Residential Building Inspector B1Footings and FoundationsMedium
An inspector observes a foundation wall that is 8 inches thick and 7 feet high, supporting a wood frame wall. The soil adjacent to the foundation wall is classified as GW (well-graded gravel). According to the 2021 IRC, what is the minimum required thickness for this foundation wall?
- A8 inches
- B10 inches
- C6 inches
- D12 inches
Show answer & explanationAnswer & explanation
Correct answer: A. 8 inches
IRC Table R404.1.2(1) specifies minimum thickness for concrete and masonry foundation walls. For an 8-inch thick foundation wall, 7 feet high, supporting a wood frame wall, the table indicates that 8 inches is acceptable for soils with a presumptive load-bearing value of 2,000 psf or more (GW soil typically exceeds this). The 8-inch thickness meets the requirement.
Why the other options are wrong
- B. 10 inches would be acceptable but is not the minimum required.
- C. 6 inches is generally too thin for a 7-foot high foundation wall supporting a wood frame wall.
- D. 12 inches is typically for taller walls or heavier loads, not the minimum here.
Foundation Wall Thickness
The minimum thickness of a foundation wall is determined by its height, the type of material, the soil conditions, and the load it supports, as specified in IRC tables.
- IRC Table R404.1.2(1) provides minimum thickness for concrete/masonry walls.
- Wall height and backfill height are critical factors.
- Soil type influences lateral pressure, affecting required thickness.
Memory trick: Wall height, soil might, dictates its might.