12 Signs of Mold Under Hardwood Floors: A Case Study 2023

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Written by: Mohammad Sameer

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Finding mold under your nice hardwood floors can be scary. But don’t freak out, mold happens a lot and you can fix it if you stay calm.

In this article, I’ll tell you the signs of mold growing under hardwood floors, so you know what to watch for. From dark spots to bad smells, I’ll cover the top clues that mean you have a mold problem.

You’ll also learn when you need to call a professional to check it out. My goal is to teach and empower you, so you see a mold issue early and take action. Hardwood flooring costs a lot, don’t let mold ruin it.

Read on to become a mold detector and protect your floors!

Why is mold harmful to health?

Mold can be harmful to health because it releases spores and mycotoxins into the air which can cause allergic reactions and respiratory issues when inhaled. Mold exposure is especially dangerous for people with asthma, allergies, or compromised immune systems.

The toxins released by mold can also affect the nervous system, lungs, and skin. Some of the most common health effects of mold exposure include coughing, wheezing, eye and skin irritation, runny nose, headaches, and general flu-like symptoms.

To avoid the harmful effects of mold, it is important to control moisture and humidity levels in your home to prevent mold growth and remove any existing mold properly. Speak to a doctor if you are concerned about the effects of mold exposure on your health.

What types of molds grow on hardwood floors?

The most common molds that grow on hardwood floors are:

  • Cladosporium – A greenish-black mold that grows on wet wood. One of the most common molds inside.
  • Penicillium – A blue or green mold that grows well in damp places. Can cause allergic reactions.
  • Aspergillus – Different coloured molds that release toxins. Can cause big health issues for people with asthma or weak immune systems.
  • Alternaria – A black mold that grows on floors damaged by too much moisture. Releases spores easily into the air.
  • Fusarium – A white or pink mold that grows well on wet wood. Harder to kill with bleach cleaners than other molds.
  • Chaetomium – A greyish-brown mold that grows where it is very damp and water-damaged. Makes spores that can infect the lungs.
  • Stachybotrys – A toxic black mold. Can cause serious health problems in babies and kids.

Keeping home humidity under 50% helps stop mold from growing on floors. Cleaning water damage fast and drying floors quickly also limits mold growth.

Signs of Mold Under Hardwood Floors: A Case Study

Hardwood floors can be susceptible to mold growth if moisture seeps underneath. As a homeowner, it’s important to detect signs of mold early before it becomes a bigger problem.

I’ll share my personal experience dealing with mold under my hardwood floors, including how I identified it and the remediation process.

Noticing the First Signs

It started with a musty smell in our home office. At first, I assumed it was coming from old books or furniture. But the smell persisted. That’s when I noticed the first sign of moisture – a dark stain on the hardwood floor behind a bookcase.

Removing the bookcase, I saw a larger dark patch. Kneeling down, I could smell mildew. The walls and baseboards also showed water damage. I realized this was likely a mold issue underneath the floors.

Confirming with Mold Testing

To confirm my suspicion, I used a home mold test kit. This included air sampling and surface swabs of the floor. I sent the samples to a lab for analysis. The results showed elevated levels of mold, specifically Aspergillus and Penicillium.

This scientific evidence proved that unseen mold was growing under the floors. I recommend testing because mold symptoms can be non-specific. Testing provides objective data to present to a remediation company.

Hiring Professionals for Remediation

Once I had confirmation of the mold problem, I called a certified mold remediation company. They used special tools like a borescope to inspect underneath the floors.

The remediation process involved:

  • Removing sections of damaged hardwood flooring
  • Killing mold with antimicrobial agents
  • Addressing the moisture source
  • Installing new janitorial flooring

In the end, the floors looked good as new. The musty smell also disappeared after remediation. My family is breathing easier now without mold spores in the home.

Takeaways for Homeowners

This personal experience taught me the importance of acting quickly when there are signs of moisture under hardwood floors. Mold can grow rapidly. If you notice suspicious smells, stains, or moisture, begin testing right away.

Professional remediation is also vital for health. Mold can worsen respiratory issues like asthma. With early intervention, you can stop mold growth in its tracks and prevent it from spreading. Acting at the first signs of moisture is key.

12 signs of mold under hardwood floors

12 signs of mold under hardwood floors
image source: affordableremediation.com

Finding mold under your nice hardwood floors can be scary. But don’t freak out, mold happens a lot and you can fix it if you stay calm. In this longer article, I’ll go really in-depth on the signs of mold growing under hardwood floors, so you can catch it early.

A musty odour in the room

A musty odour in the room
image source: depositphotos.com

One of the biggest giveaways you have a mold problem is a stale, musty smell in the room with the wood floors. Mold and mildew produce a distinct earthy, dank odor that you’ll notice right when entering the space. Don’t ignore or try to mask the smell! Follow your nose to pinpoint where the mold might be lurking under the floors so you can address it.

