Deck footings and frost depth

Every deck stands on some number of concrete footings, and the depth of those footings is not a preference. It is set by a physical process: water in soil expands when it freezes, and soil above the frost line moves upward as it does. Anything bearing on that soil moves with it, then settles unevenly when it thaws. Repeat annually for fifteen winters and a deck that started level has a noticeable fall.

The rule

Footings bear on undisturbed soil at least 12 inches below the undisturbed ground surface, or below the frost line, whichever is deeper. Bearing conditions are verified in the field by the building official before concrete is placed.

Two phrases in that do the work. Undisturbed soil means original ground, not fill placed during construction, which matters in newer Meridian subdivisions where the yard was graded before the house was finished. And whichever is deeper means the 12 inch figure is a floor, not the answer.

The frost line here

The Treasure Valley is a cold semi-arid climate at around 2,600 feet. NOAA's 1991 to 2020 normals for Boise give a January average low of about 25 degrees Fahrenheit, which is a genuine freezing season rather than a nominal one.

For a published number, the City of Boise lists its building design criteria openly: a minimum footing depth of 24 inches, alongside a 20 psf ground snow load, a 25 psf minimum uniform roof design load, seismic design category C, a 90 mph basic wind speed, and a 10 degree winter design temperature, all under the 2018 International Residential Code. Meridian sits in the same county on the same valley floor, so 24 inches is the depth valley builders work to. The Meridian building official confirms it at inspection for your specific site.

What a footing looks like

A typical residential deck footing is a poured concrete pier, commonly 12 to 18 inches in diameter, with the base bearing on undisturbed soil at depth. The post connects to it with a galvanized or stainless post base rather than sitting directly in the soil or in the concrete. That connector detail is not cosmetic: a wood post buried in concrete or soil holds moisture at exactly the point where it matters most.

Footing count comes from the beam span and the load path, not from a rule of thumb. A quote should say how many footings, what diameter, and what depth. If it says none of the three, that is a reasonable thing to ask about.

The inspection is a hold

The footing inspection happens after the holes are dug and before any concrete goes in. That is the only moment anyone can see whether the excavation reaches undisturbed soil at the right depth, which is why it is a stop rather than a formality.

In practice this means a day or two of the schedule is waiting rather than working. A crew that digs and pours the same afternoon has skipped it. On a two week deck build, the footing inspection plus concrete cure is most of the difference between the labor hours and the calendar.

Stair stringers, the detail that catches people

Stair stringers have to bear on footings placed below the frost line too, on solid ground, exactly as the deck posts do. What they must not do is land on an existing concrete patio or pad that was poured above frost depth.

A patio slab in this valley moves seasonally. When the stringer bottoms are fixed to it and the deck end is not, the whole flight racks, the risers go out of uniform height, and the handrail geometry follows. Where a stair has to land on an existing patio, the correct answer is breaking through the slab and placing a footing beneath the bearing point, not screwing a bracket to the surface.

Bearing capacity and the awkward cases

Prescriptive deck framing assumes soil with a reasonable allowable bearing capacity. Where the building official determines that in-place soils fall below that, footings get larger or an engineer gets involved. This is uncommon on flat Meridian subdivision ground and more likely on filled lots, on the foothill slopes above Boise and Eagle, and near old irrigation channels.

The other common trigger is concentrated load. A hot tub full of water with people in it is well outside the range prescriptive tables cover, and it needs its own engineered framing and footings underneath. Anyone quoting a hot tub deck off a standard framing plan is guessing.

How to read this in a quote

Three things worth having in writing: the number and diameter of footings, the depth they will be excavated to, and confirmation that a footing inspection is scheduled before pouring. None of the three is unusual to state, and a builder who works in Meridian regularly will have all of them at hand.

The cost guide covers how footings sit in the overall budget, the permit guide covers the inspection sequence, and the glossary defines frost heave, bearing, and the rest of the vocabulary that shows up on a footing plan.