How deep do footings need to be in Central Oregon?

What the frost line is

When the ground freezes, water in the soil expands. Soil that freezes under a footing can lift it, and when it thaws the footing settles again, often unevenly. The frost line, or frost depth, is how deep the ground is expected to freeze. Putting the bottom of a footing below that depth keeps the foundation out of the freeze and thaw zone.

The Deschutes County number

Deschutes County publishes its building codes design criteria for permit applicants. For frost depth and footing depth it lists 18 inches minimum. The same page sets residential seismic design category C under the 2021 Oregon Residential Specialty Code, and assumes category D for commercial buildings unless a geotechnical report says otherwise.

The state also maintains an Oregon Design Criteria Hub that reports ground snow load and frost line depth for a location under both the residential and commercial codes. Use it, or call the building department, to confirm the values for your address, especially if you are building inside a city or outside Deschutes County.

Why 18 inches is the floor

Minimums in a design criteria table are what the building department will accept without further justification. Plans drawn by an engineer, a manufacturer's foundation details for a steel building, or site conditions such as fill, slopes or poor soils can all call for deeper or wider footings. When plans and the county minimum differ, build to the more demanding one.

Snow load reaches the foundation too

Footings carry everything above them, and in Central Oregon that includes snow. Deschutes County says snow load is site specific and must be determined using the Oregon Snow Load Map, because it changes with location and elevation. The state's how to guide adds that for prescriptive design under the 2021 residential code, you use the greater of the mapped ground snow load and a code minimum of 36 psf.

A heavier roof load means heavier loads on posts, walls and footings, which is one reason a steel building and its foundation should be designed for the same site. See snow load requirements in Central Oregon.

Slabs, monolithic pours and steel buildings

Not every structure uses deep continuous footings with stem walls. Garages and steel buildings are often built on a slab with thickened edges, and pole or post buildings use individual footings. Each still has to meet the frost depth where the code requires it, and each has to carry the design snow load. Manufacturer details, the plan and the building department decide which applies.

Exempt flatwork, such as a patio or driveway that carries no building, is a different case. See permits for driveways and slabs.

Pouring footings in cold weather

Footings in Central Oregon often go in when nights are freezing. Concrete has to be kept from freezing until it reaches about 500 psi, and it should never be placed on frozen ground or against ice. See pouring concrete in winter.

Frost and the ground around the footing

Depth is only part of frost protection. Water is what freezes and expands, so drainage around a foundation matters too. Grading the ground to slope away from the building, keeping downspouts from dumping at the footing, and building on a compacted, well drained base all reduce the water available to freeze. A footing at the right depth surrounded by wet, poorly drained soil is still asking for trouble.

Backfill is the other piece. Loose fill against a stem wall settles over time and can direct water toward the foundation. Compacting backfill in lifts and grading it away from the wall keeps the site stable.

Questions for your foundation contractor

Ask what footing depth and width the plan calls for and whether it meets the county's 18 inch minimum; what the design snow load is and whether the footings were sized for it; how the base will be prepared and compacted; how reinforcement will be placed and inspected; and, if the pour is in the cold, how the concrete will be protected until it reaches 500 psi. If a steel building will sit on the foundation, ask whether the anchor layout matches the manufacturer's details.

How RCC builds foundations

RCC does its own excavation and foundations, and pours to the plan and the local design criteria. Send us your plans, or the building you are planning, and the address.