2021 IRC Chapter 4
Everything above grade depends on this inspection. Footings and foundations are where a rehab project's whole structure gets its bearing, and once concrete is poured the soil condition, the depth, and the reinforcement are hidden for good. This lesson covers the 2021 IRC's requirements for soil evaluation and footing design under Chapter 4, Sections R401 through R403: what soil a footing may bear on, how wide and deep a footing has to be, how footings step down a sloped lot, and how the foundation is anchored to the framing above it.
A footing's whole job is to spread the building's weight over enough soil that the soil does not compress or shift under it. Section R401.4.1 lets a builder skip a full geotechnical evaluation and instead assume the presumptive load-bearing values in Table R401.4.1, a table of bearing capacities by class of material. Crystalline bedrock tops the table at 12,000 pounds per square foot (psf); sedimentary and foliated rock carries 4,000 psf; sandy gravel and gravel carry 3,000 psf; sand, silty sand, and clayey sand carry 2,000 psf; and clay, sandy clay, silty clay, clayey silt, and silt sit at the bottom at 1,500 psf. A table footnote adds that where the building official determines that in-place soils below 1,500 psf are likely, the bearing capacity must be determined by a soils investigation. R401.4 also lets the building official decide whether to require a soil test where quantifiable data indicate expansive, compressible, shifting, or otherwise questionable soil is likely, regardless of what the presumptive table would otherwise allow.
Section R403.1.1 and Tables R403.1(1) through R403.1(3) size a footing from the load it carries (snow load, number of stories, basement or crawl space) and the soil's bearing value. A weaker soil needs a wider footing to spread the same load over more area, which is why the presumptive table above drives footing width directly. Whatever the tables give, a concrete footing must be at least 12 inches wide and 6 inches thick under R403.1.1, and R403.1 requires it to bear on undisturbed natural soil or engineered fill. Footing projection, how far the footing extends beyond the face of the foundation wall on each side, must be at least 2 inches and cannot exceed the footing's own thickness; a wider overhang than that turns the projecting edge into an unreinforced cantilever that can crack off under load. Where a footing carries more load than the presumptive tables cover, or the soil is weaker than assumed, the code requires an engineered design in place of the prescriptive tables.
Section R403.1.4 sets a floor under every exterior footing: at least 12 inches below the undisturbed ground surface (deck footings follow R507.3). In areas where the ground freezes, that 12-inch minimum is rarely the controlling number, because R403.1.4.1 also requires the foundation to be protected from frost, by extending below the frost line in Table R301.2, by a frost-protected shallow foundation (FPSF) under R403.3, by ASCE 32, or by bearing on solid rock. R403.1.4.1 exempts small free-standing accessory structures (light-frame, 600 square feet or less, eave height 10 feet or less) from frost protection. An FPSF works by trapping the building's own heat loss and the ground's stored heat under insulation, which only works reliably where the space above stays warm; the FPSF provisions apply only to buildings whose monthly mean temperature is maintained at not less than 64°F, and they may not be used for unheated spaces such as porches, utility rooms, garages, and carports. Footings may not bear on frozen soil unless the frozen condition is permanent, since frozen soil is temporarily stiff but loses strength as it thaws; a footing inspection on a cold morning has to confirm the bearing soil is not frozen, not just that the trench is deep enough.
Under R403.1.5, a footing's top surface must be level so the wall above bears evenly; its bottom surface is allowed to slope no more than 1 unit vertical in 10 units horizontal. On a hillside lot, that limit is reached quickly, so the footing is stepped instead: a series of level runs connected by vertical steps, each run bearing flat rather than the whole footing tilting downhill. The vertical step itself is the vulnerable point in the assembly. It needs to be poured as continuous concrete, and where the footing is reinforced, the steel has to continue through the step rather than stopping at it, so the stepped footing behaves as one continuous footing instead of a series of disconnected pads that can slide independently.
Section R403.1.7 also protects buildings on or adjacent to slopes steeper than 1 unit vertical in 3 horizontal. Under R403.1.7.2 a footing near a descending slope must be founded with enough embedment and setback for vertical and lateral support, and the setback is deemed adequate at a distance of H divided by 3, where H is the height of the slope, capped at 40 feet (the dimensions are drawn in Figure R403.1.7.1). The building official may accept alternate setbacks, and may require an engineer's investigation, under R403.1.7.4. Get too close to the edge and the wedge of soil that the footing's bearing capacity depends on can slide out from under it. Wood columns supporting girders, common in crawl space rehab work, bear on their own concrete piers or footings; R407.1 sends wood columns to R317, which calls for pressure-preservative-treated or naturally durable wood where a column is closer than 8 inches to exposed ground in a crawl space. Under R407.3 a column must be restrained at its bottom end against lateral displacement, wood columns must be at least 4 by 4 nominal, and steel columns at least 3-inch Schedule 40 pipe. The code exempts columns not over 48 inches tall on a pier or footing from the bottom-restraint rule in Seismic Design Categories A, B, and C, within under-floor areas enclosed by a continuous foundation, so both height and seismic category matter when checking this in the field.
