Engineered Hardwood in San Francisco

San Francisco Conditions That Affect Engineered Hardwood

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. Engineered hardwood exists precisely for climates like this. With San Francisco summer highs near 67°F and air conditioning drying the house out, a cross-laminated or multi-ply core moves far less across the grain than solid wood does, so seasonal gapping is much less pronounced. It is the reason we recommend engineered over wide plank widths in particular — the wider the board, the more a solid version would move.

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. Through winter the same stability works in reverse: the core resists the swelling that a solid floor would show as the house takes on moisture. Stable is not inert, though — the expansion gap at the perimeter still gets set properly.

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. For engineered hardwood in San Francisco, the conditions that matter most are under and inside the house: a subfloor confirmed dry before a glue-down, and indoor humidity inside the range the manufacturer specifies.

What San Francisco's Humidity Asks of Engineered Hardwood

In San Francisco the case for engineered hardwood rests more on plank width and slab than on climate, and the measurements show why. Hourly dewpoint readings from Oakland International Airport, 11 miles away, put that air at a median 42.3% relative humidity once it is heated to 70°F over winter — 14% of those hours under NWFA's 30% minimum — rising to 59% in summer. A solid 5-inch white oak board living through that, at roughly 8% to 10.8% equilibrium moisture content, changes width by about 0.053 inch between seasons. A cross-laminated core exists to restrain that movement; how much it restrains depends on the product's construction, so we go by the manufacturer's stated humidity range for the specific floor rather than a general rule.

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

Subfloor and Foundation Work

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.

Over a crawlspace, engineered goes down by nail, staple, glue, or float, so the method follows the subfloor rather than the other way round. We flatten the subfloor first — engineered plank is stiff, and it bridges dips instead of following them, which is what produces hollow spots underfoot.

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 housing, engineered often wins on height as much as on moisture: a thinner assembly clears existing door jambs, appliance toe-kicks, and the transitions to rooms that aren't being redone, which matters when only part of the house is getting new floor.