WA · set by county and city amendment
How deep do deck footings and fence posts have to go in Washington? Below the frost line. Here's the number, and the winter behind it.
The requirement
12″
Footing bottoms in Washington must reach 12 inches below grade — the frost line. Across Washington it runs roughly 12″–24″ by county.
Reference: 2021 Washington State Residential Code (WAC 51-51), Section R403.1.4.1 and Table R301.2 (2021 edition). Counties amend their codes — confirm the exact figure with your local building department before you pour.
Depth pre-filled to the Washington 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.
Marine air off the Pacific keeps western Washington mild, so the ground seldom freezes more than a few inches. East of the Cascades, continental cold settles in for weeks each winter and drives frost as much as two feet down.
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.
The state adopts the IRC statewide but each jurisdiction fills in its own frost line. Puget Sound cities (Seattle, Tacoma) use the 12 in. code minimum, while Spokane's official joint city/county policy requires 24 in.; high mountain areas can require more, so confirm locally.
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.
12 inches is the working number (set by county and city amendment). Across Washington it runs roughly 12″–24″ 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 Washington that means at least 24″. The post depth calculator checks both rules for you.
Below the frost line — 12″ 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.