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Frost depth and foundations in Spokane

Spokane winters are cold enough that freezing ground is a real factor in foundation design and repair. Here is what the local codes say and what it means for an existing home.

Spokane's official frost depth: 24 inches

A joint City of Spokane and Spokane County policy (BUILD-07) sets frost depth at 24 inches below finished grade, unless a soils investigation supports another approved depth. Detached residential accessory structures may use 18 inches, and frost-protected shallow foundations are allowed under the IRC.

Other local design numbers

The City of Spokane's adopted design criteria list a ground snow load of 39 pounds per square foot, "Severe" weathering, a 10°F winter design temperature and required ice-barrier underlayment. "Severe" weathering means concrete must meet higher minimum strength.

Why soil matters

Spokane County's foundation requirements sheet (BP-12) shows how soils differ. Gravels and sands drain well and have low frost-heave potential. Silts and clays drain poorly, and some have high frost-heave potential. Footings bearing on soil that holds water near the surface can move more in freeze-thaw cycles.

Signs worth an inspection

Cracks that are wider at one end, stair-step cracks in block, or horizontal cracks
Doors or windows that start sticking after winter
Sloping floors or gaps at trim
Water pooling against the foundation, or downspouts dumping next to it

Older homes, outbuildings and additions may not have footings that meet today's depth. A contractor or structural engineer can check on site.

This page summarizes public code documents and is not engineering advice.

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