Mold on Concrete

Have you ever looked at your basement foundation walls and noticed something growing on them? You’re not alone. It could be mold, or it could be a harmless mineral deposit called efflorescence, and the two need very different responses.

Below we cover how to tell what it is, why mold grows on concrete at all, how to clean a small patch safely, and when it’s time to call a professional. If you’re still not sure what you’re looking at, mold on concrete is something Mold Busters can check for you.

Does mold grow on concrete?

Yes, mold can grow on concrete, but not because it eats the concrete. Concrete is a mineral material made of cement, sand, stone and water, so it gives mold nothing to feed on by itself. What mold feeds on is the thin layer of dust, dirt, soap film, wood fibres and other organic debris that settles on the surface. As the US EPA puts it, molds can grow on virtually any organic substance, as long as moisture and oxygen are present.

Moisture is the other half. Concrete is porous, so it soaks up water from the ground, from leaks and from humid air, and it can stay damp long after the surface looks dry. Basement walls and floors are also cold, so warm, humid summer air condenses on them. Give the dust on a damp slab a few days and mold has everything it needs.

Brand-new concrete is very alkaline (a pH above 12), which holds mold back for a while. Over the years the surface slowly reacts with carbon dioxide in the air and becomes less alkaline, which is one reason older basements see more mold than new ones.

What causes mold on concrete?

Mold needs three things to survive and thrive:

1. Mold needs an organic food source to grow and multiply
2. It needs the correct environmental temperature to colonize and expand
3. Finally, mold needs a damp, wet environment to survive.

Concrete doesn’t supply the food, but it is very good at supplying the water. If there is high humidity or water coming in, the concrete holds that moisture and gives mold the water source it needs.

Keep your basement dry with a dehumidifier, and aim for relative humidity between 30% and 50%, the range Health Canada recommends. If water pools outside around your foundation, you may have a drainage problem that lets water creep inside and feed the mold.

Signs of mold on concrete?

To know if you have mold growth on your concrete, start by looking closely at the growth itself. Water moving through concrete can carry dissolved minerals to the surface and leave them behind as a white, powdery or crystal-like deposit called efflorescence. It can look a lot like white mold.

Mold or efflorescence? A quick check

• Spray it with water. Efflorescence is salt, so it dissolves or fades when wet. Mold doesn’t. As the home inspectors’ association InterNACHI puts it, “Efflorescence will dissolve in water, while mold will not.”
• Check the colour. Efflorescence is almost always white, yellow or brown. If the growth is purple, pink or black, it isn’t efflorescence.
• Feel the texture (wear a glove). Efflorescence crumbles into a dry, gritty powder. Mold tends to be fuzzy, slimy or smeary.
• Look at where it is. Efflorescence forms on the concrete itself, often along cracks, mortar lines or the bottom of a wall. Mold grows wherever dust collects, and it spreads onto wood, drywall, boxes and carpet nearby.

Efflorescence isn’t alive, but it does mean water is moving through the concrete, so the moisture still needs fixing. Our full guide to mold vs efflorescence covers how to remove it and stop it coming back, and this short video shows the difference.

Still not sure? Contact Mold Busters and one of our inspectors can check and test the area for you.

If the growth is brown, green or black, it’s much more likely to be mold. Try not to disturb it more than you need to: dry-brushing or sweeping it can send spores and dust into the air and spread the problem to other parts of the home. Read the cleaning steps below first.

Did you know?

Chaetomium is the 2nd common toxic mold type found in homes we tested?! Find out more exciting mold stats and facts inside our mold statistics page.

What does mold on concrete look like?

Mold on concrete can look like any number of different things. It depends on what classification of mold is growing on the surface of your concrete. It could be white, black or green.

Mold can be harmless or harmful regardless of color, so you should have a professional from Mold Busters visit your property for an expert inspection.

Dark spots on concrete aren’t always mold. Dirt, rust, soot and water staining can look similar. A mold colony usually has a fuzzy or velvety texture and spreads outward, often in roughly circular patches.

Outdoors, on patios, steps and shaded walls, green growth is more often algae, moss or mildew. That’s a different problem with a simpler fix, covered in the green mold section below.

Understanding concrete and mold Growth

Concrete, a mixture of cement, water, and aggregate, might seem impervious to something like mold. However, its very nature can make it susceptible. Here’s a breakdown of common locations and the reasons mold might thrive there.

Cement mold & can mold grow on cement?

