San Antonio – Austin corridor

Is your ZIP code built on soil that moves?

Foundation damage in Central Texas is mostly soil. Expansive clay swells when wet and shrinks in drought — and the more it moves, the more it pushes on your slab. This map scores every ZIP in our service area from real USDA soil shrink–swell data. Hover or tap your area to see its risk, the main driver, and what to read about it.

The Foundation Risk Map is © Motmot Foundation Repair and licensed under CC BY-NC-ND 4.0. You’re welcome to share it, embed it, or link to it with credit to Motmot Foundation Repair (motmotfoundationrepair.com). You may not sell it, charge for access to it, or publish a modified version.

Attribution line: Foundation Risk Map — Motmot Foundation Repair, https://motmotfoundationrepair.com/foundation-risk-map/ (CC BY-NC-ND 4.0)

Want the numbers instead of the picture? Download the data behind this map — every ZIP’s score as a free CSV, with the statistics we computed from it. The dataset is licensed separately under CC BY 4.0 (reuse with credit, commercial use OK); the map application above stays CC BY-NC-ND.

How to read the risk map

Every ZIP is shaded by its dominant soil’s shrink–swell potential — the USDA’s linear-extensibility measure of how much the ground expands when it soaks up water and contracts when it dries out. Darker areas sit on high-plasticity clays like Houston Black and Ferris; lighter areas sit on shallow limestone or sandier soils that barely move. The risk isn’t evenly spread across San Antonio and Central Texas. South and east of downtown San Antonio, and out across the Blackland Prairie toward the I-35 corridor, the clay is deep and active. North of the Balcones Escarpment — Stone Oak, the Hill Country edge, and much of the limestone north side — the ground is steadier, and the movement you do see is usually local (a leaking line, poor drainage, or fill around a slab) rather than the whole footprint.

What a high-risk reading actually means

A high score means the soil under your ZIP is capable of moving a slab enough to crack drywall, rack doors, and separate brick — not that your specific house is damaged. Soil sets the odds; drainage, plumbing leaks, tree roots, watering habits, and the original build quality decide whether that potential ever turns into movement. Plenty of homes on high-risk clay stay flat for decades because their drainage and moisture stay even. The map is a screening tool: use it to understand the ground you’re on, spot the seasonal pattern (clay swells in a wet spring and shrinks hard in an August drought), and know which of our drought and heavy-rain guides apply to your area.

The only way to turn an odds reading into a real answer for your address is a measured look. A free foundation inspection shoots your floor elevations, maps any cracks, and reviews drainage and moisture — so you know whether your slab is actually moving, or whether the ground under your ZIP just could. If you want the raw source behind the shading, it comes from the USDA’s public Web Soil Survey, and our Texas clay soil guide explains the science in plain English.