one side?
Usually water. A downspout or a grade that slopes toward the post softens or washes away the soil under the footing. Fix the drainage first, then assess whether the movement has stopped.
Why does my deck rise in winter?
Frost heave. The footing is above the frost line, water in the soil freezes and expands, and it lifts the concrete. There is no surface fix — the footing has to be deeper.
Can I shim a deck post to fix a settled footing?
If the movement has stopped, yes, with a non-compressible shim. If it is still moving, shimming hides the problem for one season.
Do deck blocks fail in cold climates?
Yes. They sit on the surface, entirely within the frost zone, so they heave every winter and settle on soft ground. They are only appropriate in warm climates on firm, drained grade.