How Deep Do Deck Footings Go in Utah County? (Frost Depth, Explained)

“How deep do the footings need to go?” is one of the first questions that comes up once a deck project gets past the design-and-dream stage and into a permit set. It sounds like it should have one tidy answer for the whole county. It doesn’t — and the reason why is worth understanding before you call anyone for a quote.

What “frost depth” actually means for a footing

Soil holds water, and water expands when it freezes. If a footing sits above the depth where the ground regularly freezes, the freeze-thaw cycle each winter can lift and shift it slightly — a little more each year. A footing poured below that depth sits on soil that stays at a stable temperature year-round, so it isn’t riding the seasonal freeze up and down.

That’s the entire reason “frost depth” exists as a building code concept. It’s not an arbitrary number — it’s the depth below which your local ground stops freezing in a normal winter, and it’s specific to each place because soil, elevation, and climate vary from valley floor to bench to canyon mouth.

The verified number: 30 inches — but only in two places

Utah County Community Development, which governs unincorporated Utah County, publishes a frost depth of 30 inches, or an engineer’s design if deeper, in its adopted-codes summary. Provo City publishes the same figure — a completed IRC design-criteria table listing a 30-inch frost line, alongside its weathering, termite, and decay classifications.

Here’s the detail that matters most: the international code itself does not set a frost depth. The relevant code table has a blank that each city or county is required to fill in with its own number. So “30 inches” isn’t a code minimum handed down from above — it’s a locally adopted value, and as of this writing it’s only publicly confirmed for two jurisdictions: unincorporated Utah County and Provo.

Why the same number doesn’t apply next door

Do not assume 30 inches applies across Utah County. Utah County has real elevation variation from the valley floor up into the benches and canyon mouths, and the county’s own frost-depth language — “30 inches, or engineer’s design if deeper” — already anticipates that some sites need more. Lehi, Orem, American Fork, Spanish Fork, Pleasant Grove, Springville, Mapleton, Highland, Alpine, Eagle Mountain, and Saratoga Springs each set their own building requirements, and their published figures aren’t the same as the county’s or Provo’s by default. Your city’s building department has the final, correct number for your address — it’s worth a call or a look at their adopted design criteria before you assume anything.

There’s a second wrinkle that compounds this: jurisdictions don’t always adopt the same edition of the code at the same time. Utah County’s current building page lists 2024 code adoptions, while Provo’s design-criteria page cites its tables from the 2021 edition. Section numbers move between editions, so even the reference number a contractor cites on a permit application can be technically out of date if it’s copied from the wrong city or the wrong year.

How deep must a footing actually go, in practice?

The general rule — sourced to the prescriptive residential deck guide published by the American Wood Council, which cites the international code’s foundation section — is that footings must bear on undisturbed soil at least 12 inches below the undisturbed ground surface, or below the frost line, whichever is deeper. In a jurisdiction where the adopted frost line is 30 inches, the frost line is the number that governs, not the 12-inch floor.

A few practical points that come with that rule:

• The building official verifies the actual bearing condition in the field before concrete gets placed — it’s an inspected step, not a take-someone’s-word-for-it step.
• Footing size itself is calculated from the deck’s tributary area and the soil’s bearing capacity, not just frost depth alone.
• Footings have to steer clear of septic systems, leach fields, utility lines, and meters — which is why a call to 811 before digging is standard practice, not optional.
• Post anchors set into the footing need to be galvanized or stainless so they hold up in contact with treated lumber and Utah’s freeze-thaw cycles.

Posts sit on those footings — and they have a minimum size too

The same prescriptive deck guide calls for deck posts to be 6×6 nominal or larger, measured from grade (or the top of the foundation, whichever is highest) to the underside of the beam, and centered on their footings. An 8×8 can substitute anywhere in that sizing table but is capped at 14 feet of height. A 4×4 post is only allowed with a specific approved cap connection, because there isn’t enough cross-section in a 4×4 to cut the notch detail a 6×6 can take. Note this is the prescriptive guide’s own minimum for a code-path deck, not necessarily the exact figure your city’s plan reviewer will cite — it’s a useful baseline to know walking into a conversation about your project, not a substitute for what gets stamped on your permit set.

One more detail that’s easy to miss: any cut end or notch on a post is supposed to be field-treated with an approved wood preservative at the time it’s cut. It’s a thirty-second step that’s completely invisible once the deck is built, and it’s exactly the spot — the notch carrying the beam — where untreated wood starts to fail first.

How Rooval handles the footing depth question on a real job

We pull the permit and build to your city’s footing requirement — not a number copied from a neighboring town’s page. Because the verified frost depth differs by jurisdiction and the correct figure has to come from your specific building department, that’s a call we make (or verify) before anything gets dug, on every job. If you want the fuller picture of how we approach a project end to end, our build standard page walks through it.

If you’re earlier in the process and want to understand what permitting actually involves for a Utah County deck — who requires what, and roughly how that timeline runs — we’ve laid it out separately on our deck permits in Utah County page.

What this costs

Footing depth and post sizing are a small piece of a much bigger project scope. Most of the decks we build in Utah County land somewhere in the $18,000–$35,000 all-in range, depending on size, materials, height, stairs, and site access — footing depth (and the excavation and concrete that come with it) is one of several variables that moves that number. We don’t quote per linear foot or per footing over the phone; every project gets a free on-site look first so the number reflects your actual site, your actual city’s requirements, and your actual soil.

Frequently asked questions

Rooval Deck & Beam Builders is a DBA of Rooval LLC and the sister company of Rooval Roofing. Same family of craftsmen, same honest approach.

Sources: Utah County Community Development — adopted building codes · Provo City Design Criteria · IRC Table R301.2, Utah 2021 edition (UpCodes) · IRC R507.3 footings, Utah 2021 edition (UpCodes) · American Wood Council, DCA 6 Prescriptive Residential Wood Deck Construction Guide