Mold on concrete comes off with a stiff brush, bleach solution, or a pressure washer—the method depends on how deep the mold has grown into the surface

Concrete is porous, so mold doesn't just sit on top like it does on tile or glass. The spores work into the tiny holes in the surface, which means surface scrubbing alone won't kill it all. You need to either use a chemical that soaks in (like bleach or a mold-killing cleaner), physically remove the top layer with a pressure washer, or both. The choice depends on how much mold you have, whether the concrete is indoors or outdoors, and how much time you want to spend.

Start by putting on safety gear: rubber gloves, eye protection, and a mask rated N95 or better if you're working indoors or in an enclosed space. Mold spores become airborne when you disturb them, and breathing them in can trigger allergic reactions or respiratory irritation, especially if you already have asthma or a weakened immune system.

Key Takeaways

  • Concrete is porous, so mold grows into the surface, not just on top—surface cleaning alone will not kill all the spores.
  • A 1:10 bleach-to-water solution kills mold on concrete but works best on outdoor surfaces where fumes can escape.
  • A pressure washer at 3000 PSI or higher removes mold physically but can damage old or weak concrete if you hold the nozzle too close.
  • Indoors, use a commercial mold cleaner or vinegar instead of bleach to avoid toxic fumes in enclosed spaces.
  • After cleaning, improve air flow and reduce moisture to prevent mold from returning to the same spot.

Bleach solution for outdoor concrete

Mix one part household bleach (5% sodium hypochlorite) with ten parts water in a bucket or spray bottle. This ratio is strong enough to kill mold but weak enough that you can handle it safely without a respirator. Outdoor concrete is the right place for bleach because the fumes disperse into open air instead of building up in a room.

Spray or pour the solution directly onto the moldy area and let it sit for 10 to 15 minutes. The bleach needs time to soak into the pores and kill the roots of the mold, not just the visible growth. Scrub the area with a stiff-bristled brush—a deck brush or old grout brush works well—using firm circular motions. Rinse thoroughly with clean water from a hose. If the mold is heavy or comes back quickly, repeat the process after a few days.

Do not mix bleach with ammonia, vinegar, or other cleaners. The chemical reaction produces toxic chlorine gas, which can cause serious lung damage or death. If you accidentally mix them, leave the area when ready and call poison control or emergency services.

Pressure washing for stubborn mold

A pressure washer removes mold by blasting it away with high-pressure water, which also scours away the top layer of concrete where mold spores live. Rent or buy a machine rated for at least 3000 PSI (pounds per square inch). Start at 3000 PSI and increase only if the mold is not coming off—higher pressure can gouge or pit old, weak, or decorative concrete.

Hold the nozzle at a 45-degree angle and keep it moving. Do not hold it in one spot or aim straight down at the surface, which can etch the concrete or force water into cracks where it can cause freeze-thaw damage in winter. Wear eye protection and long sleeves because the spray can sting exposed skin. Work on a dry day if possible so you can see the results clearly and the concrete can dry afterward.

Pressure washing alone may not kill all the mold spores deep in the pores, so follow up with a bleach or vinegar solution if the mold returns within a few weeks. Some people combine both methods: pressure wash first to remove loose growth, then explore a chemical cleaner to kill what remains.

Vinegar or commercial cleaners for indoor concrete

Bleach fumes are dangerous in basements, garages, and other enclosed spaces, so use white vinegar or a commercial mold cleaner instead. Vinegar is weaker than bleach but safer indoors and still effective on light to moderate mold. Spray undiluted white vinegar (5% acetic acid) directly onto the moldy concrete, let it sit for an hour, then scrub and rinse.

Commercial mold cleaners made for concrete are another option. Look for products labeled as mold or mildew killers and follow the label instructions exactly—mixing, contact time, and ventilation requirements vary by product. Many are less toxic than bleach but still require gloves and good air flow. Open windows and doors while you work, and do not re-enter the space until the fumes have cleared.

