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Overview
San Francisco receives roughly 19.6 inches of precipitation per year at an average of 58.7 F, which means it experiences infrequent rain but high sensitivity to leaks when storms hit.
San Francisco follows California wet and dry seasons, with winter rain bringing the highest leak risk.
San Francisco DBI Code Enforcement investigates building and housing code violations and accepts complaints.
This guide highlights local moisture drivers, seasonal risk, and prevention steps. For legal rights and landlord obligations, review the California mold laws guide.
Sources: [California - Climate and Wildfires | Britannica] | [Code Enforcement at DBI | SF.gov] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
Action Checklist
San Francisco Home Inspection Checklist
Common construction types in San Francisco affect where moisture problems develop. Here is how local building patterns shape what to inspect.
In San Francisco, dBI Code Enforcement handles building and housing code concerns tied to maintenance and safety.
Keeping documentation of recurring moisture problems can support follow up actions.
Sources: [Code Enforcement at DBI | SF.gov]
- With just 19.6 inches of annual precipitation, San Francisco homes need focused roof and window checks after every rainfall event.
- Roof flashing at valleys, skylights, and vent stacks.
- Exterior door thresholds and window sills.
- Bathroom fans and vent ducting.
- Kitchen cabinet bases around plumbing.
- Laundry supply hoses and shutoff valves.
- HVAC condensate drain lines.
- Closets on exterior walls with limited airflow.
Sources: [EPA Mold, Moisture, and Your Home] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
Warning Signs of Mold in San Francisco
Watch for these signs that moisture is lingering in San Francisco homes and rentals:
- Condensation forming on glass during San Francisco warm seasons (average temp 58.7 F) suggests indoor humidity needs attention.
- Musty odor near hallways or closets.
- Bubbling paint around windows.
- Condensation that returns on glass each morning.
- Dark spotting near ceiling vents.
- Soft drywall around showers.
Sources: [EPA Mold, Moisture, and Your Home] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
Mold Risk Factors in San Francisco
These are the most common moisture drivers for homes and rentals in San Francisco:
- With only 19.6 inches of annual precipitation in San Francisco, indoor moisture from plumbing and HVAC systems is the primary mold driver rather than rainfall.
- Wet season storms stress roof edges and exterior caulk lines.
- Short, intense rain can overwhelm drains and patio door thresholds.
- Slow plumbing leaks can keep wall cavities damp for long periods.
- Bathroom humidity lingers without strong exhaust.
- Clogged condensate drains can leak into closets or attics.
Sources: [California - Climate and Wildfires | Britannica] | [EPA Mold, Moisture, and Your Home] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
San Francisco Seasonal Risk Calendar
Winter
Peak rainfall. Inspect roofs, windows, and gutters after storms.
Spring
Showers taper. Fix minor leaks before the dry season.
Summer
An average of 58.7 F drives extended cooling seasons. Monitor HVAC condensate and use dehumidifiers in below-grade spaces.
Fall
First storms return. Recheck exterior caulk lines.
Sources: [California - Climate and Wildfires | Britannica] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
San Francisco Moisture Playbook
Use this short playbook to stay ahead of common moisture problems in San Francisco.
After a Leak
Dry the area quickly, remove wet materials, and keep airflow moving for 24 to 48 hours.
Monthly Maintenance
Inspect under sinks, clear condensate lines, and keep exhaust fans running after use.
Seasonal Tune Up
At just 19.6 inches of rain per year, a single heavy storm can overwhelm unprepared homes. Seal windows and prep drainage early.
Sources: [EPA Mold, Moisture, and Your Home] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
Mold Prevention in San Francisco
Use these priorities to keep indoor humidity low and avoid recurring growth:
- Despite only 19.6 inches of annual rainfall, seal window and door gaps before storms to prevent water intrusion.
- Maintain indoor humidity between 30 and 50 percent.
- Dry wet materials within 24 to 48 hours.
- Vent bathrooms and kitchens outdoors.
- Inspect roof flashing and window seals before rain.
- Keep gutters clear and downspouts flowing.
- Service HVAC condensate drains annually.
Sources: [EPA Mold, Moisture, and Your Home] | [NOAA Climate Normals - SAN FRANCISCO INTL AP, CA US]
Local Regulations
Some cities in California publish additional housing rules or enforcement guidance that can affect mold complaints.
State Laws and Rights
California mold law snapshot
California has mold-specific rules in addition to general habitability standards.
California recognizes an implied warranty of habitability in all residential leases, requiring landlords to maintain rental properties in a condition fit for human habitation. Under Civil Code Section 1941.1, landlords must ensure adequate weatherproofing, plumbing, heating, and sanitation. Since January 1, 2016, visible mold growth (except minor mold on surfaces that can accumulate moisture as part of their intended use) is explicitly classified as a substandard condition under Health and Safety Code Section 17920.3, making any dwelling with significant mold a violation of habitability standards.
Use the full California guide for tenant rights, landlord responsibilities, and remedies.