Discolored floorboards

Inspect your floorboards closely. Mold growth often appears as dark spots and black or brown stains on the wood. It can also show up as light-colored splotches of white, gray, or yellow discoloration. These color changes indicate moisture has gotten underneath and mold is growing under the floorboards out of sight.

Standing water

One of the most obvious tip-offs to a mold problem is seeing standing water on your floors. Water sitting on the surface or trapped underneath feeds mold growth. Watch for water pooling in dips or gaps in the boards after spills, leaks, flooding or high humidity. Even small amounts allow mold to thrive.

Water stains

Along with standing water, keep an eye out for water stains on your hardwood planks. Water damage often leaves behind dark, discoloured splotches and streaks on the wood.

These stains happen when moisture seeps through unfinished cracks or penetrates sealed boards over time. If you spot them, investigate where the water is coming from and how it’s getting under the floors. Address leaks promptly and dry any puddles to avoid mold.

Texture changes in the wood

Inspect the texture of your floorboards closely. Mold often causes uneven ripples, pits, bumps or distortions in the wood’s surface. You may see wavy patterns or raised areas forming. Mold eating the subfloor underneath can create these subtle changes visible on top.

Also, watch for tiny punctures or holes where mold has softened and eaten into the grain. In severe cases, very spongy spots can form where mold has decayed sections entirely. The floors may feel rough or gritty.

Catching texture issues early allows you to address mold before it penetrates deeper. Be on the lookout for other signs too:

  • Strong musty, earthy odours come from mold growth. Follow your nose.
  • Discolouration like dark stains, spots or light splotches indicating moisture issues.
  • Soft areas where moisture has degraded and rotted the subfloor.
  • Planks detach as mold deteriorates the structural integrity.

Warped floorboards

Warped floorboards
image source: hudsonfloorgroup.com

Warped and twisted boards clearly point to a mold problem. Excess moisture under the floor causes the wood to swell, expand, and distort out of shape. Mold-damaged planks can warp enough to loosen and lift from the subfloor.

Also, look for curling at board edges and buckling where planks detach. Warped floors have a very uneven, wavy appearance. Dips, ridges or sloping spots form as boards twist. Wider planks show the most dramatic warping.

You’ll also notice dark mold spots and soft areas where moisture has degraded the subfloor. Significant warping creates tripping hazards. Address it promptly before the boards crack apart.

Don’t attempt removal until inspectors check for hidden mold. Their drying equipment can eliminate moisture warping the boards. Minor warping can often be sanded and refinished if caught early.

Cupped floorboards

Cupped boards – higher at the edges than the middle – clearly indicate mold or excess moisture underneath. This happens when the bottom of the floor swells more than the top from asymmetric moisture exposure.

Check for cupping by sighting down the floor’s surface and looking for gaps of light under the curled edges. Also, run your hand across to feel for dipped centres. Wider, thinner boards cup the most.

Along with the cupped shape, you may notice dark mold colonies and detached, loose boards as the subfloor fails. Address cupping fast before the boards crack or break.

Crowning floorboards

Crowning is when the middle of your hardwood floorboards begin to rise up higher than the edges. It causes a convex bulge to form across the width of the planks. This is one of the more subtle signs of moisture or mold issues starting underneath.

As mold grows and feeds on the subfloor, it causes humps and ridges to form under the flooring above. This forces the middle of the boards to gently push up and crown. The slight arching gets more pronounced over time as the moisture damage worsens.

Along with crowning, look for related signs like warping, curling, cupping, soft spots, or detachment where mold is degrading the wood’s integrity. Dark stains or discolouration may indicate concentrated mold colonies.

Insects infestations

Another warning sign for mold is the presence of wood-eating insects like termites, carpenter ants, or powderpost beetles. An infestation points to excessive moisture allowing these pests to thrive and breed under your floors.

Where there’s moisture, there is often mold as well. Insects chew through soft, moist wood weakened by fungus and rot. Watch for tiny holes in the floors or sawdust-like frass in the gaps. Getting rid of excess moisture helps eliminate the insects along with the mold.

Allergy symptoms

If family members start sniffling and sneezing more when in the room with your wood floors, it could point to mold growth under the surface. Mold releases spores and gases into the air that provoke allergy and asthma symptoms.

Pay attention if your symptoms worsen specifically in the area with the floors, then improve after leaving the room. This is a sign you may be reacting to mold. Children and those with respiratory issues are most susceptible.

Increased Humidity Levels

One of the first warnings of a potential mold issue is high indoor humidity. Mold thrives when moisture levels are consistently above 50%. Use a hygrometer to monitor humidity near your wood floors.

