Black mold on wood can be cleaned with white vinegar, hydrogen peroxide, or a borax solution, depending on how deep the mold has grown into the wood

Black mold on wood surfaces—whether on a basement joist, window frame, or deck—starts as a surface stain but can penetrate the wood grain if left untreated. The removal method depends on how long the mold has been there and how much moisture the wood has absorbed. Surface mold (visible only on top) responds to vinegar or hydrogen peroxide. Mold that has begun to soften or discolor the wood itself may need sanding, replacement, or a stronger treatment with borax.

Before you start, put on an N95 mask and gloves. Black mold spores become airborne when disturbed, and you do not want to inhale them. Open windows or run a fan to move air out of the space. If the mold covers more than 10 square feet, or if the wood is structural (like a load-bearing beam), contact a mold remediation professional instead—the cost of removal is less than the cost of a weakened house.

Key Takeaways

  • Surface mold on wood can be removed with white vinegar or hydrogen peroxide applied directly and scrubbed with a stiff brush.
  • Wear an N95 mask and gloves when cleaning mold, because spores become airborne and can irritate your lungs and sinuses.
  • If mold has softened the wood or penetrated deep into the grain, sanding or replacing the affected section is more effective than cleaning alone.
  • After removal, dry the wood completely and address the moisture source—poor ventilation, a leak, or standing water—or the mold will return within weeks.
  • Structural wood or mold larger than 10 square feet should be assessed by a professional before you attempt removal.

Cleaning Surface Mold with Vinegar

White vinegar is the gentlest option and works well on fresh mold that has not yet stained or softened the wood. Pour undiluted white vinegar (5% acidity) into a spray bottle and spray the moldy area until it is wet. Let it sit for one hour—the acetic acid kills the mold at the root. Then scrub the area with a stiff-bristled brush, working in the direction of the wood grain to avoid pushing mold spores deeper into the wood.

Wipe the area clean with a damp cloth and dry it when ready with a towel or fan. If the mold returns within a few days, vinegar alone is not strong enough and you should move to hydrogen peroxide or borax. Vinegar works best on wood that is already dry; if the wood feels damp or soft, the mold has likely penetrated and vinegar will not stop it from spreading.

Using Hydrogen Peroxide for Deeper Mold

Hydrogen peroxide (3% solution, the kind sold in drugstores) is stronger than vinegar and kills mold that has begun to stain the wood. Spray the area generously and let it sit for 10 minutes. You will see the mold begin to bubble and lighten as the peroxide breaks down the mold cells. Scrub with a stiff brush, then wipe clean and dry thoroughly.

Hydrogen peroxide can bleach dark wood, so test it on a hidden area first. If the wood is valuable or finished (stained or varnished), stick with vinegar or call a professional. Hydrogen peroxide is safe to use indoors and does not leave toxic fumes, but wear gloves because it can irritate skin with repeated exposure.

Borax Solution for Stubborn or Returning Mold

If mold keeps returning after vinegar or hydrogen peroxide, a borax solution is more durable. Mix one cup of borax powder with one gallon of water. Spray or brush the solution onto the moldy wood and let it dry completely—do not rinse it off. The borax residue prevents mold from regrowing for several months.

Borax is toxic if swallowed, so keep it away from children and pets, and wash your hands after handling. Do not use borax on wood that will be sanded or eaten from (like a cutting board or food prep surface). For those surfaces, vinegar or hydrogen peroxide is the safer choice. Borax works best in dry spaces; if the wood is damp, the mold will return even with borax present.

Sanding or Replacing Wood That Is Soft or Deeply Stained

If the mold has discolored the wood deeply or made it feel soft or spongy, cleaning alone will not save it. The mold has penetrated the wood fibers and weakened the structure. Sand the affected area with 80-grit sandpaper to remove the top layer of wood. Wear a dust mask while sanding—mold spores are released into the air. Sand until you reach clean, firm wood underneath.

If sanding does not reach clean wood, or if the soft area is large or structural, the wood section should be replaced. A joist, beam, or load-bearing stud that is compromised by mold is a safety issue. For non-structural wood like trim, shelving, or deck boards, replacement is straightforward and inexpensive compared to the cost of a mold problem that spreads.

Finding and Fixing the Moisture Source

Mold returns quickly if you do not address why the wood got wet in the first place. Look for the source: a roof leak, a plumbing leak, condensation from poor ventilation, or standing water in a basement or crawl space. Fix the leak, improve air circulation with a fan or dehumidifier, or install a sump pump if water is pooling.

In basements and crawl spaces, mold thrives in humidity above 60%. A dehumidifier or exhaust fan can bring humidity down to 50% or lower, which stops mold from growing even if the wood was previously wet. In bathrooms and kitchens, run the exhaust fan during and for 20 minutes after showers or cooking. On window frames, wipe condensation away each morning and improve ventilation to prevent moisture from collecting.

Drying the Wood After Cleaning

Mold grows in wet or damp wood, so drying is as important as cleaning. After you finish scrubbing, use a clean towel to remove as much moisture as possible. Then open windows, run a fan, or use a dehumidifier to dry the wood completely. In a basement or crawl space, this can take several days. Do not close the space up or cover the wood until it is bone-dry, or mold will return when ready.

You can check if wood is dry by touching it—it should feel cool and firm, not damp or spongy. If the wood is still damp after three days of air circulation, the moisture source has not been fixed and you need to find and stop the leak before the mold problem will resolve.

When to Call a Professional

Mold on structural wood, mold covering more than 10 square feet, or mold in HVAC ducts or insulation should be handled by a mold remediation company. These situations require containment to prevent spores from spreading through your home, and professional equipment to remove mold safely. A structural engineer may also need to assess whether the wood can be saved or must be replaced.

If you have health problems—asthma, a weakened immune system, or a mold allergy—avoid cleaning mold yourself. Exposure to mold spores can trigger serious respiratory symptoms. Let a professional handle the work while you stay in a different part of the house or leave temporarily.

Frequently Asked Questions

Can I paint over black mold instead of cleaning it?

No. Paint seals the mold underneath and traps moisture, which makes the problem worse. The mold will continue to grow and spread under the paint, and the wood will weaken. Always clean and dry the wood first, then paint if you want to protect it.

Is black mold dangerous to my health?

Black mold produces spores that can irritate your lungs, sinuses, and skin, especially if you have asthma or allergies. Some people have no reaction; others develop coughing, wheezing, or sinus congestion. Wear a mask when cleaning mold and ventilate the space well. If you develop symptoms that do not go away, see a doctor.

How long does it take for mold to come back after I clean it?

If the wood is still damp or the moisture source is not fixed, mold can return within days or weeks. If you dry the wood completely and fix the leak or ventilation problem, mold should not return. Borax treatment can slow regrowth for several months even if some moisture remains.

What is the difference between black mold and other mold colors?

Black mold is usually Stachybotrys, but many mold species are dark gray or black. Green, white, or orange mold is usually a different species. The color does not determine how dangerous it is—all mold should be cleaned and the moisture source fixed. If you are unsure what you are looking at, a professional can identify it.

Do I need to replace wood after mold is cleaned?

Not always. If the mold was surface-level and the wood is still firm after cleaning and drying, it can stay. If the wood is soft, discolored deep into the grain, or structurally important, replacement is safer and more cost-effective than trying to save it.