4.2 Core Workflows and Decision Points
Key Takeaways
- Footing width is read from Table R403.1(1) by story count and soil bearing value; the default 12 in covers one- and two-story frame on 2,000+ psf soil.
- Sill anchorage (R403.1.6) has five testable parts: 1/2 in bolts, 6 ft o.c. max, 7 in embedment, within 12 in of each plate end, minimum two bolts per plate section.
- Footing thickness must be at least equal to the footing projection beyond the wall face, and never less than 6 inches.
- Footings bear below the frost line; in non-frost regions, at least 12 inches below undisturbed ground surface.
- Anchor bolts must sit in the middle third of the plate width with a nut and washer tightened on each.
Sizing the footing
Footing width is read from Table R403.1(1) by two inputs: the number of stories the footing supports and the load-bearing value of the soil. For conventional light-frame construction the defaults are forgiving - a one-story or two-story dwelling on soil of 2,000 psf or better takes the 12-inch minimum width. The width only grows on weak soil: a two-story frame house on 1,500 psf soil needs about 13 inches, and a three-story on 1,500 psf needs about 16 inches. Read the value straight from Table R403.1(1) for these prescriptive cases.
A hard floor applies under R403.1.1: footing width and thickness shall not be less than 12 inches wide and 6 inches deep, even when a calculation would allow less.
The projection rule
A plain (unreinforced) concrete footing must be at least as thick as it projects past the face of the wall it supports. If a 16-inch-wide footing sits under an 8-inch wall, the projection is 4 inches per side, so the footing must be at least 4 inches thick - but the 6-inch floor still controls. This prevents the footing from cracking off in cantilever. On the exam, watch for a wide footing on a thin wall: the projection, not the table minimum, may drive the thickness.
Depth and frost protection
Under R403.1.4, the bottom of exterior footings must be at or below the frost line for the jurisdiction (the local value is inserted into Table R301.2(1)), and in all cases not less than 12 inches below the undisturbed ground surface. The code accepts four ways to satisfy frost: (1) extend below the frost line, (2) build a frost-protected shallow foundation (FPSF) per R403.3 with the prescribed insulation, (3) construct in accordance with ASCE 32, or (4) bear on solid rock. Heated and unheated buildings use different FPSF tables - an unheated detached garage cannot use the heated-building insulation values.
Sill anchorage - the five-part check
R403.1.6 is the single most tested foundation rule. Memorize all five parts as one block:
| Anchorage rule | Value (R403.1.6) |
|---|---|
| Bolt diameter | >= 1/2 inch |
| Maximum spacing | 6 ft on center |
| Embedment in concrete | >= 7 inches |
| Distance from each plate end | <= 12 inches |
| Bolts per plate section | >= 2 |
| Bolt position across plate | middle third of plate width |
| Hardware | nut + washer tightened on each bolt |
A short sill piece (say a 4-foot jog) still needs at least two bolts, each within 12 inches of an end - the 6-foot spacing alone does not satisfy a short plate. In Seismic Design Categories D0-D2 (R403.1.6.1), a plate washer of at least 0.229 in x 3 in x 3 in is required on each bolt.
From cue to decision
Every footing/foundation question hides a workflow cue in the stem. Train yourself to extract five things in order, because the right answer is the one that respects all five:
| Step | Question to ask | Where the answer lives |
|---|---|---|
| Trigger | What condition is being inspected (size, depth, anchorage, moisture)? | The stem's nouns |
| Construction type | Frame or masonry? One, two, or three stories? Plain or reinforced? | R401, R404 |
| Soil / site | Bearing value, frost line, water table, slope? | Tables R401.4.1, R301.2; R403.1.4 |
| Authority | Which IRC section/table controls? | Table R403.1, R403.1.6, R402.2, R404, R405, R406 |
| Verdict | Comply or cite the deficiency by section | Compare field value to code value |
Worked example
Stem: A continuous footing for a one-story frame house on 2,000 psf soil is poured 11 inches wide and 6 inches thick; the frost line is 0 (warm climate). Walk it: trigger = footing size; type = one-story frame; soil = 2,000 psf; authority = Table R403.1(1) + R403.1.1; verdict = non-compliant - even though the table at 2,000 psf one-story would allow a narrow footing, R403.1.1 sets an absolute floor of 12 inches wide, so 11 inches fails. The thickness (6 in) and the no-frost depth (>= 12 in below grade still applies) must also check out.
Where field errors cluster
The predictable handoff failures are: footing poured before the inspector verifies depth/width (cover-up risk); anchor bolts wet-set out of position or missing end bolts; rebar with too little lap or cover; and backfill placed before dampproofing/drainage. Catch these at the pre-pour (footing/foundation) inspection and the anchorage check before the sill is concealed.
Decision points that flip a verdict
A few specific checks change the answer more often than any others. First, the R403.1.1 absolute minimums override a favorable table value - a footing can never be narrower than 12 inches or thinner than 6 inches even on strong soil. Second, the projection rule can force a thicker plain footing than the 6-inch floor when the footing is wide relative to its wall. Third, frost depth can override the 12-inch minimum depth wherever the frost line is deeper.
Fourth, missing or out-of-position anchor bolts (R403.1.6) fail the wall-to-foundation tie regardless of footing size. Fifth, seismic design category layers on extra anchorage hardware (plate washers) and bracing.
When a stem mentions any of these conditions explicitly, treat it as the controlling cue and let it override the generic prescriptive default - that is almost always why the question was written.
A two-story wood-frame dwelling bears on soil with a presumptive load-bearing value of 1,500 psf. Per IRC Table R403.1(1), the minimum footing width is closest to:
An inspector finds a sill plate anchored with 1/2-inch bolts at 6 feet on center, embedded 7 inches, with the nearest bolt 18 inches from one plate end. What is the code deficiency under R403.1.6?