Home Inspection in Union City, CA

The City's own environmental analysis puts it in geographic terms rather than hazard terms, and that is the most useful way to understand Union City. The Hayward fault divides the low-lying, gently sloping, and nearly level alluvial and estuarine landforms that surround San Francisco Bay from the strongly sloping and steep upland forms of the northwest-trending East Bay hills. West of the Hayward fault the land is urbanized, whereas east of the fault development is sparser. Everything that follows on this page comes from which side of that line a house sits on. The flat urbanized west is a clay and groundwater inspection. The sparser east is a slope and access inspection. They are not variations on one job. On the flats we are reading shrink-swell movement, drainage, and slab behaviour. Going east we are reading retaining walls, surface water that has been collected or simply dumped over a bank, fill placed after construction, and distress in framing and foundation that tracks the downhill direction. We tell clients which inspection they are buying before we arrive, because the time it takes is different.

The Station District environmental analysis reproduces the Natural Resources Conservation Service soil mapping for the area, and the numbers are worth knowing before you read a single crack. Rincon clay loam covers roughly 51 percent of that planning area in the northern portion, Clear Lake clay about 27 percent across the central and eastern portion, and Yolo silt loam about 21 percent in the central western and southwestern portion, with a small amount of Danville silty clay loam in the east. The analysis then states plainly that the Clear Lake, Danville, and Rincon clay and clay loam soils are moderately to highly expansive, and that expansive soils can shrink and swell in response to the presence of water, causing foundation and wall cracks, heaving sidewalks, and creating flaws in paved areas. Expansive clay produces a seasonal signature rather than a single event, and reading it is a pattern-recognition job. We look at whether stucco cracking radiates from stress points like window corners, whether a slab has heaved at one location or settled across a plane, whether interior finish cracking lines up with anything visible outside, whether doors bind at the top on one side of the house and the bottom on the other, and whether irrigation and landscaping have left perimeter moisture wildly uneven from one elevation to the next. We report what the structure shows. We do not classify a soil.

This is where Union City differs from the bayside cities south of it. The City's environmental analysis records that portions of central and southeast Union City are susceptible to high and very high levels of liquefaction, and for the Station District planning area specifically it describes regional mapping from the Association of Bay Area Governments showing areas of moderate and very high liquefaction susceptibility, with most of the area at moderate susceptibility and very high susceptibility in the westernmost extent and in the southeasternmost Gateway subarea. It adds the honest caveat that regional mapping is only a general analysis and that site specific analysis would identify specific liquefaction risk, which is influenced by the depth to groundwater and soil type. The same document flags lateral spreading where high liquefaction susceptibility sits adjacent to an open face or slope. For a buyer that means the answer genuinely changes by neighbourhood, which is not true everywhere. For us it means the liquefaction-related findings get weighted by where in the city the house is, and the parcel-level question goes to a current soils report and a qualified geotechnical engineer rather than to our report.

The City's analysis is careful here and we will be too. It notes that Alameda County has not experienced land sinking due to low ground water levels and that USGS mapping of historical and current subsidence does not identify Union City as an area that has experienced subsidence. It then says that the alluvial soils west of the Hayward fault zone, where the flat half of the city is, are considered susceptible to subsidence, and concludes that due to the composition of the soils underlying the planning area there is a moderate potential for subsidence to occur. That is a different finding from the neighbouring city of Newark, whose Safety Element states that subsidence is not considered a risk there, and it is the sort of distinction that gets flattened when someone writes one page for the whole Tri-City area. Subsidence is slow, broad and usually invisible at the scale of a single lot, so an inspection does not detect it directly. What an inspection can do is document whether the building sits level, whether floor slopes are consistent across the structure or localized to one area, whether utility connections at the perimeter have been strained, and whether exterior flatwork has moved relative to the building rather than with it.

The City's analysis states that a rupture of the Hayward fault is considered capable of generating a moment magnitude 7.4 earthquake, and that an earthquake matching that scenario is estimated to be capable of generating very strong to violent seismic shaking in the area, in Modified Mercalli terms MM-VIII to MM-IX. The historical record behind that is local and specific: the largest earthquake on the Hayward fault occurred in 1868 with an epicenter south of San Jose, estimated at magnitude 6.8, and surface fault rupture occurred along the Hayward fault from Fremont to San Leandro. Union City sits on that stretch. The USGS working group figure the City cites is a 72 percent chance of a magnitude 6.7 or greater earthquake in the Bay Area between 2014 and 2043, with 14 percent of that attributed to the Hayward and Rodgers Creek faults. The fault also creeps, meaning it slips slowly and continuously between earthquakes, though the clearest documented evidence of that sits in Hayward and Fremont rather than here and those pages cover it properly. What we inspect is anchorage and condition: mudsill bolting where the foundation is visible, cripple wall bracing in any raised-foundation house, post and girder bearing, chimney condition above and below the roofline, and water heater strapping.

Union City incorporated on January 26, 1959, formed from two much older communities that had been neighbours since 1870. Alvarado on the west was the first county seat of Alameda County from 1853 to 1865 and was the site of the California Beet Sugar Company, the first successful sugar beet mill in the United States, which began operating in 1870 and was demolished in 1977. Decoto on the east side was founded in 1870 as a railroad town. Around and between those two cores sits a city that mostly went up after incorporation, through the decades that followed. That produces two distinct inspections in one set of city limits. In the older Alvarado and Decoto blocks we expect raised foundations, cripple walls that may or may not have been bolted and braced, original knob and tube or early cloth-sheathed wiring surviving in parts of an otherwise updated house, galvanized supply piping, and additions layered on over a century. In the post-1959 tract stock the findings shift to slab behaviour on expansive clay, service capacity in a panel sized for a smaller decade, aluminum branch wiring in the mid-1960s to mid-1970s band where it appears, and bath fans discharging into an attic rather than through the roof. Same city, same inspector, two different checklists.

Union City covers roughly 19 square miles in southern Alameda County with no bay frontage of its own, bounded by Hayward to the north and Fremont and Newark to the south, with the East Bay hills rising behind it east of the fault and the BART Station District as its centre. It is worth naming what belongs to the neighbours rather than to this page. Hayward's story is the fault running under its downtown and the 1930 City Hall it destroyed by creep. Fremont's is an Alquist-Priolo earthquake fault zone crossing five of its districts with offset curbs to prove it. Newark's is a Safety Element that places the entire city inside a liquefaction hazard zone with no landslide areas and no subsidence risk at all. What Union City owns is the combination here: a single fault acting as the physical divider between a flat urbanized half and a hillside half, moderately to highly expansive clay and clay loam under most of the flat side, liquefaction that is high to very high only in named portions rather than everywhere, a moderate potential for subsidence on the alluvial soils, and two nineteenth-century town cores embedded in post-1959 tract stock. Soils, slope, groundwater and construction history still vary lot to lot, and parcel-level questions belong to a current soils report, current hazard mapping and the seller's disclosures.