Why posts and piers lift: frost heave vs. adfreeze

A footing inside the frozen zone gets lifted by ice lenses; a deep straight pier can still be gripped and jacked up by frozen soil (adfreeze). Go below the frost line, bell the bottom, and backfill with gravel.

Three cross-sections of a wood post set in a concrete pier, with frozen soil shaded above a dashed frost line. A, marked wrong: the pier is too shallow and sits inside the frozen zone; ice lenses grow beneath it and push it up off its original seat. B, marked wrong: a straight pier reaches below the frost line, but frozen soil bonds to its sides and jacks it upward, which is called adfreeze. C, marked correct: the pier goes below the frost line and ends in a wider bell in unfrozen soil that resists uplift, with free-draining gravel backfill around the shaft in the frozen zone so there is less wet soil to grip it. — Source: Frost heave requires freezing temperatures, frost-susceptible soil and water; IRC R403.1.4.1 frost protection

Use this diagram free on your site — grab the embed codeGet the code

What this diagram shows

Three cross-sections of a wood post set in a concrete pier, with frozen soil shaded above a dashed frost line. A, marked wrong: the pier is too shallow and sits inside the frozen zone; ice lenses grow beneath it and push it up off its original seat. B, marked wrong: a straight pier reaches below the frost line, but frozen soil bonds to its sides and jacks it upward, which is called adfreeze. C, marked correct: the pier goes below the frost line and ends in a wider bell in unfrozen soil that resists uplift, with free-draining gravel backfill around the shaft in the frozen zone so there is less wet soil to grip it.

Use this diagram — free

You're welcome to use this diagram on your own site, blog, handout, or lesson at no cost. The only condition is a visible credit link back to the source page. Copy the snippet below and paste it into your page — the credit is already included.

Embed code (HTML)

<figure style="margin:0;max-width:800px">
  <a href="https://remodelcalculators.com/construction-diagrams/frost-heave-adfreeze" target="_blank" rel="noopener">
    <img src="https://remodelcalculators.com/api/figure-embed/frost-heave-adfreeze" alt="Three cross-sections of a wood post set in a concrete pier, with frozen soil shaded above a dashed frost line. A, marked wrong: the pier is too shallow and sits inside the frozen zone; ice lenses grow beneath it and push it up off its original seat. B, marked wrong: a straight pier reaches below the frost line, but frozen soil bonds to its sides and jacks it upward, which is called adfreeze. C, marked correct: the pier goes below the frost line and ends in a wider bell in unfrozen soil that resists uplift, with free-draining gravel backfill around the shaft in the frozen zone so there is less wet soil to grip it." width="800" height="520" style="width:100%;height:auto;border:0" loading="lazy">
  </a>
  <figcaption style="font:14px/1.4 -apple-system,sans-serif;margin-top:6px;color:#555">
    Diagram: <a href="https://remodelcalculators.com/construction-diagrams/frost-heave-adfreeze" target="_blank" rel="noopener">Why posts and piers lift: frost heave vs. adfreeze</a> — courtesy of <a href="https://remodelcalculators.com" target="_blank" rel="noopener">RemodelCalculators.com</a>
  </figcaption>
</figure>

Please keep the credit link intact and don't alter the diagram itself. Redistribution as part of a competing diagram library or template pack isn't permitted. Questions? Reach us via the site.

Was this diagram helpful?