How Plumbing Problems Lead to Mold—and How to Stop It

Under-Sink Leaks

The most common source of mold we see. These leaks come from:

• Loose connections at supply lines or drains
• Aging shutoff valves or supply hoses
• P-trap connections that have loosened over time
• Minor drips that only occur when water is running

Because cabinets hide the plumbing, these leaks can soak wood and drywall for months before anyone notices. By then, mold has often established itself inside the cabinet walls.

Check yours: Open every cabinet with plumbing and look for water stains, warped wood, or musty smells. Feel the cabinet floor—it should be dry.

Toilet Leaks

Toilets leak in subtle ways that are easy to miss:

• Water slowly seeping at the base (failed wax ring)
• Condensation forming on the tank exterior
• Internal parts allowing water to run continuously
• Supply line connections dripping

The base leak is the worst—water seeps under flooring and into the subfloor, creating ideal conditions for mold you’ll never see until the floor feels soft or starts to smell.

Check yours: Look for discoloration around the toilet base. Feel the floor—any softness is a warning sign. Listen for running water when the toilet hasn’t been used.

Appliance Connections

Appliances that use water are frequent culprits:

• Dishwashers — Supply lines, drain connections, door seals
• Washing machines — Supply hoses, drain standpipes, pump seals
• Refrigerators — Ice maker supply lines (often in walls)

These appliances are installed in tight spaces where small leaks go unnoticed until damage appears. Washing machine supply hoses are particularly notorious—rubber hoses can fail suddenly after years of service.

Check yours: Pull appliances out periodically and inspect behind them. Replace rubber washing machine hoses with braided stainless steel.

Water Heater Issues

Water heaters can release moisture from:

• T&P (temperature and pressure) relief valve discharge
• Drain valve connections
• Tank corrosion and eventual failure
• Condensation on the tank or nearby pipes

Because water heaters live in garages, closets, or utility areas, leaks often go unnoticed for long periods. A drain pan with a proper discharge line is essential—but many homes don’t have one.

Check yours: Look for rust, water stains, or moisture around the base. Verify there’s a drain pan underneath, especially if the unit is in a finished space.

Showers and Tubs

Bathrooms are designed to handle water—but only in the right places. Problems occur when:

• Grout or caulk fails, allowing water behind tile
• Shower pans crack or lose their seal
• Drain connections leak below the floor
• Overflow drains aren’t properly connected

These problems often cause mold inside walls rather than on visible surfaces. By the time you see it, the damage is extensive.

Check yours: Look for soft spots in flooring near tubs and showers. Check for loose tiles or deteriorated grout. Note any musty smells after showering.

Condensation and Ventilation

Not all moisture comes from leaks. Condensation builds up when:

• Cold water pipes run through warm, humid spaces
• Bathrooms lack adequate exhaust ventilation
• Humid air has nowhere to escape

In our climate, we see this particularly in crawlspaces and bathrooms without functioning exhaust fans. Over time, repeated condensation is just as damaging as a slow leak.

Check yours: Look for water droplets on cold water pipes. Verify bathroom exhaust fans actually vent to the exterior (not just into the attic). Check crawlspace moisture levels.

What You See

• Stains or discoloration on walls or ceilings
• Warped or swollen cabinets
• Peeling paint or bubbling drywall
• Flooring that’s buckling or feels soft
• Rust or mineral buildup around fixtures
• Visible mold (black, green, or white spots)

What You Smell and Feel

• Musty or earthy odors
• Rooms that feel humid or sticky
• Cabinets that smell damp when opened
• A “wet basement” smell in any room

What Your Bills Tell You

• Higher-than-normal water bills
• Unexplained increases in usage
• Water meter spinning when nothing is running

Step 1: Stop Using Water in That Area

If you’ve identified a likely source, reduce water use there until the leak is found and fixed. This slows the moisture accumulation.

Step 2: Have the Plumbing Inspected

A licensed plumber can locate hidden moisture by:

• Checking supply and drain connections
• Inspecting behind fixtures and appliances
• Using leak detection equipment when needed
• Identifying condensation issues

The plumbing problem must be fixed before addressing mold. Otherwise, the mold will return.

Step 3: Assess the Mold Situation

If moisture has been present for a while or you see/smell mold, consider a professional mold assessment. They can evaluate:

• How far moisture has spread
• Whether mold exists in hidden areas
• What remediation steps are needed

Plumbers and mold professionals often work together—we fix the source, they handle the remediation.

Step 4: Follow the Right Order

1. Stop the moisture source — Fix the plumbing
2. Dry affected areas completely — This may require professional drying equipment
3. Remove damaged materials — Wet drywall and insulation often can’t be saved
4. Remediate mold — If present
5. Repair and restore — Only after everything is dry

Skipping steps—or doing them out of order—allows mold to return.

Monthly Habits

• Open cabinet doors and check for moisture
• Look behind toilets for water or staining
• Listen for running water when nothing is on
• Note any new musty odors

Annual Maintenance

• Inspect washing machine hoses (replace rubber with braided stainless)
• Check water heater for rust or leaks
• Verify bathroom exhaust fans work and vent outside
• Have a plumber inspect accessible plumbing

Smart Upgrades

• Water leak detectors — Battery-powered sensors that alert you to moisture ($20-$50 each)
• Automatic shutoff valves — Stop water flow when a leak is detected
• Braided stainless hoses — Replace rubber hoses on all appliances
• Drain pans — Under water heaters and washing machines

These aren’t expensive, and they can save you thousands.