Yes. Cement is the binder in concrete, and the same rules apply: cement itself isn’t food for mold, but cement floors, walls and blocks hold moisture and collect dust, and that combination is enough. Unsealed cement and concrete absorb water more readily than sealed surfaces, so they stay damp longer.

Concrete slabs and mold

A concrete slab sitting on soil can draw ground moisture up through it, especially in older homes built without a vapour barrier underneath. Outdoor slabs like patios and walkways stay damp when they’re shaded or don’t drain well. Indoors, anything laid on the slab, like carpet, rugs, flooring or cardboard boxes, traps that moisture against the concrete.

Common places for mold growth

Mold doesn't discriminate. Whether it's a floor, wall, or basement, if the conditions are right, mold will find its way.

Mold on concrete floor

Basement floors are the most common spot. Mold tends to start where dust and moisture collect: along the edges of the floor, under and around stored items, near the furnace and laundry area, and anywhere water has pooled. Cardboard boxes are a classic culprit, and Health Canada advises never placing them directly on the basement floor. Use plastic bins on shelves instead. In a garage, white growth on the floor is often efflorescence or road-salt residue rather than mold; our guide to mold in garages covers garage floors in detail.

Mold under carpet or flooring on a concrete slab

Carpet, underpad, laminate and vinyl laid straight onto a basement slab can hide mold for years. Moisture rising through the concrete gets trapped under the flooring, and the dust and fibres underneath give mold something to feed on. A musty smell, dark staining at the edges, or flooring that feels damp or starts to lift are the usual clues.

To check whether moisture is coming up through the slab, tape a square of clear plastic sheet (about 60 cm by 60 cm) tightly to the bare concrete and leave it for a day or two. Droplets under the plastic mean moisture is coming through the slab; droplets on top mean the room air is too humid. Health Canada suggests considering removing carpets from the basement floor, and says mouldy carpets and underpads should be cut out and discarded, with the surface underneath cleaned and dried. More on this in our guide to mold in carpet.

Mold on concrete wall

Poured concrete walls get damp from the soil outside and from condensation inside. In a finished basement, the drywall, wood framing and insulation in front of the wall are where mold usually does the real damage, because those materials are food for it. If the wall is painted or coated with a waterproofing paint, mold can grow on the paint film, and the coating can blister or peel when moisture keeps pushing through from behind.

Mold on basement walls

Basement walls are in contact with damp soil all year. Concrete block (cinder block) walls are the most prone: the hollow cores fill with moisture and wick it up the wall, so you’ll often see efflorescence along the mortar lines as well as mold where dust collects. Good grading around the house, working eavestroughs and downspouts, a sump pump or an interior footing drain all help keep water out. If the wall stays wet no matter what you do, that’s a job for a waterproofing contractor.

Types of mold on concrete

Here’s how the common colours of growth on concrete compare, and what each one usually tells you.

Black mold on concrete

Dark spots or patches are what most people mean by black mold. The species with the scariest reputation, Stachybotrys chartarum, needs cellulose (paper, wood, drywall) to grow, so on bare concrete, dark growth is usually one of the common household molds, or dirt and staining. Black growth on a basement wall is still worth taking seriously, because it means moisture, and the drywall or wood next to it may be affected too. Our black mold guide has more on identifying it.

Is black mold on concrete dangerous?

Any mold growing indoors is worth removing, whatever its colour, and colour alone doesn’t tell you which species it is. Some people are more sensitive to mold than others, including people with mold allergies. If you think your health might be affected, Health Canada advises talking to your health care professional as soon as possible. For the mold itself, follow the cleaning steps below or call us for an assessment.

White mold on concrete

White mold on concrete is usually fuzzy or cottony, and it often shows up on basement floors under or around stored items, or at the base of walls. Because efflorescence is also white, do the water test in the signs section first. Mold is still fuzzy once the spot dries; efflorescence dissolves and comes back as a dry, crusty powder. Our white mold guide covers it in more depth.

Green mold on concrete

Indoors, green growth on concrete is usually mold, and it’s handled the same way as any other colour. Outdoors it’s a different story. Green on patios, steps, retaining walls and the shaded side of a foundation is most often algae or moss, sometimes with mildew mixed in. All of them need the same things: shade, moisture and something to cling to.

The fix for outdoor growth is to make the surface drier. Trim back plants and branches that shade it, point downspouts away from it, fix low spots where water sits, and scrub or gently pressure-wash the surface on a dry day so it dries quickly afterwards. Outdoor algae is mostly a slip hazard and a cosmetic problem. If green growth shows up inside, on basement walls or floors, treat it as mold.