If humidity stays elevated, condensation forms on windows and surfaces. This damp environment allows mold colonies to grow quickly in dark, enclosed spaces under the floors. Ventilate and run dehumidifiers to keep your home’s moisture under control.

Spongy Flooring

Along with high humidity, spongy or soft spots in your flooring indicate mold damage. Press down carefully with your foot and feel for areas that seem abnormally soft or sink down. This happens when moisture rots and weakens the wood and subfloor underneath.

Advanced mold decay causes sections of flooring to feel almost hollow or springy when stepped on. The surface may also look warped or uneven. Soft areas often correlate with musty smells from mold growth.

How to Remove Mold under Hardwood Floors?

Finding mold growing under your beautiful hardwood floors can be worrying. If addressed promptly, you can often remove minor mold and prevent major damage. This comprehensive guide covers safe, effective methods for DIY mold removal under hardwood floors.

Using Vinegar to Remove Mold

For small areas of mold growth, you can use undiluted white vinegar to clean and disinfect. Vinegar is a natural antibacterial that kills mold spores. Dip a soft cloth or scrub brush in vinegar and scrub affected areas of the floor and subfloor underneath.

Allow the vinegar solution to sit for 10-15 minutes to fully penetrate and neutralize the mold. The acidity in vinegar destroys the mold at the roots. Repeat as needed for stubborn growth.

Ventilate the area well during and after use. Vinegar can help eliminate musty odours from mold as it kills fungi. Its natural formula is safe for you and most surfaces.

Use Bleach to Kill Mold

For disinfecting and removing mold on your hardwood floors, bleach is very effective. Choose an EPA-registered bleach product specifically labelled for killing mold. Follow all label instructions for concentration and safe use.

Put on protective gloves, goggles and a face mask. Scrub bleach solution onto affected floorboards and down into subfloor cracks with a stiff brush. Let it soak 5-10 minutes then rinse clean with water. Repeat weekly to prevent mold from recurring.

Bleach can damage wood finishes, so spot test first. Rinse surfaces well. Ensure proper ventilation while using bleach and avoid mixing it with other cleaners.

Use Mold Remover Products

Look for dedicated mold remover products made for wood, drywall and other porous surfaces. These usually contain bleach along with detergents formulated to cut through mold and lift stains.

Apply to affected areas by spraying or scrubbing. Let the remover penetrate for 10-15 minutes before wiping away. Rinse thoroughly. HEPA vacuuming after application cleans deep into cracks. Mold removers help prevent future outbreaks.

Is mold on hardwood floors dangerous?

Yes, mold growth on or under hardwood floors can be dangerous to health, especially for those with allergies or asthma. Mold releases spores and gases that trigger allergic reactions when inhaled. It can also damage the structural integrity of the wood over time.

Certain toxic varieties like black mold are severely hazardous if inhaled or touched. Extensive mold growth hidden under flooring and subfloors can release high volumes of spores into living spaces.

That’s why it’s critical to take action at the first signs of mold. Identify and fix moisture issues promptly. Address small growths quickly before mold spreads and becomes a health issue.

What causes mold on hardwood floors?

Excessive moisture is the root cause of mold growing on or under hardwoods. Common moisture sources include:

  • Flooding, leaks, high humidity, condensation
  • Spills, pets, wet shoes on the floors
  • Improper installation over concrete without vapour barriers

Mold needs consistent dampness to thrive. Wet conditions allow mold spores floating in the air to settle and grow on wood. Hardwoods are porous and prone to absorbing ambient moisture.

Preventing indoor humidity over 50% is key. Address any leaks and spills immediately. Ventilate and use dehumidifiers to control moisture.

Can mold grow under hardwood floors?

Yes, it’s very common for mold growth to occur under hardwood flooring, even if you don’t see the signs above. The dark, enclosed area under the floor is ideal for mold. Slow leaks or high humidity provide enough moisture for extensive colonies to grow unseen.

The subfloor becomes damp and mold takes hold. Spores release gases that cause stains or discolouration visible on floorboard surfaces. Warping and soft subfloors are signs of advanced infestation.

If you suspect hidden mold under your floors, hire a professional mold inspector. Special cameras detect mold you can’t see just by looking at the floor’s surface. Identifying and addressing underlying mold prevents serious damage.

How To Remove Mold On Wood Furniture

For small mold spots on finished wood furniture, mix 1/4 cup baking soda and 1/4 cup water into a light scrub. Let bubble on the mold for 15 minutes before wiping clean.

On unfinished wood, lightly sand the affected area with 150 grit sandpaper to remove the moldy top layer. Wipe with hydrogen peroxide, vinegar or a wood-safe mold killer. Consider calling a professional refinisher for serious mold.

