According to typical building codes, what is the minimum slope required for positive drainage away from building foundations?
Correct Answer
C) 5% (5/8 inch per foot)
Most building codes require a minimum slope of 5% (or 6 inches in the first 10 feet) away from the foundation to ensure positive drainage and prevent water from ponding against the foundation wall.
Why This Is the Correct Answer
Most model building codes (IBC, IRC) require a minimum 5% slope — approximately 6 inches of fall over the first 10 feet (5/8 inch per foot) — away from the foundation. This slope ensures positive drainage and prevents water from pooling against the foundation wall, which can cause hydrostatic pressure and moisture infiltration.
Why the Other Options Are Wrong
Option A: 2% (1/4 inch per foot)
A 2% slope (1/4 inch per foot) is the minimum grade used for paved surfaces like driveways and patios, not for soil grading away from foundations. It is insufficient to reliably drain water away from a building foundation.
Option B: 10% (1.2 inches per foot)
A 10% slope (1.2 inches per foot) far exceeds code requirements and is not a standard threshold. While steeper slopes drain better, they are not required and can cause erosion.
Option D: 1% (1/8 inch per foot)
A 1% slope (1/8 inch per foot) is too shallow to ensure positive drainage in soil conditions. It is below the minimum acceptable threshold and would risk water ponding near the foundation.
Memory Technique
Foundation = 5 feet away at 5%. The '5 and 5' rule: at least 5% grade for the first ~10 feet away from the foundation.
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