Pink or orange growth on damp concrete, especially in showers and around floor drains, is often bacteria rather than mold. See our guide to pink mold.

Is mold on concrete dangerous?

Mold on concrete is worth dealing with, but the concrete itself is rarely the real worry. Any mold growing indoors should be cleaned up and its moisture source fixed. If you think your health might be affected, Health Canada advises talking to your health care professional as soon as possible.

Some people are more sensitive to mold than others; our guide to the health effects of mold explains more. For your home, mold on concrete usually means three things:

• There’s a moisture problem. Water is getting in, condensing or not drying out. Fix that and the mold stops; skip it and the mold comes back after every clean.
• Nearby materials may be affected. Concrete doesn’t rot, but the drywall, wood framing, carpet, insulation and stored belongings next to it can, and those are much harder to clean.
• It can spread through the house. Spores and musty odours from a damp basement travel to the rest of the home, especially through a forced-air system.

If the growth is more than a few small patches, keeps coming back, or you can smell mold but can’t find it, Mold Busters can inspect the space, find the moisture source and remove the mold with proper containment.

How to identify mold on concrete?

If you think you have mold growing on your concrete surfaces, check a few things to see whether it’s really mold:

Colour. White, black, brown or green growth could be a mold colony. White could also be efflorescence, so do the water test.

Shape and texture. Fuzzy, velvety or slimy growth in roughly circular patches, or growth climbing from the floor up the wall, points to mold. Crusty, crystal-like deposits along cracks and mortar lines point to efflorescence.

Smell. A musty, earthy smell in the basement is often the first sign of hidden mold under flooring, behind finished walls or inside stored boxes.

If you see stains or growth on your concrete and still aren’t sure what it is, contact a professional at Mold Busters to confirm whether it’s mold. If you’re in the Montreal or Ottawa area, our teams are ready to help with mold inspection and removal.

How to prevent mold on concrete?

As mentioned earlier, mold needs three conditions to thrive:

1. Water source
2. Food source, and
3. Proper temperature.

If you remove one or more of those, you stop the mold from growing.

Prevention is always better than cure. Here’s a table detailing various preventive and treatment measures for mold on concrete, along with their effectiveness.

To keep your basement safe, you can do some easy steps to ensure your home is mold-free.  First, make sure you have a dehumidifier and it is operating correctly.  A dry basement is a healthy basement.  Keep the temperature a little warmer in the basement since relative humidity (RH) decreases as temperature increases.

Keep your basement clean and clear of unnecessary organic materials like dirt, old newspapers and cardboard. Vacuum or wipe the floor regularly so dust doesn’t build up into mold food, and store the things you keep in sealed plastic bins, up off the floor.

Additional tips

• Fungal growth: Apart from mold, other types of fungal growth can appear on concrete. It’s essential to identify the type of growth correctly to treat it effectively.
• Excess moisture: Excess moisture is the primary cause of mold growth. Ensure proper drainage, fix leaks, and use tools like sump pumps and French drains to manage water.
• Poor ventilation: Areas with poor ventilation, like basements, are more prone to mold growth. Ensure adequate airflow and consider using fans or dehumidifiers.
• Sealing: Once the concrete is clean and has dried completely, a penetrating concrete sealer reduces how much moisture the surface soaks up. Seal only after the water problem is fixed. A sealer or paint over concrete that is still pushing moisture through will blister, peel and trap the problem underneath.

How to remove mold on concrete?

It depends on how much there is. Health Canada says you can generally clean small and medium sized areas of mould yourself, and recommends professional help for extensive growth. It sorts mold into three sizes:

• Small: 1 to 3 patches, each smaller than 1 square metre (about 10 square feet). You can clean it yourself.
• Medium: more than 3 patches, or patches bigger than 1 square metre but smaller than 3 square metres (about 32 square feet). Usually still a do-it-yourself job with proper protection, though a professional assessment is a good idea.
• Extensive: any single patch larger than 3 square metres. Get a professional.

Call a professional too if the mold keeps coming back after you clean it, if it’s hidden behind finished walls or under flooring, if the area was soaked by a flood or sewage backup, or if anyone in the home has health concerns.