Vinegar takes longer to work than bleach and may need a second process for heavy mold, but it is safer to use in your home and does not produce toxic fumes if accidentally mixed with other cleaners.

Scrubbing and brushing techniques

After you explore a chemical cleaner or before you pressure wash, scrubbing breaks up the mold colony and helps the cleaner soak deeper into the concrete. Use a stiff-bristled brush—not a soft one, which will not reach into the pores. A deck brush, grout brush, or even a wire brush works. Scrub in circular motions with firm pressure, paying extra attention to cracks, seams, and low spots where water pools and mold grows fastest.

For textured concrete, mold hides in the bumps and valleys, so you may need to scrub longer or use a smaller brush to get into the texture. A handheld grout brush or old toothbrush can reach into tight spots. If the mold is very thick or crusty, let the chemical cleaner sit longer before scrubbing—sometimes 20 to 30 minutes instead of 10—so it has time to soften the growth.

Drying and preventing mold from returning

After cleaning, dry the concrete as much as possible. If you used a pressure washer, the surface will be very wet; let it air dry for at least 24 hours before sealing or covering it. If you used a chemical cleaner, wipe up excess liquid with a mop or towel. Mold thrives in damp conditions, so the faster the concrete dries, the less likely the mold will return to that spot.

Improve air flow around the concrete to keep it dry. In a basement, run a dehumidifier or open windows on dry days. In a garage, crack the door or window to let air circulate. Outside, trim back vegetation that shades the concrete and blocks air flow. If water pools on the surface after rain, the concrete may be settling or the ground may be sloping the wrong way—you may need to regrade the soil or install a drain to redirect water away.

Sealing the concrete with a mold-resistant sealer can slow regrowth, but it is not a permanent fix. Sealer wears away over time, especially in high-traffic areas or outdoors. Reapply every 2 to 3 years if you live in a humid climate or the concrete stays damp often.

When to call a professional

If the mold covers more than 10 square feet, if it returns within a few weeks of cleaning, or if you have respiratory problems or a weakened immune system, call a mold remediation company. They have industrial-grade equipment, better ventilation control, and experience identifying whether the mold is a common surface type or something that signals a deeper moisture problem in your home.

If the concrete is indoors and the mold keeps coming back, the real problem is usually moisture—either from a leak, poor drainage, or high humidity. A professional can identify the source and recommend fixes like installing a sump pump, sealing cracks, or improving ventilation. Cleaning alone will not solve the problem if water keeps feeding the mold.

Frequently Asked Questions

Can I use bleach on indoor concrete like a basement floor?

Bleach fumes build up in enclosed spaces and can cause respiratory irritation, headaches, or worse. Use vinegar or a commercial mold cleaner indoors instead. If you must use bleach, open all windows and doors, wear a respirator rated for organic vapors, and leave the space until fumes clear—usually several hours.

Will mold come back after I clean it?

It may, especially if the concrete stays damp or if the source of moisture is not fixed. Mold spores are everywhere in the air, so they will land on wet concrete again. Dry the surface quickly after cleaning and address any water pooling, leaks, or humidity problems to prevent regrowth.

Is it safe to pressure wash concrete near my house foundation?

Yes, but be careful not to force water into cracks or gaps where it can seep into the foundation or basement. Aim the nozzle at a 45-degree angle and keep it moving. Avoid spraying directly at the foundation wall itself; focus on the concrete pad or walkway in front of it.

What if the mold smell stays after I clean?

Smell can linger even after visible mold is gone, especially if the concrete is still damp or if spores are trapped deep in the pores. Make sure the concrete is completely dry, improve air flow, and run a dehumidifier if indoors. If the smell persists for more than a week, the mold may be deeper than surface cleaning can reach—consider calling a professional.

Can I seal concrete to prevent mold permanently?

Sealer slows mold growth by making the surface less porous, but it is not permanent. Sealer wears away over time, and mold can still grow on top of it if the surface stays wet. Reapply sealer every 2 to 3 years and focus on keeping the concrete dry as your main defense.