How Deep Do Deck Footings Need to Be in Boise?

Why does Boise use 24 inches?

Boise's adopted design criteria for one- and two-family homes list a frost line depth of 24 inches, along with an air freezing index of 894 and a winter design temperature of 10°F. Water in soil above that line freezes, expands and lifts whatever sits on it, then lets it settle when it thaws. A footing that stops short of the frost line rides that cycle up and down, and the deck racks, pulls on its ledger and opens up its stairs.

The cycle is not rare here. NOAA's 1991–2020 normals for the Boise Air Terminal record 108 nights a year at or below 32°F but only 16 days that stay below freezing all day. That means most winter days freeze at night and thaw by afternoon, which is exactly the pattern that works a shallow footing loose. The median last spring freeze is April 24 and the first fall freeze is October 21.

Meridian's shed handout uses the same 24-inch frost line, and Star publishes 24 inches in its design criteria. Other Treasure Valley jurisdictions may differ, so confirm the number with the city that issues your permit; our Boise deck permit guide lists who to call there.

What does the 2018 IRC require for deck footings?

The deck section of the 2018 IRC, Section R507, as adopted in the Idaho Residential Code, covers footings in three steps:

• R507.3 requires decks to sit on concrete footings or another approved structural system designed for the loads in Section R301, sized to carry the deck's load to the ground, with depth set by Section R403.1.4.
• R507.3.1 sets the minimum footing size using Table R507.3.1, based on the tributary area each footing supports and the soil's allowable bearing pressure from Table R401.4.1.
• R507.3.2 requires deck footings to extend below the frost line in Table R301.2(1), which in Boise is 24 inches.

R507.3.2 carries one exception: a freestanding deck whose joists bear directly on precast concrete pier blocks at grade, with no beams or posts, no more than 200 square feet, and a walking surface no more than 20 inches above grade within 36 inches of its edge, does not need frost-depth footings. Note that in Boise a deck that low may not need a permit at all if it stays at or under 12 inches, but the exception is about structure, not paperwork.

The American Wood Council's DCA 6 deck guide, which mirrors the IRC's prescriptive deck rules, adds a practical floor: footings bear on undisturbed soil at least 12 inches below the undisturbed ground surface or below the frost line, whichever is deeper. In Boise that is always the frost line.

How big does each footing need to be?

Depth is fixed; size depends on load. The IRC table is based on the higher of a 40 psf live load or the ground snow load (Boise's is 20 psf, so 40 psf governs), plus a 10 psf dead load. The table below reproduces the 40 psf rows for two common soil bearing values.

Tributary area is the patch of deck a footing holds up. Take a deck with joists spanning 12 feet from the house ledger to a beam, and posts under that beam every 8 feet. Each interior post carries half the joist span times the post spacing: 6 feet × 8 feet = 48 square feet, plus any cantilever past the beam. The table footnotes allow interpolation, so on 1,500 psf soil that post needs a round footing of roughly 17 inches, between the 16-inch and 19-inch values in the 40 and 60 sq ft rows.

Soil bearing value matters. Table R401.4.1 assigns presumptive values by soil class, and 1,500 psf is the lowest presumptive value the table lists. Unless a soils report says otherwise, the conservative design assumes 1,500 psf. Footings on lots in Boise's Hillside overlay are different: the #406 checklist requires stamped engineering for footings and foundations there.

Piers, post bases and the post connection

The most common Boise deck footing is a cylindrical pier formed with a cardboard tube, often with a flared base form to widen the bearing area at the bottom where the table's diameter applies. Poured spread footings with a short pier on top work too, as do engineered helical piers. Boise's #406 checklist notes that helical or push piers need an ICC Evaluation Service report showing third-party code verification, so bring that paperwork if you go that route.

The connection from post to footing has its own rule. IRC R507.4.1 requires lateral restraint by a manufactured connector or by embedding the post at least 12 inches in surrounding soil or a concrete pier, and it bars relying on surrounding soil where soils are expansive, compressible or shifting. Most builders here use a galvanized standoff post base set in the wet concrete. It holds the post in place, lifts the end grain off the concrete so it can dry, and makes a post replaceable later.

Post size has limits too. Table R507.4 caps a 4×4 post at 6 feet 9 inches, a 4×6 at 8 feet, and 6×6 and 8×8 posts at 14 feet, measured to the underside of the beam and based on a 40 psf live load. Taller posts, or posts carrying a roof, are outside the prescriptive tables and need design. Our multi-level deck ideas page covers where those limits start to shape a design.

Treasure Valley ground conditions to plan for

• Backfill at the house. The soil next to a foundation was dug out and refilled when the house was built. DCA 6 shows deck footings near the house bearing at the same elevation as the house footing so they don't sit on that looser fill.
• Utilities and irrigation. DCA 6 says not to build footings over septic systems, leach fields, utility lines or enclosed meters, and to call 811 before digging. Valley yards are crossed by irrigation mainlines that 811 may not mark.
• Slopes. In the Foothills, a 24-inch hole on the uphill side and the downhill side are different structures. Hillside overlay lots in Boise need engineered footings.
• Rock and cobble. Parts of the valley are full of river rock. Plan for an auger or hand digging and keep the hole at full diameter all the way down.

Can deck footings be poured in winter?

Yes, with protection. The cold-weather concrete interpretation signed by Boise and other Southwest Idaho jurisdictions defines cold weather as three consecutive days averaging below 40°F. Forms, steel and subgrade must be at least 35°F when concrete is placed, snow and ice must be removed from the holes, and the concrete must be kept at 50°F for 72 hours; below a 20°F daily average it must be heated. Inspectors approve placement only for the day of the inspection and check for frozen ground and blankets on site.

Boise's inspection page describes the footing inspection the same way for every structure: holes excavated and forms in place before concrete. Schedule it, then order the truck.

How we handle footings

On a Valley Decks build, footing layout comes from the framing plan, not the other way around. We calculate tributary area for each post, size footings from Table R507.3.1 at 1,500 psf unless a soils report supports more, dig to at least 24 inches on undisturbed soil, set standoff bases in the pour, and book the footing inspection before concrete arrives. Where a lot is in the Hillside overlay or carries a hot tub or roof, an engineer sizes the footings.

Shortcuts to avoid

1. Setting a raised deck on precast pier blocks. The IRC exception covers only small freestanding decks with joists bearing directly on the blocks; posts and beams on blocks are not covered.
2. Stopping at 12 or 18 inches because the auger hit rock. A shallow footing in Boise heaves.
3. Bearing on backfill or loose spoil instead of undisturbed soil.
4. Burying a post in concrete with no base. Water collects at the post and rots it from inside the pier.
5. Moving posts farther apart during construction without resizing the footings. Tributary area changes with spacing.
6. Pouring onto frozen ground or skipping blankets in a cold week.
7. Reusing old piers on a replacement without checking their depth and diameter. Our repair-or-replace checklist covers how to tell.

Related resources

Footings are the bottom of the load path that starts with snow and people on the boards; our deck snow load guide explains the 40 psf design load the table above uses. If you're planning around the calendar, see the best time to build a deck in Boise. For a footing-to-rail build, start with custom deck building or request an estimate.