Red Clay, Triassic Clay and Fill: The Ground Under Raleigh Homes

Most of Wake County's natural soil is Piedmont clay weathered from granite, gneiss and schist, led by Pacolet, Cecil and Wedowee. Toward the west, soils from the Triassic basin, such as Creedmoor and Green Level, cover about a tenth of the county; the USDA rates Creedmoor high in shrink-swell potential and describes Green Level's subsoil as very plastic clay.

Piedmont clay over weathered rock

Across most of the county, the subsoil is clay formed in place from granite, gneiss and schist that has rotted to soft saprolite below. Pacolet, Cecil and Wedowee together make up about a quarter of the major soil acreage in the Wake County survey. Cecil, the best known, is described by the USDA as well drained. These soils usually carry a house well. Trouble tends to start where water collects against a footing or where a lot was cut into a slope.

The Triassic basin and its shrink-swell clay

About 220 million years ago, rifting opened basins that filled with sediment, now reddish-brown sandstone, siltstone and mudstone. The Deep River basin, the largest in North Carolina, includes part of Wake County, and the NC Geological Survey says soils from these rocks drain poorly and can shrink and swell. Green Level soil has its official type location in Wake County, about five miles west of Raleigh near Green Level Church Road, with very slow permeability and slick, very plastic clay. Creedmoor, also formed from Triassic rock, carries a high shrink-swell rating. The two cover roughly 53,000 acres of the county.

How does the soil survey rate Wake County for houses?

The USDA rates each soil for building. For houses without basements, the county's rated land splits:

• Not limited: 40.6%
• Somewhat limited: 32.4%
• Very limited: 27.0%

The reasons cited most are a shallow saturated zone, slope, shrink-swell (about 72,000 acres) and flooding. For houses with basements, the very limited share rises to 43.4%. About 16% of the county's soil acreage is urban land the survey does not rate, because building has already reshaped it.

What to do with this

Look up your own lot in the USDA's Web Soil Survey; the map unit name tells you which soil you're on. The NC Geological Survey notes that a geotechnical engineer can identify expansive soil, which is worth doing when bids disagree about why a house moved. On any of these soils, steady moisture helps: long downspout lines, soil graded away from the walls. Read about sealing and judging foundation cracks and leveling a settled house, and compare figures in the 2026 Raleigh price guide.