Frost Depth and What It Sets

Why this matters

Frost does not crack a footing set too shallow. It lifts it, a fraction of an inch every winter, and the deck stairs pull off the landing, the shed door binds every February, and the water line to the barn freezes in the first hard January and splits. The fix is always the same, a deeper hole, and it is always cheaper the first time. The depth comes from one number in the jurisdiction's design table, and the most common way to get it wrong is to carry over the number from wherever the crew learned the trade.

What the frost line number is

Frost depth is how deep the ground freezes in a cold winter. The frost line in the code is a design value the jurisdiction sets and publishes, not a measurement from this winter. It appears in the climatic and geographic design criteria table of the International Residential Code (Section R301.2, as adopted and filled in by your jurisdiction; numbered R301.2(1) in the 2018 edition), in the column for frost line depth. Some jurisdictions list one value; some vary it by area or elevation.

It is measured from finished grade, the ground surface after landscaping is done, not from the ground as you find it. That distinction is where most frost failures start (see the worked example).

The design value is usually conservative for lawn under snow cover, which insulates the ground. It can be too shallow for ground kept bare all winter: a plowed driveway, a shoveled walk, a patio slab. Those freeze deeper than the lawn beside them.

Why frozen ground lifts things

Frost heave needs three things at once: frost-susceptible soil, a supply of water, and freezing temperatures reaching into that soil.

• Frost-susceptible soils are fine-grained: silts, silty clays, very fine sands. Their small pores pull water upward toward the freezing front by capillary action.
• At the freezing front, that water freezes into ice lenses, horizontal layers of nearly pure ice that keep growing as long as water keeps arriving. The ground rises by roughly the thickness of the ice, which is far more than water's 9 percent expansion on freezing would explain.
• Clean gravel and coarse sand drain freely and do not wick water to the front, so they heave little even when frozen.

Heave acts on a footing in two ways. Uplift pushes on the bottom of anything whose base sits in the freezing zone. Adfreeze is frozen soil gripping the sides of a post, pier or foundation wall and carrying it up with the ground. A footing set below the frost line defeats uplift, but a rough-sided pier through silty backfill can still be jacked by adfreeze. That is why smooth form tubes, a flared or belled footing base below the line, and free-draining backfill all appear in good practice.

When the ground thaws, it thaws from the top, and the lifted object rarely settles back to where it was. Soil slumps into the void beneath. That ratchet is why frost damage gets worse each year rather than cycling.

What the number sets

Footings for structures. IRC R403.1.4.1, as adopted, requires footings to extend below the frost line unless they are protected another way. The IRC also sets a minimum depth of 12 inches below undisturbed ground for exterior footings regardless of frost (R403.1.4). The alternatives the code allows, under the conditions your edition lists:

• Frost-protected shallow foundations (R403.3), which use horizontal and vertical insulation sized to the site's air-freezing index to keep the ground under the footing from freezing. The method relies on heat from the building, and an unheated building needs a different insulation design, so it is a design, not a shortcut.
• Bearing on solid rock.
• Exceptions for small freestanding accessory structures under size and height limits the code states, and, in recent editions, for freestanding decks not attached to the house. A deck attached by a ledger is not freestanding: its piers must move with the house, which means they must not move at all.

Water piping outside the building. The International Plumbing Code's protection-from-freezing provision (Section 305, as adopted) places exterior water supply piping at least 6 inches below the frost line and at least 12 inches below grade. A frost-free yard hydrant drains its riser through a valve that must also sit below the frost line, or it freezes with the water standing in it. Some water utilities set their own service line depth, which governs from the meter to the street.

What the frost line does not set. Buried gas piping cover comes from the fuel gas code (Section 404 of the International Fuel Gas Code, as adopted, commonly 12 inches), not from frost depth. Buried electrical wiring cover comes from NEC Table 300.5, as adopted, and depends on the wiring method, not on frost. Where a water line shares a trench with electrical or gas, the trench goes to the deepest requirement among them, and any separation the local code or utility requires is kept within it. A condenser pad set on grade is not a footing; it will float with the ground a little each winter, which is fine as long as the line set and electrical whip have the slack to move with it.