Table R402.2 sets 2,500 psi at 28 days as the minimum specified compressive strength for concrete not exposed to weather, such as basement walls and interior slabs, and for any concrete in a negligible weathering region. Where the weathering potential in Table R301.2 is moderate or severe, exposed vertical concrete must be at least 3,000 psi, and porches, carport slabs, steps, and garage floor slabs at least 3,000 psi (moderate) or 3,500 psi (severe). Concrete in moderate or severe weathering that is exposed to the weather must be air-entrained with a total air content of not less than 5 percent and not more than 7 percent. Air entrainment works by creating microscopic voids that give freezing water somewhere to expand into instead of spalling the concrete apart from the inside; too little gives no protection and too much cuts into the concrete's strength. Concrete placed when the temperature will drop well below freezing carries its own separate risk: hydration, the chemical reaction that develops strength, effectively stops near freezing, and ice crystals forming in fresh concrete disrupt the paste structure permanently. That is why cold-weather placements need protection and, often, heating measures, not just a longer cure time.
A footing and foundation wall only do their job if the framing above is actually tied to them. Section R403.1.6 sets the standard sill-plate anchorage: bolts at least 1/2 inch in diameter (or approved anchors or straps giving equivalent anchorage), extending at least 7 inches into concrete or grouted masonry cells, spaced no more than 6 feet on center, and located in the middle third of the plate width. Each plate section needs a bolt no more than 12 inches, and no less than seven bolt diameters, from each end, and at least two bolts per section regardless of how short the piece is; one bolt near the center lets the plate pivot around it, which defeats the connection entirely. Every anchor bolt gets a properly sized nut and washer, tightened, since that is what actually transfers uplift and shear from the plate into the bolt. Loose nuts and missing washers are among the most common foundation-anchorage rejections in the field, right alongside a plate section anchored with only one bolt.
In the field
On a Gulf Coast rehab, frost depth is almost never the controlling number, the region rarely freezes deep enough for it to matter, but the 12-inch minimum depth below grade still applies everywhere, on every lot, regardless of climate. A GCHS inspector opening an old pier-and-beam foundation for repair should still confirm the footing bears on undisturbed soil or documented engineered fill, not loose fill or organic material raked over from an old yard, and should check that any replaced or added footing meets the 6-inch minimum thickness and 2-inch minimum projection even when it is tucked under an existing sill. On a site with expansive clay, common across much of the region, a presumptive 1,500 psf value from the table may not be conservative enough if the soil report calls for something more restrictive; when in doubt, ask for the geotechnical report rather than defaulting straight to the table.
Here is a simplified comparison of how soil class changes footing sizing (the real Table R403.1(1) also accounts for snow load, stories, and basements, and never goes below 12 inches wide). Assume a wall footing must support a load of 3,000 pounds per linear foot of wall, and the footing width is sized simply as load divided by allowable soil bearing pressure. On clay soil at the presumptive 1,500 psf, the required width is 3,000 divided by 1,500, or 2 feet, 24 inches. On sandy gravel at the presumptive 3,000 psf, the same 3,000-pound load only needs 3,000 divided by 3,000, or 1 foot, 12 inches. The load on the wall has not changed at all between the two houses; only the soil's bearing value has, and it alone doubles the footing width the weaker soil needs to carry the identical load.
1. Exterior footings must extend at least how far below the undisturbed ground surface under Section R403.1.4?
2. Under Table R401.4.1, what is the presumptive load-bearing value for clay, sandy clay, silty clay, and clayey silt?
3. Two identical houses sit on different soils, one on clay and one on sandy gravel. Compared with the sandy-gravel house, what does the clay house need?
4. What is the minimum thickness of a concrete footing under Section R403.1.1, regardless of width?
5. A footing is 10 inches thick. Under Section R403.1.1, its projection beyond the face of the foundation wall may be:
6. On a sloping site, why are stepped footings used instead of one continuously sloped footing (R403.1.5)?
7. A footing is placed near the top of a descending slope steeper than 1 unit vertical in 3 horizontal. Under R403.1.7.2 and Figure R403.1.7.1, the setback from the slope face is deemed adequate at:
8. A wood column supports a girder in a crawl space, bears on a concrete pier, and is closer than 8 inches to exposed ground. What does the IRC require of the column?
9. Concrete for a basement wall not exposed to weather must have a minimum specified compressive strength, under Section R402.2, of at least:
10. Where concrete is exposed to weather in a moderate or severe weathering region, Table R402.2 requires air entrainment with a total air content of:
11. Sill plate anchor bolts must be at least 1/2 inch in diameter, embedded at least 7 inches, and spaced no more than:
12. A sill plate section 4 feet long is anchored with a single bolt near its center. What should an inspector do?
13. A footing is placed in an area with a 36-inch frost line in Table R301.2, and the footing bottom sits 24 inches below grade. What should the inspector do?
14. Under Section R403.3, the IRC's frost-protected shallow foundation provisions apply where the building's monthly mean temperature is maintained at not less than 64°F, but may NOT be used for: