Floor Repair in San Francisco

San Francisco Conditions That Affect Floor Repair

NOAA's Comparative Climatic Data humidity table lists San Francisco (station 23272, period of record 1945-2018) at a mean July relative humidity of 91% in the morning and 71% in the afternoon, with annual means of 87% and 69%. The same table gives Sacramento a July afternoon mean of 29% and Stockton 27%. San Francisco's morning-to-afternoon spread of about 20 points is among the narrowest in the table; Sacramento's is roughly 42. With San Francisco summers near 67°F, a share of the damage we're called for is seasonal rather than structural — gaps that open in dry summer conditions and close again later. Distinguishing seasonal movement from a real defect is the first thing we do, because the honest answer is sometimes that the floor does not need repairing.

NOAA 1991-2020 normals at San Francisco Downtown give a January average high of 57.8°F and an average low of 46.6°F. Winter lows sit well above freezing. Annual precipitation normal is 22.9 inches; December is the wettest month at 4.76 inches, with January close behind at 4.40. Winter brings the opposite call: boards that have swelled tight or peaked at a joint, usually because the floor was installed without the room it needed to move.

The marine layer holds summer temperatures far below the inland valleys. The July normal high at San Francisco Downtown is 66.3°F and August is 67.9°F — about 15°F cooler than Redwood City, some 23 miles down the peninsula, which normals at 81.0°F in July. The city's warmest month is September, at 70.2°F. In San Francisco, telling seasonal movement from a real defect starts with the conditions the floor actually lives in, measured in the house rather than inferred from the gap.

Movement or Damage? The San Francisco Climate Baseline

A repair diagnosis starts with what the climate alone would do to a floor. Using 79,943 hourly readings from Oakland International Airport, a house in San Francisco heated without humidification sits near 42.3% relative humidity in winter, with 14% of heating hours below NWFA's 30% floor, and near 59% in summer — a swing of about 2.8 points in the moisture content of the wood. On white oak that accounts for about 0.024 inch per 2¼-inch strip board and 0.053 inch per 5-inch plank. Gaps inside those figures that follow the seasons are movement. Gaps well beyond them, gaps that stay open through summer, and cupping or crowning are the ones with a cause worth tracing, and the cause is usually moisture from below.

Source: Hourly ASOS dewpoint observations 2015–2025, Iowa Environmental Mesonet (Iowa State University); equilibrium moisture content from USDA Wood Handbook FPL-GTR-190 Eq. 4-5; movement from FPL-GTR-282 Eq. 13-3 and Table 13-5

Finding the Actual Cause

California's Earthquake Brace + Bolt program, administered by the California Residential Mitigation Program, states that 'an EBB seismic retrofit is only done on wood-framed, pre-1980 homes with a raised foundation,' targeting cripple walls in crawl spaces beneath the structure. Twenty-eight San Francisco ZIP codes appear on the 2025 EBB Northern California eligibility list, from 94102 through 94158. Eligibility is set by ZIP code, so the list shows where the state offers that retrofit, not how many homes have raised foundations, and no source publishes a measured raised-versus-slab split for San Francisco. Alongside this, 76.5% of the city's 414,602 housing units predate 1980 and 45.2% predate 1940 per ACS. Separately, San Francisco's mandatory soft-story retrofit program applies to 'wood-frame structures, containing five or more residential units, having two or more stories over a soft or weak story, and permitted for construction prior to January 1, 1978' — evidence about multi-unit buildings, not about single-family foundation type, and not used here as a proxy for one.

Raised floors are where most structural floor repair happens: soft spots, squeaks, and bounce that trace back to the subfloor, the joists, or the crawlspace rather than to the finish floor. We go underneath and look. Fixing a soft spot from above without addressing what softened it is a temporary repair sold as a permanent one.

Source: NAHB analysis of the Census Bureau Survey of Construction, foundation type by region (2024)

San Francisco Homes We Work In

ACS 2020-2024 five-year estimates count 414,602 housing units, of which 45.2% (187,447 units) were built in 1939 or earlier — by count, more pre-war housing than any other city in this service area (Berkeley's share, 46.3%, is statistically the same). A further 8.4% date to 1940-1949, and 76.5% of all units predate 1980; construction from 2000 onward accounts for 14.4% of the stock. 69.4% of units are in buildings with two or more units. In San Francisco's pre-1940 and 1940s homes, repairs frequently run into discontinued products and subfloor assemblies that predate current practice. Sourcing a match — or working out where a transition can hide a change of material — is often the harder half of the job.