Mold and mildew usually not a threat to lumber — unless you cover it before it’s dry - The Washington Post

2022-09-03 16:27:23 By : Mr. David Xu

I’m having a new room addition built. While inspecting the framing lumber, I noticed black mildew and mold on different pieces of wood. One joist is totally black with it. Is the structural integrity of the wood compromised? Should the lumber be replaced? What causes this to happen as some of the lumber looks perfect? What’s a sensible course of action at this point?

You’re not alone. Many homeowners experience mold and mildew on their lumber as houses are built or room additions are constructed. The good news is the lumber is going to be fine and there’s rarely any damage to the wood. If it’s just surface mildew, it will clean off. Wood rot can weaken wood, but it’s easy to tell if wood is rotten.

There are many reasons why the black mildew and mold appeared on the lumber. Understand that it can happen to just about any lumber any time. Lumber that’s treated with chemicals that contain copper or borates are less susceptible to mold and mildew growth because these elements and chemical compounds are natural biocides.

The mold and mildew spores are just about everywhere. They are on the wood surfaces or can be transported there by wind and rain. Once in place, all they need is water to start growing and flourishing. Some lumber provides plenty of food for the mold and mildew to grow rapidly.

When you see the black mold cover wide areas of lumber, usually this happens because the wood got wet and stayed wet while it was being stored. If it’s warm and humid, the growth of the mold and mildew can be rapid.

Spotty outbreaks can sometimes be traced to food or liquids that nourish mildew or mold. For instance, If you shake up a bottle of soda containing sugar or high fructose corn syrup and spray it randomly on the lumber, you’ll probably see black spots appear in short order wherever the liquid contacted the lumber and dried.

You can test for structural integrity yourself. First, make sure the lumber is dry. Once the room addition is under roof, the wood should dry rapidly unless you’re in a very long damp spell.

Take an 8-penny nail with a sharp tip and see if you can push it into the wood with just your hand. If you meet immediate resistance, the wood is fine. If the nail, using hand pressure, penetrates deeper than one-quarter inch, then you could have wood rot.

Cleaning the mildew and mold from the wood is a good idea. You don’t want it covered up. Cleaning can be accomplished in several ways.

You may be able to remove the black stains using liquid dish soap and water and a scrub brush. Don’t worry: The water is not going to harm the wood since it’s able to dry rapidly.

Chlorine bleach is also very effective at cleaning up the mold and mildew. You can mix a 50-50 solution with regular chlorine bleach and water. Some people don’t do well with the fumes, so be careful. Wear old clothes, as the chlorine bleach will ruin dyed fabrics. Wear goggles and gloves when using chlorine bleach or any chemical.

You also can use oxygen bleach. It’s a commonly available powder you mix with water. Stir until the powder is dissolved in warm or hot water. Pour this solution in a hand-pump garden sprayer and spray the black areas on the wood. Allow the solution to work. Keep the wood wet with the solution for up to an hour.

The oxygen bleach has no odor and is color safe. It’s not going to hurt your clothes. You’ll want to spray all the wood in case there are any spores that have yet to grow. Stubborn mildew and mold stains may require some scrubbing. Rinse all the wood once it’s clean, hose the dirty water outdoors, squeegee the floor of the room addition and set up fans in the space to air dry the wood as rapidly as possible.

You do not want to cover up this contaminated wood. It needs to be cleaned before any insulation is installed. Never cover this stained wood with drywall or paneling. Anyone with asthma or other respiratory challenges could suffer from the hidden spores.

Tim Carter is a columnist for Tribune Media Services. He can be contacted through his Web site at

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