How to tell if a subfloor is moldy?

Author: Prof. Lilly Krajcik PhD  |  Last update: Monday, July 28, 2025

You might find it under carpets, vinyl, or laminate. The mold can look like black, green, or white marks on plywood or OSB, which are common in homes since the 1950s. A musty smell is a sign of subfloor mold. Sometimes, the only hints are bubbling in vinyl floors or a smell of mildew.

How to tell if a subfloor has mold?

Signs of Mold under Hardwood Floors

You might notice patches of discoloration (usually black or green, although mold can grow in a variety of different colors) with a slimy or fuzzy texture. Other times, mold isn't so easily seen if it's growing under the floorboards, on the floor joists, or on the subfloor.

Can you kill mold on a subfloor?

Use a mixture of water and detergent or a commercial mold cleaner to scrub the affected area. Ensure the area is well-ventilated and dry the subfloor thoroughly. Consider using a dehumidifier to prevent future mold growth.

What are signs of mold sickness?

Although symptoms can vary, the most common symptoms seen in people exposed to mold indoors include:
  • Nasal and sinus congestion.
  • Eye irritation, such as itchy, red, watery eyes.
  • Wheezing and difficulty breathing.
  • Cough.
  • Throat irritation.
  • Skin irritation, such as a rash.
  • Headache.

How long does it take for mold to grow in the subfloor?

mildew and mold will develop within 24-48 hours of water exposure. Even worse, it will continue to grow until steps are taken to eliminate the source of moisture, and effectively deal with the mold problem. Use the diagram on the facing page to assess the extent of mold in your home.

3. Discovering mold and cleaning it up

How to test for mold under flooring?

What Are The Signs That Mold Is Growing Under Your Home's Floor?
  1. Check underneath the flooring for mold. Carefully pull up the planks and check whether you can spot mold. ...
  2. Test for mold using a mold detector. A mold kit can detect spores in any area and how many they are in the air.

How much does it cost to replace a subfloor?

For an average-sized room of 300 square feet, expect to spend about $900 to $3,000 on the cost to replace subfloors, or $3 to $10 per square foot. Most homeowners spend about an average of $1,560. Laying down a new style of flooring, like marble or linoleum, might require a new subfloor.

Can you clean up mold yourself?

Scrub mold off hard surfaces with detergent and water, and dry completely. Fix plumbing leaks and other water problems as soon as possible. Dry all items completely. Absorbent or porous materials, such as ceiling tiles and carpet, may have to be thrown away if they become moldy.

How to test yourself for mold exposure?

There are several methods available to test for mold exposure:
  1. Blood Tests: Blood tests for mold exposure detect specific antibodies produced by the immune system in response to mold exposure. ...
  2. Skin Prick Tests: This method involves exposing the skin to small amounts of common molds to check for allergic reactions.

What does harmless black mold look like?

Although the name implies a uniform coloring, black mold may appear black, dark green, or dark brown. It typically grows as a cluster of round spots with a distinctly wet, slimy appearance. As stated, black mold typically appears fuzzy or furry.

What does black mold on plywood look like?

Toxic black mold on wood appears as dark green or black patches. It often has a slimy or wet texture and may spread across the surface in a spotty pattern. The affected wood might appear discolored, soft, and damp.

How do you disinfect subflooring?

Besides water, all you'll need is one ingredient that you likely have in your pantry - vinegar. Here's what to do: Mix one part of vinegar with one part of water. Moisten a cloth or mop with this vinegar/water solution.

Does kilz kill mold?

“Does Kilz kill mold?” Despite the brand name “Kilz,” which sounds like it would kill mold, Kilz does not kill mold but does proactively prevent it once it is applied over a mold-free surface. This makes it a good choice for damp areas.

Do I need to replace moldy subfloor?

When Should You Replace Subfloors? There are many signs that your damaged subfloor should be replaced with a new subfloor. These include squeaks and other noises, cupping or warping, cracked or popped tiles, sagging or unevenness, the smell of mold or mildew, or bubbling tile.

Does homeowners insurance cover mold?

Homeowners insurance doesn't usually cover mold damage or removal unless it's the result of a covered peril. If the mold in your residence was caused by something sudden or accidental, such as a bursting pipe or some other covered incident on your policy, you may be protected.

How do I know if my subfloor needs replacing?

9 Common Signs of Subfloor Damage
  1. Your floor is uneven or sunken-in. ...
  2. Your floorboards squeak — loudly. ...
  3. The room smells musty. ...
  4. Your floors shift or bounce when you walk on them. ...
  5. Your toilet is rocking or loose. ...
  6. Your flooring has cracked. ...
  7. Your hardwood floor is cupping. ...
  8. Your linoleum floor is bubbling up.

How do you detect mold if you can't see it?

Musty Smell

If you don't see signs of mold, but you can smell it, it may be hidden in your walls. A musty smell indicates that mold is growing in your home. The smell of mold feels earthy, like rotting leaves or decaying wood in a dense, damp forest.

What are the symptoms of mold sickness?

Symptoms
  • Sneezing.
  • Runny or stuffy nose.
  • Cough and postnasal drip.
  • Itchy eyes, nose and throat.
  • Watery eyes.
  • Dry, scaly skin.

Is there a DIY test for mold?

The Do It Yourself Mold Test Kit can be used for indoor or outdoor mold testing. Tests indoor air quality for unseen mold in the home. There are three testing methods: (1) Indoor Air Quality Test Method (2) Air Conditioning & Heating Sample Method (3) Surface Sampling Testing Method (when visible mold is present).

What kills 100% of mold?

Hydrogen peroxide, commonly used to disinfect wounds, is active against a wide variety of micro-organisms including mold. It acts by releasing oxygen at a rapid rate, which kills mold by oxidizing the surface and breaking down proteins and DNA.

Will mold just wipe off?

Mold, on the other hand, is alive and can reproduce. Unlike dust, simply wiping it away doesn't solve the problem; mold spores can linger and regrow.

Does a dehumidifier help with mold?

Mould will easily grow and spread if the right conditions are present. So, dehumidifiers don't kill mould, but they can help to prevent it from growing inside your home by reducing humidity. However, the best way to deal with it is to get rid of its source.

Is subfloor covered by insurance?

Does homeowners insurance cover subfloor? In general, homeowners insurance covers subfloor water damage. However, it must be caused by one of the perils listed in your insurance policy.

Is my subfloor rotting?

It's difficult to assess if the sub-floor is rotting until carpets or tile are pulled back and an inspection is conducted. However, if the sub-floor feels spongy and sags when walking across the floor, this is a sign of a rotted sub-floor.

Who to hire to replace a subfloor?

Given the complexity of subfloor repairs, you should really stick with a carpenter for this project. But if you want to install a new subfloor or underlayment over sound floor joists, hire a local flooring contractor for this step.

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