Frost Depth in San Antonio, Texas: Effectively Zero

San Antonio Frost Profile (USDA Zone 9a)

• Avg annual min temp: 20–25°F (-6 to -4°C)
• Last spring frost: ~March 15
• First fall frost: ~November 15
• Frost-free period: ~245 days

❄️ Frost Depth Line?

None. Ground rarely freezes. Mediterranean/subtropical climate — soil temp stays >40°F (4°C) at depth. No structural frost-depth requirement for foundations (unlike North/Central US).

🏗️ Building Implication

• Shallow footings typical.
• No need for frost-protected shallow foundations (FPSF).
• Focus on expansive clay soils (montmorillonite) — moisture-driven movement > frost heave.

Check local building codes (San Antonio amendments to IRC) for exact footer specs.