Q&A / 

Mold and Mildew on Lumber

mold and mildew

The horizontal collar tie under the roof rafters is black with mold and mildew. Stain Solver oxygen bleach will remove it with ease. CLICK THE PHOTO now to order Stain Solver. © 2017 Tim Carter

Mold & Mildew on Lumber TIPS

DEAR TIM: I'm having a new room addition built and 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? Tracy K., Chicago, IL

DEAR TRACY: 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 using a certified organic oxygen bleach like Stain Solver.

Wood rot can weaken wood, but it's easy to tell if wood is rotten. What's more, wood will not rot in the short time it takes to build a new home or room addition. It typically takes years for wood rot to advance.

CLICK HERE to get FREE & FAST BIDS from local cleaning companies that can remove mold and mildew.

Any Lumber Anytime

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. But mold and mildew can, and does, grow on treated lumber.

Spores Everywhere

The mold and mildew spores are just about everywhere. They're 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.

Sugar aerosols broadcast by trees and bushes is food for mildew. Dust is also a food. It's nearly impossible to keep the lumber clean as you build. Some lumber provides plenty of food for the mold and mildew to grow rapidly.

Mildew Within 48 Hours

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.

Mildew can start to grow in as little as 48 hours if conditions are favorable.

Construction Food & Drink

Spotty outbreaks can sometimes be traced to food or liquids that are food for 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.

CLICK HERE to get FREE & FAST BIDS from local cleaning companies that can remove mold and mildew.

Test For Rot

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 pretty 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.

Stain Solver Cleaner

Oxygen Bleach

Stain Solver is MADE in the USA with USA ingredients that are food-grade quality. CLICK THE IMAGE to order some NOW.

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.

I recommend you use Stain Solver certified organic oxygen bleach mixed with a little bit of liquid dish soap

The Stain Solver is a pure powder Made in the USA with USA ingredients. It dissolves in warm or hot tap water with a little bit of stirring.

Once mixed and all the powder is dissolved, just pour the solution into a garden hand-pump sprayer and squirt it on all the lumber that's got mildew and mold on it.

Allow it to soak for about 15 minutes keeping the lumber wet with the solution the entire time.

Don't worry, the water is not going to harm the wood since it's able to dry rapidly.

Toxic Chlorine Bleach

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.

The other issue with chlorine bleach is that any solution that gets on, or near, trees, flowers, grass, bushes, etc. will kill it. Chlorine bleach is very toxic.

Don't Cover It

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.

If you're building during a wet spell and the builder is trying to move the job along, never allow him to cover up the wood if it's still wet. Trapping water and moisture in wood will cause mold and mildew to grow.

Air Dry Is Fast

It doesn't take long for wood to air dry, especially once the roof is on and there are high-performance vapor barriers in place in crawlspaces and under concrete slabs. Don't be fooled by a builder that says the wood will dry out on it's own in the wall.

IMPORTANT TIP:  Don't allow a builder to apply a vapor barrier on wet lumber. This will trap moisture and mold and mildew will become a huge issue. A vapor barrier traps the moisture in the wall cavity.

CLICK HERE to get FREE & FAST BIDS from local cleaning companies that can remove mold and mildew.

Column 947


12 Responses to Mold and Mildew on Lumber

  1. I noticed that you made a point of saying the Stain Solver was made in the USA with USA ingredients. Does that mean that if it were made in Canada with Canadian ingredients it would be sub-standard. Lord knows I don't want and sub-standard Canadian products.

    • I do it to save time because in many past columns people ask me in the comments where it is made. There are countries in the world that do happen to have a reputation for making poor-quality products. Canada, to the best of my knowledge, is not one of them. I think it's most interesting that you take offense at that statement of fact in my column. Are you feeling uneasy about Canadian products for some reason????? 😉 Or, do you bleed Canada? Eh?

      • Well-made reply. @Mike Kemper, what is your reasoning for making such a targeted question? Tim gave no indication whatsoever in his original article that would indicate he was talking about Canada. If anything, I would think the majority of readers would assume him to mean China, considering 1) a vast majority of products are made in China, and 2) China has well established themselves as exporters of inferior products.

  2. I have recently encapsulated my crawlspace in order to condition the air into the rest of the house. It was previously only accessible by an outside entrance. I laid 10 mil poly and 2" rigid insulation on the floor and exterior walls and a combination of rocksul, rigid and spray foam in the joist cavities. This past spring I had flooding (there was no sump pit and I've installed one now) and subsequent black mold. I've now cleaned the joists (detergent and vinegar) waited one day, sprayed with vinegar and repeat. I've even painted some areas with a mildew resistant paint. I've read that bleach doesn't penetrate a porous surface to attack the migrating spores whereas vinegar will. My question: there is a lingering smell that wasn't even there before. The crawlspace is the same temp as the house now and I'm hoping this smell will dissipate. It's been a week and with winter approaching, the house will need to be closed up from the elements. Any thoughts on dealing with this smell would be appreciated. I don't want to make myself sick by my house. Thanks.

    • I am no expert - just starting some furniture clean up from my Mom's home - but I would assume that if there is still an odor then you still have a problem. But it seems hard to determine which odor my be associated with the mold and mildew, wet wood still needing to dry out, or just old wood. I am using Mold Control by Concrobium. It is expensive for such a large area as yours. Home Depot carries it in the gallon size too.
      All the the "encapsulation of the crawl space" you did may have to come out and be redone - just a thought if you are still having problems.
      Your homeowners insurance co. was of no help? It sounds like the space is much too big for you to manage on your own and calling in the the professionals would be in order. I know they are expensive but better the cost now and not all the damage to re-mediate later. Good luck - this is not fun I know.

  3. I had a front porch put in end of September 2016. They used oak wood. Now several of the boards are showing mold. I live in west Texas. Is this normal? After I use the stuff you say to get rid of it what should I do to keep it away? Somehow I even have an oak tree growing from it!!

    • I think you need to go back up and re-read the column. I tell you all you need to know to STOP mold growth in the column. As for the tree growing, well, you know how to stop that.

  4. I bought a lot of lumber, it was stored wet. I also bought a bunch of fresh milled lumber, that was also stored wet. I have black, gree, and white colored molds. Is there a easy way to treat the wood? I have stacked all the lumber with spacers between them to help dry them out. Will this help kill the molds? If not, how do I treat so much lumber?

  5. Tim I have mold on the metal fascia and vinyl siding on the west side of my cabin. Can I use Stain Solver and my pressure washer to get rid of the mold. Peak of the fascia is about 20 feet from ground level and I would just like to use the pressure washer to apply the solution (your recommendation for concentration since I have the 9lb container).


    • Gerry,

      No, that's not the way to do it. Mix up the solution as you see here:


      Pour the mixed solution into a pro hand-pump sprayer. Adjust nozzle so you can spray the areas from the ground. Allow it to soak for 30 minutes. Re-spray every 5 minutes so the surfaces STAY WET with the solution.

      Work in the SHADE. After the wait period, it works BEST if you can RUB the boards with a cleaning brush like you'd use for cleaning an RV.

      It's NEVER a good idea to spray UPWARDS from the ground with a pressure washer as the water stream can get behind siding overlap and fascia overlaps.

      Good luck.

  6. If bleach were used to clean the wood, and then not rinsed, what would cause the wood at this point to pull moisture from the environment? Problem we are seeing.

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