How to clean a small patch of mold off concrete

1. Find and fix the moisture first. A leak, a drainage problem, condensation or high humidity. Otherwise the mold will be back.
2. Protect yourself. Wear rubber gloves, safety goggles and an N95 mask, and keep children, pets and anyone sensitive to mold out of the area.
3. Clear the area. Move stored items away, and throw out mouldy cardboard, rugs and other porous things that can’t be cleaned. Bag them in plastic before carrying them through the house.
4. Scrub with soap and warm water. For concrete, Health Canada says to scrub the surface using a brush with an unscented soap mixed with warm water. A stiff nylon brush works well on rough concrete. Don’t dry-brush or sweep the mold first.
5. Rinse and dry fast. Wipe or rinse with clean water, then dry the area completely with fans and a dehumidifier. Concrete holds water, so give it longer than you think.
6. Keep an eye on it. Mold that comes back after cleaning usually means the moisture source hasn’t been fixed. That’s the time to call a professional.

You don’t need bleach. Health Canada advises against using it to clean up mould, and the US EPA says bleach is not recommended as a routine practice during mold cleanup. On porous concrete it can lighten the stain without removing the growth. Never mix bleach with ammonia or acidic cleaners: the combination gives off dangerous gases.

When it’s a job for professionals

For larger areas, professional removal works differently. The work area is closed off with containment so spores don’t travel to other parts of the home, technicians wear full personal protective equipment, materials that can’t be cleaned (like moldy drywall and carpet) are removed, and the concrete is cleaned and HEPA-vacuumed. Every Mold Busters remediation project ends with clearance testing, lab air sampling or on-site swab tests depending on the job, included in your quote.

If your mold problem is bigger than a small patch, Mold Busters can find the moisture source and remediate the mold growing in your home. If you’d like an expert opinion first, we also offer a free virtual mold inspection service to help identify potential mold issues.

Frequently asked questions

Will bleach or vinegar kill mold on concrete?

They can kill some mold on the surface, but killing it isn’t the goal. The mold has to be cleaned off, and the moisture that fed it has to be fixed. The US EPA puts it plainly: “It is necessary to clean up mold contamination, not just to kill the mold.” On concrete, bleach isn’t recommended (Health Canada advises against using it for mould cleanup), and vinegar doesn’t clean a porous surface any better than soap and water.

Scrubbing with unscented soap and warm water, then drying the concrete thoroughly, is what Health Canada recommends for concrete surfaces. For larger areas, or mold that keeps returning, a Mold Busters professional team can remove it with proper containment.

Should you paint over mold on concrete?

Painting over mold is never a good idea on any surface. Paint doesn’t remove the mold or fix the moisture that caused it, so the growth comes back through or under the paint, often bigger than before. Clean the mold off, fix the moisture, let the concrete dry, and only then paint or seal.

How can I prevent mold growth on concrete surfaces?

To prevent mold growth on concrete surfaces, it’s essential to control excess moisture. Ensure proper ventilation, especially in basements, use dehumidifiers, and address any water leaks promptly. Regularly inspecting outdoor concrete, like patios, for water pooling and ensuring good drainage can also help.

Is wet concrete more susceptible to mold growth?

Yes, wet concrete can be more susceptible to mold growth. Concrete is porous, and when it remains wet or damp, it can provide the ideal conditions for mold to thrive, especially if there’s organic material present. It’s crucial to let concrete slabs dry thoroughly after construction or cleaning to prevent mold.

Can mold-resistant paint help in preventing mold on concrete surfaces?

It can help on surfaces that see occasional dampness, like basement walls with seasonal humidity, because it makes it harder for mold to take hold on the paint film. It won’t stop water coming through the wall, though, and paint over concrete that’s still damp will blister and peel. Clean off any existing mold and fix the moisture first.

What’s the best way to clean mold from a concrete surface?

Scrub it with a stiff brush and unscented soap or dish detergent mixed with warm water, rinse, and dry the area completely. That’s the method Health Canada gives for concrete surfaces, and it doesn’t call for bleach. Wear rubber gloves, goggles and an N95 mask while you work. For anything more than a few small patches, or mold that keeps coming back, consider professional mold removal services.

Are there health hazards associated with mold on concrete?

Mold growing indoors, on concrete or anywhere else, can affect indoor air, and some people are more sensitive to it than others. Our guide to the health effects of mold explains more. If you think your health might be affected, Health Canada advises talking to your health care professional as soon as possible. For the mold itself, clean small areas as described above and call a professional for anything larger.

Sources

• Health Canada: Reduce humidity, moisture and mould
• Health Canada: Addressing moisture and mould in your home
• US EPA: A Brief Guide to Mold, Moisture and Your Home
• InterNACHI: Efflorescence

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Published: August 9, 2021 Updated: October 23, 2026

Written by:
Steven Adams
AMRT, WRT, ASD
Mold Busters

Fact checked by:
Michael Golubev
CEO
Mold Busters