CO · set by local jurisdictions
How deep do deck footings and fence posts have to go in Colorado? Below the frost line. Here's the number, and the winter behind it.
The requirement
36″
Footing bottoms in Colorado must reach 36 inches below grade — the frost line. Across Colorado it runs roughly 30″–48″ by county.
Reference: City and County of Denver Residential Code (2021 IRC as amended), Section R403.1.4.1 (36-inch frost depth); Colorado has no mandatory statewide residential code (2021 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Colorado frost line. The hole bottom has to reach it — a few inches deeper seats the base cleanly below.
| Bag size | Exact | Buy (with spare) |
|---|
The buy column adds a 10% spare margin and rounds up. Running out mid-pour is the one mistake you can't fix.
Colorado's continental climate brings long stretches of subfreezing weather that push frost about three feet into Front Range soils. High mountain valleys stay colder far longer, so the ground there freezes four feet deep or more.
The depth tracks how much freezing a winter actually delivers. NOAA's Air Freezing Index keeps a running score of how far and how long air temperatures sit below freezing; deeper scores mean deeper frost, and building codes translate that climate record into the footing depth above. Water expands about 9% when it freezes — when that happens under a footing, the ground lifts it. That's frost heave, and it cracks foundations, tilts fence lines, and pops deck piers. Getting below the frost line puts your concrete where the ground never freezes.
Colorado is a home-rule state, so every jurisdiction sets its own depth: Denver and Aurora require 36 in., Colorado Springs and several Front Range counties 30 in., and mountain towns like Vail 48 in. or more. Always confirm with the local building department before digging.
Treat this page as the starting number, not the permit. County and city amendments are where the real precision lives, and inspectors enforce the local figure — one phone call to the building department settles it.
36 inches is the working number (set by local jurisdictions). Across Colorado it runs roughly 30″–48″ by county. Confirm the exact figure with your local building department — county amendments are common.
Bury one third of the above-grade height, never less than 24″, and never shallower than the frost line — in Colorado that means at least 36″. The post depth calculator checks both rules for you.
Below the frost line — 36″ minimum, bearing on undisturbed soil beneath it. Size the diameter with the deck footing calculator, then figure bags with the depth pre-filled above.
Yes — 811 covers every state, and the locate is free. Call a few business days before any post hole or footing dig and crews mark the buried utilities.