Seeing dark stains or mold in an attic is stressful—and many homeowners assume it’s a roof leak. Sometimes it is. But in many Bay Area homes, the root cause is condensation from indoor moisture and poor ventilation.
How to tell condensation from a roof leak
Clues it’s condensation
- Widespread staining or mold across large areas (not one point)
- Mold on nail tips or metal fasteners
- Problems worse during cold nights (warm indoor air meets cold roof deck)
- Bathroom/kitchen fans venting into the attic
Clues it’s a roof leak
- One localized stain that grows after rain
- Wet insulation in a specific bay
- Staining that lines up with a valley, chimney, skylight, or vent penetration
- Drips during storms, especially with wind direction
Fast checks you can do
- Timing: Does moisture appear only after rain—or also during dry cold nights?
- Location: Is it near a penetration (vent, skylight, chimney) or spread evenly?
- Vent terminations: Confirm all exhaust fans terminate outside.
- Air leaks: Look for gaps around can lights and attic hatches.
Fixes that address condensation
- Seal ceiling air leaks to reduce moisture migration
- Confirm soffit intake is open and not blocked by insulation
- Add baffles at eaves to keep airflow pathways clear
- Improve exhaust with ridge/roof vents where appropriate
Fixes that address true roof leaks
- Rebuild flashing at the leak source (not just seal it)
- Repair valley details and underlayment if water concentrates
- Replace failing vent boots and re-integrate properly
Next step
If you’re unsure, a documented inspection can determine whether it’s a moisture management problem or an exterior leak path. Start with roof repair or request an inspection.