California / San Mateo County
Score 20 · moderate · #21 of 48 in CA
Crawl space moisture in San Mateo County, CA
52% of San Mateo County's soil falls in hydrologic group D or a dual D group: clays and high water tables that shed water slowly. Where a seasonal water table is reported — 6% of acres — it averages 0 ft below grade, shallow enough to reach a typical crawl-space footing.
- 1Water table 0 ft
- 2Wet soils 5%
- 3Moisture score 20
- 4EPA radon zone 2
Reading the San Mateo County soil survey
The score of 20 combines four parts, each from the USDA survey: wet drainage classes contribute 6.8 points of 100 on their own scale, group D soils 61.6, a shallow water table 6.2, and flooding 1.6; the weights are on the methodology page. The dominant hydrologic group is D (50% of acres): very slow infiltration, high runoff (clays, high water table).
A water table under two feet is the single most important number here. A crawl-space floor is usually 18 to 36 inches below the joists and often below outside grade, so in a wet season groundwater can stand at or above the crawl-space floor on the soils that report a water table — about 6% of San Mateo County's acres. Venting cannot fix that; it needs drainage — a perimeter drain to daylight or a sump — before any liner goes down.
San Mateo County against the rest of California
San Mateo County ranks 21 of 48 California counties, 2 points above the state median of 18. That places it in the middle range, nationally and for the state. Statewide, 11% of mapped soil is in the wet drainage classes; here it is 5%, below the state figure — the score here comes from heavy group D soils rather than drainage class alone. After well drained, the next most common class is moderately well drained at 9%.
Flooding. 0.8% of acres with a flooding class are rated frequent; 0.9% of acres with a flooding class are rated occasional; 1.1% of acres with a flooding class are rated rare. Flood-prone soils are a minority here, concentrated along streams; the score mostly reflects what happens between floods.
What it means for a crawl space here
In a moderate county the soil record is mixed rather than uniformly wet, so conditions vary from lot to lot. Where the lot drains, the model-code default — a vented crawl space with 1 sq ft of net vent per 150 sq ft of floor, or 1 per 1500 with a Class I vapor retarder on the ground — is a reasonable starting point; where the sections above point to a shallow water table or flood-prone soil on your lot, plan as for an elevated county. Either way, check the site first: grading that falls away from the foundation, gutters and downspouts that discharge well clear of it, and a ground cover over bare earth. A wet crawl space in a moderate county points to one of those first.
Radon. EPA radon zone 2. The zone is a county-level prediction; EPA says elevated levels occur in every zone and every home should be tested.
Near San Mateo County by score
Sources: USDA NRCS SSURGO via Soil Data Access (muaggatt: drclassdcd, hydgrpdcd, flodfreqdcd, pondfreqprs, wtdepannmin), acreage-weighted (survey areas CA637, CA689; drainage covers 58% of 428,551 mapped acres); EPA Map of Radon Zones, county table (June 2024 JSON); IRC R408.3 Unvented crawl space, as adopted in S.C. Code Regs. 8-1221 (LII). Checked 2026-10-23.