Spot the Violation

FramingDecksLevel 32018 IRC

Footing hole ready for inspection

Looking down into a freshly augered round footing hole about 12 in. across, cut through the lawn into brown clay, with a ring of loose dirt around it. A tape measure blade is hooked at the bottom of the hole, runs up the far wall, and bends over the rim onto the grass toward its yellow case; the mark at the rim reads 24. A fallen yellow leaf lies on the lawn. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45
Deck footing inspection, Dakota County, Minnesota

This hole is for a deck post. The deck's other side will hang on a ledger bolted to the house. What's wrong here?

  1. The hole is only 24 in. deep; a deck footing in Dakota County must reach at least 42 in. below grade to get below frost
  2. Deck footings must be at least 24 in. in diameter regardless of the load
  3. The hole is fine at 24 in.; the IRC only requires footings 12 in. below undisturbed ground
  4. Nothing. This installation is compliant
Start this drill
Show the answer and the code reference

The answer

The footing hole is only 24 in. deep. A deck attached to the house in Dakota County (frost Zone II) needs footings at least 42 in. (3-1/2 ft) below grade.

Correct choice: 1. The hole is only 24 in. deep; a deck footing in Dakota County must reach at least 42 in. below grade to get below frost

R507.3.2 requires deck footings to extend below the frost line, following R403.1.4.1 and the frost depth for the jurisdiction. The IRC's shallow-footing exceptions apply only to freestanding decks, never to a deck hung on a ledger from the house. Minnesota sets frost depth by rule instead of leaving it to each city: Minn. Rules 1303.1600 requires footings at least 5 ft deep in Zone I (northern counties) and 3-1/2 ft (42 in.) in Zone II (southern counties, including Dakota County and the rest of the Twin Cities). A 24 in. footing sits in soil that freezes and heaves every winter. The posts jack up while the ledger end stays put, racking the frame and loosening the ledger and post connections.

How to fix it

Dig the hole to at least 42 in. below finished grade (5 ft in a Zone I county) on undisturbed soil, keeping the footing size from Table R507.3.1, then call for the footing inspection again before pouring.

Code references

  • IRC 2018 R507.3.2 — Minimum depth

    Deck footings must extend below the frost line specified in Table R301.2(1), in accordance with R403.1.4.1. The exceptions are limited to certain freestanding decks (for example, small, low decks whose joists bear on grade or on pier blocks).

    Drills citing R507.3.2

  • Minn. Rules 1303.1600 — Footing depth for frost protection

    Sets the minimum footing depth for frost protection in Minnesota: 5 ft in Zone I (the northern counties) and 3-1/2 ft in Zone II (the remaining southern counties), measured from grade.

    Drills citing 1303.1600

  • IRC 2018 R403.1.4.1 — Frost protection

    Except where otherwise protected from frost, foundation walls, piers and other permanent supports of buildings and structures must be protected from frost, for example by extending below the frost line specified in Table R301.2(1), building a frost-protected shallow foundation, or bearing on solid rock.

    Drills citing R403.1.4.1

2024 IRC:not yet checked Confidence: low

Minnesota

Minnesota amendmentConfidence: medium

Minnesota adopts the 2018 IRC with Minnesota amendments as the Minnesota Residential Code (Minn. Rules ch. 1309). Minnesota amends R403.1.4.1 (frost protection) in Minn. Rules 1309.0403 (reported from search-result excerpts in another drill's verification), and the frost depth itself is set statewide by Minn. Rules 1303.1600 (5 ft in Zone I, 3-1/2 ft in Zone II) rather than by local entries in Table R301.2(1). Dakota County is in Zone II. Whether Minnesota's R403.1.4.1 keeps the model exception for decks not supported by a dwelling was not confirmed; it does not matter here because this deck hangs on a ledger. No Minnesota amendment to R507.3.2 itself is known. DLI is adopting the 2024 IRC as the next edition; see the 2024 IRC note.

About the Minnesota Residential Code

Citation under review. Verified from the verifier's knowledge; Minnesota and ICC text could not be fetched. The 1303.1600 values (5 ft Zone I, 3-1/2 ft Zone II, Dakota County in Zone II) agree with the 42 in. footing depth in Twin Cities deck handouts. Still to confirm: (1) the exact text of Minn. Rules 1309.0403 as it amends R403.1.4.1, including any deck exception; (2) the 2018 R507.3.2 exception list (freestanding decks on grade or on pier blocks, 200 sq ft, 20 in. high).

Why the other choices are wrong

  • Deck footings must be at least 24 in. in diameter regardless of the load. Footing size comes from Table R507.3.1, based on the tributary area the footing carries and the soil-bearing value; many deck footings are 12 to 18 in. across.
  • The hole is fine at 24 in.; the IRC only requires footings 12 in. below undisturbed ground. The 12 in. figure in R403.1.4 is the minimum depth below undisturbed ground surface. In a cold climate, R507.3.2 and R403.1.4.1 also require the footing to go below the frost line, which in Minnesota is set by Minn. Rules 1303.1600.
  • Nothing. This installation is compliant. A deck tied to the house cannot sit on shallow footings; frost heave would jack the posts while the ledger end stays put, racking the frame and loading the ledger. The footing must reach the Minnesota frost depth.
Safety hazardLevel 3 · Master#deck#footing#frost depth#minnesota#exterior

More in Decks

Decks (5)Framing drillsAll drills