Floor Refinishing in San Francisco

San Francisco Conditions That Affect Floor Refinishing

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. San Francisco summers near 67°F change the finishing schedule more than the sanding. Waterborne finishes flash off faster in hot, dry air, which shortens the working window and makes lap marks easier to leave behind; we adjust application pace and airflow to the conditions in the house that day rather than to a fixed timetable.

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. Cold, damp conditions do the reverse and stretch cure times, which mostly costs patience — a floor that is walkable is not yet a floor that is ready for furniture or rugs.

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. Cool, damp days in San Francisco lengthen finish cure times compared with a hot, dry one — a scheduling factor rather than a quality one.

Seasonal Gaps in San Francisco Floors, by the Numbers

Seasonal gaps between boards are a common reason people ask about refinishing, and San Francisco's measured climate shows how much of that gapping is simply the wood moving with the seasons. Measured at Oakland International Airport, then brought to 70°F, the air averages 42.3% relative humidity over the heating season and 59% over summer, which settles wood at roughly 8% and 10.8% moisture content. For a traditional 2¼-inch oak strip floor that is about 0.024 inch of seasonal shrinkage per board (about 1/42 inch); for 5-inch plank, about 0.053 inch. Gaps in that range that open in winter and close by summer should not be filled — filler packed into a winter gap gets crushed out, or pushes the boards toward cupping, once the wood takes its moisture back.

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

What's Under the Finish

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 the best refinishing candidates live: solid wood over a crawlspace, often with decades of wear and plenty of thickness left. It is also where cupping comes from. Sanding a cupped floor flat before the crawlspace moisture is fixed produces a floor that goes concave the other way once it dries — so the moisture reading comes first and the sanding comes second.

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. San Francisco's pre-1940 and 1940s stock is where refinishing pays off most: homes of that period often have original wood under carpet, and when the wear layer is intact, refinishing keeps a floor that cannot be bought new. We check thickness at a vent or a doorway before recommending it.