How long does it take for mold to grow after water damage?

Your basement took on water yesterday. The carpet is still damp, and you can’t see any mold. Are you already too late? Probably not. But not seeing mold does not tell you much, and that is the part most people get wrong.

Nearly every article on this topic says the same thing: mold starts growing in 24 to 48 hours. Here is where that number comes from, what the testing shows, and what it means for your house.

What the 24 to 48 hour rule really means

Here is what the EPA actually says: if wet or damp materials are dried 24 to 48 hours after a leak or spill, in most cases mold will not grow. The CDC gives the same target for floods, telling homeowners to clean up and dry out fully and quickly, within 24 to 48 hours.

Both are telling you when to finish drying. Neither says mold appears at hour 24 or hour 48. A trade article in Cleanfax made the same point, noting that the guidance documents do not state that mold begins to grow in that window, even though restoration workers often repeat it that way.

24 to 48 hours is when you should be finished drying. It is not when mold shows up.

What tests on wet building materials found

We looked for actual tests rather than repeating the rule. These are the ones we could read:

Why this does not mean you have two weeks

Please do not read that table as permission to wait. There are good reasons not to:

• These are only a few tests. Most used clean water and clean materials. Your water may be neither.
• Visible is not the same as started. Growth begins before you can see it, and it can happen where you cannot look, such as the back of drywall.
• Warmth and dirt sped things up in the Accuserve test. Real houses are not clean test panels.
• Mold is not the only concern. The EPA warns that standing water and wet materials are a breeding ground for viruses and bacteria too. Flood water may contain raw sewage.
• You often do not know how long it has been wet. Accuserve points out that materials have often been wet longer than people think.

The useful takeaway is this: you cannot judge by looking. A wall that looks fine tells you nothing, so go by how wet it is and how long it has been wet.

What mold needs to grow

The EPA says mold needs moisture and food, and that moisture is the most important factor indoors.

• Moisture. It does not take much. A little condensation can be enough.
• Food. The CDC lists paper, cardboard, ceiling tiles, wood, dust, paint, wallpaper, insulation, drywall, carpet, fabric and upholstery. Most houses are full of it.
• Spores. They are already there. The EPA says they cannot be eliminated from indoor spaces, but they will not grow without moisture.

You may see lists that add temperature. The EPA says that in most cases temperature is not the issue, because some molds grow in warm areas and others in cool ones. So moisture is the thing you can actually control. As the EPA puts it, the key to mold control is moisture control.

How to tell if mold is growing after water damage

• A musty smell. The EPA says a moldy odor suggests mold is growing and should be investigated.
• Spots. The CDC says mold can look like spots of many different colors.
• Check where it was wettest for longest. Accuserve advises starting close to where the water came in, on materials mold can feed on, such as wood baseboard, the wood sill plate or drywall paper.

The EPA says mold can hide on the back of drywall, wallpaper or paneling, on the top of ceiling tiles, under carpet and pad, and inside walls around leaking pipes. Be careful about opening things up to look. Pulling off wallpaper with mold behind it, for example, can release a lot of spores. If you think you have hidden mold, the EPA suggests considering an experienced professional.

What to do in the first 48 hours

1. Make it safe. Stay out of standing water until the power is off. The CDC says to turn off the main power only from a dry location, and otherwise to call an electrician.
2. Stop the water. The EPA says to fix plumbing leaks and other water problems as soon as possible. If the water came from a burst pipe, see our guide on frozen and burst pipes.
3. Take photos and call your insurer. Michigan’s insurance regulator advises taking photos or video, and not getting rid of damaged items until your insurer says you can.
4. Get the water out. Once it is safe, the CDC suggests a wet-dry vacuum or a pump.
5. Move the air and dry it. Open windows, run fans blowing outward and use dehumidifiers. If water reached your heating or cooling system, have it checked and cleaned before you turn it on.
6. Deal with what cannot dry. The CDC says to remove or replace carpets and upholstery that were soaked and cannot be dried right away. Check with your insurer first.
7. Watch the humidity. The EPA says to keep it below 60 percent, ideally 30 to 50 percent. A small humidity meter from a hardware store will tell you.
8. Check inside, not just on the surface. Materials can feel dry on the outside and still hold water. This is what moisture meters are for, and a restoration company will have them.

If the water was sewage or flood water, skip the cleanup and call a professional with experience in contaminated water.

If you already have mold

When you can clean it yourself

The EPA says that if the moldy area is less than about 10 square feet, roughly 3 feet by 3 feet, you can in most cases handle it yourself. Fix the water problem first, or the mold will most likely come back. Then:

• Wear an N-95 respirator, goggles and gloves, and avoid touching mold with bare hands.
• Scrub hard surfaces with detergent and water, then dry them completely.
• Expect that absorbent materials such as ceiling tiles and carpet may have to be thrown away.
• Do not paint or caulk over mold. Clean and dry the surface first.

What about bleach?

This is another place where official advice differs. The CDC says mold can be removed from hard surfaces with household products, soap and water, or a solution of no more than 1 cup of laundry bleach per gallon of water. The EPA says bleach and similar chemicals are not recommended as a routine part of mold cleanup. Both agree on what matters: the mold has to come off, and the moisture has to be fixed. The EPA adds that dead mold can still cause allergic reactions. Never mix bleach with ammonia or other cleaners, because it can create toxic fumes.

When to call a professional

• The mold covers more than about 10 square feet, or there was a lot of water damage
• The water was sewage or otherwise contaminated
• You smell mold but cannot find it
• You suspect it is in your heating and cooling system. The EPA says not to run the system, because it could spread mold through the house.
• You have health concerns. The EPA says to talk to a health professional before you start cleanup.

When you hire someone, the EPA suggests checking references and making sure they have experience with mold and follow guidelines from a professional or government body, such as the EPA or IICRC. It is worth asking, because the State of Michigan does not certify mold remediators or mold testing companies. If you are in our area, here is how we handle mold removal in Grand Rapids.

Do you need a mold test?

Usually not. The EPA says sampling is usually unnecessary if you can see mold, and that no federal limits exist to compare results against. The CDC says you do not need to know the type of mold, because any mold should be removed. Michigan’s health department does not test homes and does not recommend testing. You can call them with questions at 800-648-6942.

Testing can still have a place, for example when a professional is checking that an area has been properly cleaned. If you are paying for a test, ask what the result will change.

Is mold dangerous?

It affects people differently. The CDC says exposure may cause a variety of health effects, or none at all. Some people get a stuffy nose, sore throat, cough or wheeze, burning eyes or a skin rash. People with asthma or mold allergies may react more severely, and people with weak immune systems or chronic lung disease may get lung infections.

The EPA also notes that these symptoms can have other causes, so do not assume mold is to blame. If you are worried about your health or your family’s, talk to a doctor.

Does insurance cover mold after water damage?

Mold is usually treated as part of the water claim. If it grew because a covered pipe burst and the house was not dried in time, it is often included. If it grew from high humidity or a leak nobody fixed, it usually is not. Many policies also put a lower dollar limit on mold, sometimes listed under the word “fungi”, so look for that in your policy and ask your agent.

Our guide to what homeowners insurance covers in Michigan goes into more detail.

Frequently Asked Questions