Learn the most common causes of mold in Southern California homes and how to prevent them. Clear Coast Restoration provides expert mold prevention and dry-out services in Rancho Cucamonga and surrounding cities.

Mold can start growing in as little as 24–48 hours when moisture is present — and most of the time, homeowners don’t know it’s happening until they smell it or see staining. Whether it’s a slow leak or high humidity, mold is almost always caused by one thing: moisture finding a place to hide.

Here are the top causes of mold growth and what to watch for to protect your home.

1. Slow Plumbing Leaks

Small leaks under sinks, behind walls, inside cabinets, or behind appliances are the #1 cause of mold.

Even tiny drips create enough moisture for mold to grow behind surfaces long before you notice anything on the outside.

High-risk areas:

• Under kitchen/bathroom sinks

• Behind dishwashers

• Around fridge water lines

• Inside wall cavities

2. Roof Leaks

A damaged roof, missing shingles, clogged gutters, or old flashing can let water enter the home and soak insulation or drywall.

Because attic spaces stay warm and dark, mold spreads FAST here.

3. Poor Ventilation

Rooms like bathrooms, laundry rooms, and kitchens generate humidity.
Without proper ventilation, moisture lingers in the air and settles on walls, ceilings, or behind cabinets — a perfect environment for mold.

Common signs include:

• Foggy mirrors

• Peeling paint

• Musty smells

4. High Indoor Humidity

Southern California gets humidity spikes, especially during summer. Homes without good airflow or AC control can quickly reach humidity levels where mold grows (60%+).

Humidity tends to gather in places like:

• Closets

• Bathrooms

• Behind furniture

• Basements and garages

5. Previous Water Damage That Wasn’t Dried Properly

If you’ve had a water leak, flood, or pipe break in the past, and the area wasn’t professionally dried, leftover moisture inside the walls or flooring can lead to mold months later.

This is one of the main reasons DIY drying is risky.

6. Condensation Problems

Condensation happens when warm, moist air hits a cold surface — like windows, pipes, or HVAC ducts.

Over time, this creates small wet spots that turn into mold, especially in:

• Window tracks

• HVAC closets

• Behind drywall near pipes

7. Leaking Water Heaters or Washing Machines

Laundry rooms and water heater closets are major mold hotspots because leaks often go unnoticed.

A slow drip from a hose, drain line, or tank can create mold under flooring, behind baseboards, and in wall cavities.

How To Prevent Mold in Your Home

You can lower your risk significantly by:

• Fixing leaks immediately

• Using exhaust fans during showers

• Keeping humidity between 30%–50%

• Getting professional dry-out after any water incident

• Checking under sinks and behind appliances every few months

What To Do If You Think You Have Mold

Mold spreads quickly, and the source is almost always moisture you can’t see.

Clear Coast Restoration specializes in:
✅ Mold prevention
✅ Moisture inspections
✅ Water damage dry-outs
✅ Structural drying
✅ Leak-related cleanup

Serving Rancho Cucamonga, Chino, Upland, Ontario, Fontana, Walnut, and all surrounding areas.