Frost depth in Washington
Washington has two code depths: 12 inches in 24 inches — and which one applies depends on the county you build in. Add 6 inches of bearing and that becomes a dig depth of 18" / 30". The numbers below are the code values an inspector measures, not climate averages — the local building department always has the final word.
| Area / zone | Frost depth | Dig depth | Set by |
|---|---|---|---|
| Seattle & west of the Cascades | 12" | 18" | Washington State Residential Code Table R301.2(1) |
| Spokane & east of the Cascades | 24" | 30" | Spokane Municipal Code Table R301.2(1) |
Two rows, two numbers: the county you build in decides which one governs your permit. Never take the shallower value to a counter in a deeper zone.
Why Washington's number looks the way it does
Washington's frost rule is the Cascade crest: 12 inches in Seattle and everywhere the Pacific moderates, 24 in Spokane and the east side, where winters turn continental.
From frost depth to dig depth in Washington
Footings bear on undisturbed soil below the frost line, so the hole goes 6 inches deeper than the code number. In Washington that means:
| Code frost depth | Dig depth | Shortest Sonotube |
|---|---|---|
| 12" | 18" | 4 ft tube |
| 24" | 30" | 4 ft tube |
A 4-inch gravel base at the bottom of the hole is common practice for drainage, and if you use one, add it to the tube length rather than the dig depth — the footing still has to reach the full 12–24 inches below grade.
How much concrete a footing takes in Washington
Bags of 80-lb mix per footing, for a solid pier with no post in the hole, at Washington's code depth:
| Frost depth | 10" hole | 12" hole | 16" hole | 20" hole |
|---|---|---|---|---|
| 12" (18" dig) | 2 bags | 2 bags | 4 bags | 6 bags |
| 24" (30" dig) | 3 bags | 4 bags | 6 bags | 10 bags |
The bags calculator converts any total into 80, 60, 50 or 40-pound bags, and the concrete calculator handles slab and footing volumes.
Washington frost depth FAQ
How deep is the frost line in Washington?
Washington has two code depths, and the county you build in decides which one an inspector measures: 12 inches in Seattle & west of the Cascades, and 24 inches in Spokane & east of the Cascades. These are code values, not the average depth the ground actually froze last winter.
How deep do footings have to go in Washington?
Bearing sits below the frost depth, so dig to the code depth plus 6 inches: 12 inches of frost means a 18-inch dig, and 24 inches of frost means a 30-inch dig in Washington. The extra 6 inches is the bearing layer that keeps the footing from sitting on frost-susceptible soil, and it is what Washington State Residential Code Table R301.2(1) expects to see on a permit drawing.
How do I know which Washington zone my county is in?
The citation in the table above names the authority that drew the lines — Washington State Residential Code Table R301.2(1); Spokane Municipal Code Table R301.2(1). Your county or city publishes its own amendment to IRC Table R301.2(1) (or its own zone map), and that local document, not a state table, is what a permit reviewer checks. One call to the building department settles it before you rent an auger.
Plan a pour in Washington
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Last updated: 2026-11-02