How Do You Prevent Mold On Wood?

The key to preventing mold on wood is controlling moisture:

  • Maintain indoor humidity under 50%. Use dehumidifiers if needed.
  • Address any moisture sources like leaks or floods quickly.
  • Allow airflow around wood materials and furnishings.
  • Clean spills promptly.
  • Ensure new wood dries fully before sealing or installing.
  • Use vapour barriers under wood flooring on cement.
  • Apply mold-resistant coatings to unfinished wood.

With vigilance against moisture, you can prevent mold damage and keep your wood surfaces pristine!

How To Test For Moisture In Wood

Excess moisture is the root cause of mold growth on wood. To check for dampness issues:

  • Use a moisture meter to scan wood. Look for readings above 15% which indicate excessive moisture.
  • Check for condensation on windows and surfaces which raises humidity and allows mold growth.
  • Look for dark water stains or cupping on wood floors signaling moisture damage.
  • Feel for cold, damp areas on wood materials or furnishings.

Controlling indoor humidity is crucial to prevent mold on wood. Address any leaks or floods promptly.

How To Test For Mold On Wood

Signs of mold include:

  • Musty, earthy odor – use your nose to pinpoint moldy areas.
  • Allergy symptoms like coughing and irritated eyes when around the wood.
  • Visible mold growth like fuzzy or slimy spots, often green, black or white.
  • Discoloration such as dark stains or lighter splotches on the surface.
  • Wood decay like soft, crumbly or warped sections.

Calling in a professional mold inspector is recommended if you suspect extensive mold inside wood materials or under flooring.

What Kills Mold On Wood?

What Kills Mold On Wood?
image source: istockphoto.com

For minor mold on exterior wood surfaces:

  • Scrub with a diluted bleach solution and rinse well after 10 minutes. Allow to fully dry.
  • Apply Concrobium Mold Control, a non-toxic enzyme cleaner that kills and prevents mold.
  • Use hydrogen peroxide or vinegar solutions to kill mold naturally.
  • Sand mold-damaged exterior wood lightly then treat with mold killer.

Heavily mold-infested wood may need replacement. Always fix any moisture issues to prevent recurrence. With prompt action, you can often save moldy wood!

Why Does Mold Grow On Wood?

Finding mold growing on your beautiful wood floors, furniture, or building materials can be worrying. Understanding exactly why mold readily colonizes wood can help you prevent and control infestations.

Mold needs three key elements to grow – moisture, nutrients, and a suitable surface. Wood provides ideal conditions for mold.


The main reason mold thrives on wood is its porous, absorbent structure. Wood readily absorbs ambient moisture and retains it. High humidity levels in the air condense on wood surfaces allowing mold colonies to establish and spread.

Small leaks, floods, and damp basements create moisture reservoirs that feed extensive mold growth on wood over time. Even gradual seepage of ground moisture through the concrete into wood framing or floors provides enough dampness for mold.


Wood contains many nutrients that mold needs to flourish, like cellulose, lignin and natural sugars. Hardwoods with higher starch content, such as oak and elm, provide extra nutrition for molds and fungi.

Mold enzymatically breaks down and digests components of the wood cell walls as food sources. This is why mold can damage, decay, and destroy wood over time if left uncontrolled.

Ideal Surface

The fibrous, layered grain of wood creates an ideal surface for mold spores and hyphae to adhere and colonize. Cracks, holes, and rough areas on wood trap spores and give them protected, humid environments to germinate and spread.

The natural texture of wood allows mold structures to readily grab hold and anchor into the surface. Mold then penetrates deeper into the wood to access more food and moisture.

wood’s porous, nutritious structure coupled with exposure to excessive moisture makes it highly vulnerable to mold growth. Controlling indoor humidity levels is key to preventing colonization by these destructive fungi.


If you see dark spots, lifted boards, humidity, or smell a musty odour in your home, you may have mold under the hardwood floors. This is bad for your health and your home.

Look closely in hidden areas like under vents. Use a moisture meter to check for damp boards. Hire an inspector to test the air and find mold you can’t see.

Stop any leaks right away. Scrub and disinfect moldy areas. Remove and replace very damaged boards. Keep the air flowing under the floors. Use fans and dehumidifiers to prevent moisture. Mold means there’s a problem you need to fix or it will spread.

Call a professional immediately if you think you have mold. Your family’s health depends on getting rid of it completely.

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About Mohammad Sameer

My name is Mohammad Sameer, the founder of SoundproofGears. My hypersensitive hearing turned me into a lifelong seeker of silence. After years of research, I've become an expert on soundproofing techniques and materials. In November 2022 I launched this site to share my knowledge and help others find acoustic sanctuary. About More