How Deep Should Pole Barn Posts Be in Spokane? Frost Line, Embedment, Backfill and Treated Posts (2026)

Short answer: a pole barn post has to go at least below your local frost line, which is 24 inches in Spokane and Spokane County, 30 inches in Stevens and Pend Oreille counties and 36 inches in Deer Park. That's only the minimum. The real depth comes from the engineering: how hard wind pushes sideways on the building, how much it tries to lift the roof, and how firm your soil is. Here's how post depth actually gets set, what the building code says about backfill and treated posts, and the mistakes that let posts heave or rot.

Step one: get below the frost line

When wet soil freezes, ice lenses grow and push upward on anything with a rough face, which is called frost heave. The National Frame Building Association (NFBA) says the first defense is to put the base of every embedded post below the frost line. Local minimums around Spokane:

• City of Spokane, Spokane Valley, Liberty Lake, Airway Heights and unincorporated Spokane County: 24 inches to the bottom of the footing, under the City and County joint frost depth policy
• Colville and rural Stevens County: 30 inches
• Pend Oreille County (Newport area): 30 inches below finish grade
• City of Deer Park: 36 inches

Our frost depth guide has the code citations for each one.

Step two: the engineering usually goes deeper

A pole building has no foundation wall. The posts are the foundation, so they have to resist three things at once:

• Downward load: roof snow and the building's weight, carried by the footing at the bottom of the hole
• Sideways load: wind pushing on the walls makes each post act like a lever against the soil
• Uplift: pole buildings are light and often have big door openings, so the NFBA notes that wind can apply significant uplift to the foundation

The International Building Code (IBC Section 1807.3) says embedded posts and poles must be designed by its own formulas or by the farm-building standard ASABE EP 486.3. In those formulas, embedment depth is the answer the engineer solves for, using the post size, the wind load, the height of the load above grade and the soil's allowable lateral bearing pressure. That's why there isn't one honest number for "how deep." Taller walls, higher wind exposure and softer soil all push posts deeper.

The NFBA lists the ways to make a post foundation stronger when soil stress runs too high: a thicker post, deeper embedment, a bigger footing or collar, restraint at grade, or stronger backfill. It adds that going deeper than frost depth is often the least expensive fix for lateral loads.

A slab helps more than you'd think

Tying the posts to a concrete floor keeps them from sliding sideways at grade. According to the NFBA, when a post can't move laterally at grade, it takes more than three times the applied load to cause a soil failure, with everything else unchanged. If you plan to pour a floor, tell your designer early, because it changes the post math. Our shop slab thickness guide covers the floor itself.

What goes back in the hole

IBC 1807.3.3 allows three backfill methods around posts that aren't embedded in a poured footing:

• Concrete: at least 2,000 psi, in a hole at least 4 inches larger than the post. For a square post that's 4 inches larger than the corner-to-corner diagonal, not the face, so a 6x6 needs a hole close to 12 inches across
• Clean sand: thoroughly compacted by tamping in layers no more than 8 inches deep
• Controlled low-strength material (CLSM): a flowable, self-compacting fill

Concrete gives the most sideways resistance, because the whole collar acts as the post's width. The NFBA's warning is that concrete backfill can be lifted by frost in poorly drained silt and clay, especially against rough hole walls or in holes that narrow toward the bottom. Many Spokane-area lots have silty soil, so the hole shape and drainage matter.

Two drainage mistakes that cause heave

• The sump effect: if a hole goes into a dense, slow-draining layer but not through it, coarse backfill can collect groundwater at the bottom of the hole. The NFBA suggests backfilling with the same tight soil that came out, or giving the water another path
• Low spots at the posts: backfill settles, and the NFBA says to fill those depressions and grade water away from the building

Uplift anchors

To keep wind from pulling a post up, the post needs an anchor below grade. The NFBA lists concrete footings, cast-in-place concrete collars, treated wood blocks or cleats, steel angles and concrete backfill. A collar has to be attached to the post so they move together. Ask to see the uplift detail on your plans.

Treated posts, or posts on piers

The IBC requires wood posts embedded in the ground to be pressure-treated to AWPA Use Category 4B, which is ground-contact treatment for structural posts. Lumber stamped for above-ground use doesn't qualify, even if it's labeled "treated."

Some owners skip buried wood entirely and set the posts in brackets on concrete piers, like a sonotube pier. The pier still has to reach below frost and be engineered for uplift and lateral load, but no wood sits in the soil. We pour both kinds.

Inspection before backfill

Spokane County's BP-32 pole building handout calls for a footing inspection before the post holes are backfilled. Don't fill a hole until it's been signed off. Our permit guide explains when a pole barn needs a permit, and our snow load guide covers the roof load your posts carry.

What it costs

Raven's current pricing for an installed pole building is about $60 to $75 per sq ft. That doesn't include electrical, plumbing or utility hookups, which are priced separately. Our pole barn cost guide breaks down what changes the total, and our red iron vs. pole barn comparison helps if you're still choosing a system.

Talk to Raven about your pole building

We dig, set and pour our own post footings and build to stamped engineering for your parcel. See our pole building services, call 509-350-2016, or request a free quote with your address and building size.

Sources

• Pole building pricing: Raven Buildings' current rates
• IBC 1807.3, Embedded Posts and Poles (UpCodes)
• IBC 1807.3.3, Backfill (UpCodes)
• NFBA, Post and Pier Foundation Design
• NFBA, Post Foundation Design Considerations
• NFBA, Post-Frame Building Terms (footings, collars and uplift anchors)
• City of Spokane / Spokane County Joint Policy BUILD-07, Official Frost Depth
• Stevens County Building Code Ordinance 02-1984 (frost depth)
• Pend Oreille County Geographic Design Criteria
• City of Deer Park Municipal Code 15.04, Building Codes
• Spokane County BP-32, Pole Buildings