IRC Foundations and Footings: What the B1 Exam Tests

Start With the Sequence, Not the Table

A footing is inspected after the excavation is complete and before concrete is placed, which is why R109.1.1 requires a footing or foundation inspection request before pouring. The exam uses that sequence constantly: what must an inspector verify before the pour, and what must be corrected before an approval is issued. The order is soil and bearing capacity, then depth and elevation, then width and thickness, then reinforcement and concrete strength, then drainage and dampproofing after the walls are up, and finally anchor bolts and the sill plate before framing starts.

Presumptive Soil Bearing Values

Table R401.4.1 gives the presumptive load-bearing values an inspector may use when a geotechnical report is not required. These are the numbers the exam quotes.

Two caveats matter on the job and on the exam. Table values may only be used when the building official determines that the soil conditions are suitable, and a value must be reduced or a geotechnical investigation required where the soil is soft, filled, organic, or where the excavation bottom has been disturbed. Never accept a footing bearing on loose backfill: the code requires bearing on undisturbed soil or engineered fill.

Footing Width, Thickness and Depth

The footing size comes from Table R403.1 for conventional light-frame construction, which is the default method the exam tests.

• Minimum width: 12 inches for a one-story dwelling, 15 inches for two stories, and 18 inches for three stories of conventional light-frame construction, with the load-bearing value taken from R401.4.1.
• Thickness: a minimum of 6 inches, and never less than the projection beyond the foundation wall. A footing projecting more than 2 inches beyond the wall requires a design that accounts for the projection.
• Depth below grade: footings are placed not less than 12 inches below the undisturbed ground surface, and always below the frost line. In cold jurisdictions that drives footing depth to 36, 42 or 48 inches or more, which is why frost-depth questions dominate northern examinations.
• Concrete strength: not less than 2,500 psi specified compressive strength at 28 days.
• Stepped footings: where the ground slopes, footings may be stepped, with the vertical rise between steps limited and the bearing maintained at every step.

Frost protection goes beyond depth. The code also requires that surface drainage, foundation insulation or similar measures be used to prevent frost heave where the soil is frost-susceptible, because a footing placed below the frost line in a poorly drained silt will still move when the water in the soil freezes.

Foundation Wall and Slab Requirements

Concrete and masonry foundation walls are covered in R404 and the associated tables, which set minimum wall thickness and reinforcement based on wall height, unbalanced backfill and soil class. Three numbers are tested repeatedly: the top of the foundation wall must extend at least 6 inches above the finished grade where the wall is of masonry or concrete, exterior slab-on-grade floors require a minimum 3.5-inch slab thickness with a vapor retarder beneath, and a crawl space requires a minimum of 18 inches of clearance to the bottom of the floor joists and 12 inches to the bottom of the girders.

Drainage, Dampproofing and Waterproofing

R401.3 is the simplest rule in Chapter 4 and one of the most frequently cited on site: surface drainage must direct water away from the foundation, with a fall of not less than 6 inches within the first 10 feet. Foundation drainage under R405 with crushed stone and a filter membrane is required where the water table or the soil conditions make it necessary, and dampproofing is required on foundation walls that retain earth and enclose interior spaces below grade, unless the wall is designed to hold water back and is waterproofed instead. Waterproofing is required where a high water table exists or other conditions make it necessary. Inspectors are asked to distinguish dampproofing, which resists soil moisture, from waterproofing, which resists hydrostatic pressure.

Anchor Bolts and the Sill Plate

R403.1.6 is short and heavily tested. Sill plates must be anchored to the foundation with bolts or approved anchors spaced no more than 6 feet on center, with at least two bolts per plate section and one bolt located within 12 inches of each end of the plate. Bolts are a minimum of 1/2 inch in diameter and extend at least 7 inches into the concrete or masonry. Where plate washers are required by the seismic or wind provisions, they must be a minimum of 3 inches square with a thickness of at least 0.229 inch. In addition, the sill plate must be positively anchored for uplift in high-wind areas, which is where many inspectors find the missed detail.

How the Exam Writes These Questions

B1 foundation questions come in three shapes. The first gives you soil and a building height and asks for the minimum footing width. The second gives you a climate and asks for the minimum footing depth or describes a footing detail and asks what is wrong with it. The third describes an inspection finding and asks for the code reference or the required correction. Because the exam is open book, your speed at navigating to Table R401.4.1 and Table R403.1 matters as much as knowing the numbers: tab the tables you use most, and practise reading a table row out loud with the code book in front of you.

Inspect Practice builds foundation and footing items with the code section cited in every rationale, so a missed question becomes a navigation drill instead of a dead end. See how it feels with free ICC practice questions — no signup needed.