The first big storms of the season are when hidden moisture problems show up. A roof flashing that held all summer starts to drip. A crawlspace that was dry in August has standing water. A closet on the north wall starts to smell musty.
Mold needs two things: moisture and something to feed on. In a house, food is almost always there (wood, drywall, dust), so the real problem is the water. Sonoma County's Healthy Homes program puts it the same way: mold is a symptom of a moisture problem. Fix the moisture, or the mold comes back.
What to check after a storm
- Ceilings and upper walls: new stains, bubbling paint, sagging drywall
- Around windows and doors: wet sills, swollen trim, peeling paint
- Attic: damp insulation, dark spots on the roof sheathing, daylight around vents or flashing
- Crawlspace: standing water, wet soil, a musty smell coming up through the floor
- Closets and corners on outside walls: these stay cold, so condensation and mold collect there
- Gutters and downspouts: overflowing gutters or downspouts that empty against the foundation
- Your nose: a persistent musty smell is a strong clue even if you can't see anything
Do you need a mold test?
Not always. The EPA's guidance is that if mold is visible, sampling usually isn't needed. Find the water and fix it. Testing earns its cost when you smell mold but can't find it, when someone in the home is sensitive, when you need documentation for a sale, a landlord or an insurance claim, or when you want to confirm a cleanup actually worked. Mold testing done right uses proper air-sampling equipment and an accredited lab. Cheap plate kits don't give reliable answers.
Who to call
- Active leak: a roofer or plumber first. Stop the water.
- You can't tell where the moisture is coming from: an independent inspector who can trace it with infrared and moisture readings and test the air if needed. That's what I do, and I don't sell remediation.
- Mold covering a large area: a licensed remediation contractor, ideally after independent testing so you know the scope.
- Renters: report the leak to your landlord in writing and keep a copy. If nothing happens, your city or county code enforcement office can inspect for weatherproofing problems.
Juan de la Cruz, CMI, is an independent home inspector in Santa Rosa and a former firefighter and code enforcement officer.
Sources: Sonoma County Healthy Homes Program · US EPA: Is Sampling/Testing for Mold Necessary?