Mold grows where moisture stays. The U.S. Environmental Protection Agency (EPA) says the key to mold control is moisture control. When mold follows a roof leak, a missing flashing or another construction defect, cleaning alone will not solve it. You need to find and fix the water source, dry the area, and then clean or remove the moldy material.
Last reviewed: September 2026. Sources are listed at the end of this page.
Why does mold follow water intrusion?
Mold control depends on moisture control, and a building leak brings moisture in. EPA warns that if you clean up mold without fixing the water problem, the mold will most likely come back.[1]
Construction defects can supply that water:
- Building Science Corporation (BSC) says wood above about 25 to 30 percent moisture content is in the range where mold grows.[7]
- The U.S. Department of Energy’s Building America Solution Center (BASC) warns that missing kickout flashing at a roof-wall joint can lead to mold inside wall cavities. Fiber cement, vinyl siding and brick veneer can hide the evidence for years.[6]
- University of Minnesota Extension says moisture from ice dams can cause mold and mildew.[8]
To trace the leak, see the water intrusion guide and the roof and flashing defects guide.
How fast do you need to dry water damage?
EPA says to dry water-damaged areas and items within 24 to 48 hours to prevent mold growth.[1] Do not run a heating or cooling system you know or suspect has mold in it. EPA says it could spread mold through the building.[1]
Keep indoor humidity low after the leak is fixed. The two agencies differ slightly:
- EPA: below 60 percent relative humidity, ideally 30 to 50 percent.[1]
- Centers for Disease Control and Prevention (CDC): as low as you can, and no higher than 50 percent.[2]
If a builder or warranty program may be responsible, photograph the damage before you clean it up. Some programs limit repairs. New Jersey’s New Home Warranty Program says it will not reimburse repairs made without its written authorization.[9] See how to document a construction defect.
Should you test for mold?
Usually not. EPA says sampling is unnecessary in most cases when you can see mold growing.[3] CDC does not recommend mold testing. It says health effects vary from person to person and good sampling can be expensive.[2]
EPA also notes that no standards or threshold limit values exist for airborne mold, and EPA has no regulations for airborne mold.[3] EPA says surface sampling may help confirm that an area was properly cleaned.[3]
Is black mold more dangerous than other mold?
CDC says you do not need to identify the type of mold. All molds should be treated the same for health risk and removal.[4]
The mold often called black mold is Stachybotrys chartarum, a greenish-black mold that grows on materials containing cellulose where moisture is constant. CDC says it and other molds may cause symptoms that are nonspecific.[4] CDC also says a possible link between this mold and acute idiopathic pulmonary hemorrhage in infants has not been proven.[4] The most common indoor molds are Cladosporium, Penicillium and Aspergillus, according to CDC.[2]
What health effects can mold cause?
Effects range from none to allergy-type symptoms. CDC says damp, moldy places may cause a range of health effects, or none at all.[2] In sensitive people, CDC lists a stuffy nose, sore throat, coughing or wheezing, burning eyes and skin rash.[2] EPA adds hay fever-type symptoms such as sneezing and a runny nose.[1]
CDC names people with asthma or mold allergies, people with weakened immune systems, and people with chronic lung disease as more at risk.[2]
Can you clean up mold yourself?
For small areas, yes. The table shows where EPA and CDC draw the lines.
| Situation | Agency guidance |
|---|---|
| Moldy area under about 10 square feet (roughly 3 feet by 3 feet) | In most cases you can handle it yourself (EPA).[1] |
| Mold over 10 square feet, or a lot of water damage | Consult EPA’s guide Mold Remediation in Schools and Commercial Buildings (EPA).[1] |
| Protective gear | At least an N95 respirator approved by the National Institute for Occupational Safety and Health (NIOSH), plus gloves and goggles (CDC).[5] |
| Who should stay out | People with allergies, weakened immune systems, lung disease, asthma or chronic obstructive pulmonary disease (COPD) (CDC).[5] |
| Bleach | No more than 1 cup of bleach per gallon of water (CDC).[5] Never mix bleach with cleaners that contain ammonia, because toxic fumes can form (EPA).[1] |
This page is general information, not medical advice. If you have health concerns, talk to your doctor. Have a qualified professional assess your home.
Related guides
- Water intrusion and building envelope defects
- Roof and flashing defects
- How to document a construction defect
- Asbestos, lead paint, radon and problem drywall
- New-home builder warranties
Sources
- U.S. Environmental Protection Agency, “A Brief Guide to Mold, Moisture and Your Home,” last updated February 18, 2026.
- Centers for Disease Control and Prevention, “Mold,” September 26, 2024.
- U.S. Environmental Protection Agency, “Mold Testing or Sampling,” last updated January 14, 2026.
- Centers for Disease Control and Prevention, “Facts About Stachybotrys chartarum,” May 29, 2024.
- Centers for Disease Control and Prevention, “Mold Clean Up Guidelines and Recommendations,” February 16, 2024.
- U.S. Department of Energy, Building America Solution Center, “Step and Kick-Out Flashing at Roof-Wall Intersections,” last updated September 14, 2020.
- Kohta Ueno, Building Science Corporation, “PA-1904: How to Look at a House like a Building Scientist (Part 4: Water),” August 5, 2021.
- University of Minnesota Extension, “Dealing with and preventing ice dams,” reviewed 2022.
- New Jersey Department of Community Affairs, “New Home Warranty Program Homeowner’s Booklet,” revised September 2025.