What Black Mold Looks Like and Where It Grows
Black mold is a dark fungus that thrives in damp spaces. It appears as black or dark green spots, often with a slimy or powdery texture. The most common species is Stachybotrys chartarum, though several other molds look similar. You cannot identify a mold by color alone — black mold can look greenish, and other molds can look black — but the cleaning method is the same regardless.
Black mold grows where moisture stays: around leaking pipes, under sinks, in bathrooms with poor ventilation, around windows where condensation builds up, in basements after flooding, and inside air conditioning units. It spreads through spores that float in the air and land on damp surfaces. Once it takes hold, it grows quickly.
Small patches (smaller than a square foot) can be cleaned by a homeowner. Larger infestations, mold inside walls or ductwork, or mold caused by sewage or contaminated water should be handled by a professional mold remediation company.
Key Takeaways
- Black mold grows only on damp surfaces, so the first step is stopping the water source — fix leaks, improve ventilation, or run a dehumidifier.
- For small patches, scrub with a solution of detergent and water, a commercial mold cleaner, or diluted bleach (one part bleach to ten parts water), then dry the area completely.
- Wear gloves, eye protection, and an N95 mask when cleaning to avoid breathing spores or getting them in your eyes.
- Do not paint over mold or seal it in — it will continue growing underneath and spread further.
- If mold returns within a few weeks, the moisture source is still active and needs to be found and fixed.
Stop the Water Source First
Mold cannot grow without moisture. Before you clean, find and stop what is keeping the surface wet. Check for leaking pipes, cracks in the foundation, condensation from poor ventilation, or water seeping in from outside. If you do not fix the source, the mold will return within days or weeks.
In bathrooms, run the exhaust fan during and for 20 to 30 minutes after showers. In basements, use a dehumidifier to keep humidity below 50 percent. Around windows, wipe condensation away each morning and improve air circulation. Under sinks, look for slow leaks from pipes or the drain — even a small drip will feed mold growth.
If the mold is caused by a major leak, flooding, or structural water damage, stop cleaning and call a professional. Water inside walls or under flooring requires specialized equipment to dry properly.
Gather Your Cleaning Supplies
You need gloves (rubber or nitrile), eye protection, and an N95 mask or respirator. Mold spores are microscopic and float in the air when you disturb the mold — the mask keeps you from breathing them in. The eye protection prevents spores from landing in your eyes.
For the actual cleaning, you have three options: a commercial mold and mildew cleaner (follow the label), a solution of one part household bleach to ten parts water, or a detergent-based cleaner like dish soap mixed with water. Bleach works quickly but is harsh; detergent is gentler and works well for routine cleaning. Do not mix bleach with ammonia or other cleaners — the combination creates toxic gas.
You will also need a stiff brush or scrub pad, clean rags or paper towels, and a spray bottle. Have a fan running to move air out of the room while you work.
Clean the Mold Step by Step
Put on your gloves, mask, and eye protection before you start. Spray the moldy area with your chosen cleaner — bleach solution, commercial cleaner, or detergent mix. Let it sit for 10 to 15 minutes so the cleaner can kill the mold.
Scrub the area firmly with a stiff brush. For porous surfaces like drywall or wood, scrub harder because mold roots can penetrate the material. For tile, grout, or non-porous surfaces, scrubbing removes the visible mold. Wipe away the dead mold and cleaner with a clean rag or paper towel.
Spray the area again with the cleaner and let it dry completely. Do not rinse with water unless you are using a detergent cleaner — rinsing leaves moisture behind, which can feed new mold growth. If you used bleach or commercial cleaner, let it air-dry. Open windows and run fans to speed drying.
Once the area is dry, inspect it the next day. If you see mold returning, the moisture source is still active and must be found and stopped before cleaning again.
When to Call a Professional
Do not attempt to clean mold yourself if the patch is larger than about 10 square feet, if mold is inside walls or ductwork, if it is caused by sewage or contaminated water, or if you have respiratory problems or a weakened immune system. Mold remediation companies have equipment to contain spores, dry hidden moisture, and remove contaminated materials safely.
If you have cleaned the area and mold returns within two to three weeks, the moisture problem is not solved. At that point, a professional inspection may be worth the cost — they can use moisture meters and thermal imaging to find water sources you cannot see.
Prevent Black Mold From Coming Back
Keep humidity below 50 percent. In bathrooms, run exhaust fans and wipe down wet surfaces. In basements, use a dehumidifier and seal cracks in the foundation. Around windows, improve air circulation so condensation does not build up. Fix leaks as soon as you notice them — a small drip under a sink can grow a large mold colony in a few weeks.
Check problem areas monthly: under sinks, around the toilet, in the shower, around window frames, and in the basement. Catching mold early means a quick cleaning instead of a larger problem.
Frequently Asked Questions
Is black mold dangerous?
Black mold produces toxins that can irritate the respiratory system, especially in people with asthma, allergies, or weakened immune systems. Healthy people often have no reaction. The risk is real but not as extreme as some sources claim. Wear a mask when cleaning to avoid breathing spores, and improve ventilation to reduce exposure over time.
Can I use vinegar or hydrogen peroxide instead of bleach?
Yes. White vinegar and hydrogen peroxide both kill mold, though they work more slowly than bleach. Spray the area, let it sit for an hour or more, then scrub and wipe dry. They are less harsh than bleach and safer to use around children and pets, but they do not work as quickly on heavy mold growth.
What if the mold is on drywall?
If the mold has penetrated the drywall (more than surface growth), the drywall should be removed and replaced. Mold roots grow into drywall and cannot be fully cleaned out. If the mold is only on the surface, clean it as described, but watch the area closely — drywall that stays damp will grow mold again quickly.
Do I need to throw away items that had mold on them?
Porous items like carpet, insulation, or upholstered furniture that have mold growing in them should be discarded. Hard, non-porous items like plastic or glass can be cleaned and reused. Clothing and linens can be washed in hot water with detergent.
Will painting over mold stop it from growing?
No. Paint seals the mold in and traps moisture, which makes it grow faster underneath. The mold will spread into the wall and eventually break through the paint. Always clean and dry the surface completely before painting.