The section, with the numbers

The layout from the worked example below, in a jurisdiction whose table lists 42 inches:

FINISHED GRADE ======================================
                       |    |
  backfill: clean      |    |  smooth tube; frost
  gravel, not the      |    |  grips rough sides
  silt dug out         |    |  (adfreeze) and lifts
                       |    |
  42 in  - - - FROST LINE (county table) - - - - - -
                    +--+----+--+
  48 in             | FOOTING  |  base wider than
                    +----------+  the pier
  48 in  water line  o   6 in below the line

A worked example

A shop is building a ledger-attached deck and running a water line 60 feet to a yard hydrant. The county table lists a 42 inch frost line. Soil from the first test hole is gray silt that smears between the fingers, which is frost-susceptible.

The trap. The homeowner's landscaper plans to lower the grade along the deck by 6 inches to set pavers. The crew digs the pier holes to 42 inches from the existing grade and is about to call for the footing inspection.

• After the landscaper's cut, finished grade is 6 inches lower. 42 inches minus 6 inches leaves 36 inches of cover over the footing bottoms.
• 36 inches is 6 inches above the 42 inch frost line. The footings now sit inside the freezing zone in silt, and in a cold winter they heave.
• The inspector may pass the holes, because on inspection day the grade has not been cut. The defect is built in, invisible, and the shop's name is on the deck.

The fix, before concrete. Measure from finished grade. The landscape plan says the cut is 6 inches, so 42 inches below finished grade is 48 inches below existing grade (42 plus 6). The code wants the footing below the line, not resting on it, so the shop sets the footing bottoms at 48 inches below finished grade, which is 54 inches below the ground the crew is standing on (48 plus 6), and records both figures on the inspection request so the inspector is not left measuring from a grade that will move. The crew adds a flared footing form at the bottom, uses smooth tubes, and backfills with gravel rather than the silt that came out.

The water line. The plumbing code puts it at least 6 inches below the 42 inch frost line, so at least 48 inches below finished grade, and the hydrant's drain valve at that depth or below. Across the 60 foot run, the paved area along the deck will be cleared of snow, so that part of the ground freezes deeper than the lawn. The crew holds 48 inches through the whole run and adds rigid foam board above the pipe under the paved section; going deeper there would be the other option.

The dig itself. Before any hole, the shop calls 811 so the utilities are marked; the federal one-call number routes to the state center, and state law sets how many business days of notice are required and how close to a mark you may dig with a machine. Under 29 CFR 1926 for construction work, a 48 inch trench is 4 feet deep, which is where a safe means of egress is required for anyone in it (1926.651(c)(2)), and a competent person must decide about the walls before anyone enters, because without a protective system a trench under 5 feet is allowed only where that person finds no sign of a possible cave-in (1926.652(a)(1)). In loose silt that sloughs, the crew slopes or shores it rather than stepping in. If the excavator snags a marked or unmarked gas line, nobody tries to plug or crimp it: everyone moves upwind and away, no engines started, no switches or phones used near it, and the call to 911 and the gas utility goes out from a distance.

What would change the answer. If the soil had been clean, free-draining gravel to below 42 inches, the heave risk at the same depth would be much lower, but the code depth still applies to the attached deck. If the deck were freestanding and your edition carries the freestanding-deck exception, shallower footings may be allowed under that exception's conditions, though the stairs and landing still need to stay where they were built. If the site had shallow bedrock, bearing on the rock would satisfy the frost requirement and the hole might stop at 20 inches.

What heave looks like when someone guessed

The cues on a house you are called to:

• Seasonal binding. A door or gate that sticks in late winter and frees up in late spring is moving with the ground.
• Separation that grows every year. A gap between a deck ledger and the band joist, or between the top stair and the landing, wider each spring.
• A pier with a void under it or with its form tube rising out of the ground.
• Fence posts at different heights along a run that started level.

When you find these on a job you did not build, say so before you attach to it. A new railing bolted to a heaving deck inherits the movement, and the callback lands on whoever touched it last.

References

• International Residential Code, R301.2 design criteria table, R403.1.4 and R403.1.4.1 footing depth and frost protection, and R403.3 frost-protected shallow foundations, as adopted by your jurisdiction
• International Plumbing Code, Section 305 protection of piping, and International Fuel Gas Code, Section 404, as adopted
• OSHA 29 CFR 1926.651 and 1926.652, excavations in construction work
• See related: Reading the Place Before the Quote; Soil Types Around the House; The Footing That Heaved the First Winter