Ask the Builder: Removing mold from vinyl siding

Q. My house is now 25 years old. It’s covered with vinyl siding. Every summer, mold and mildew develops on the north side of my house. Every few years I remove it. Is there some coating I can apply to the siding to stop the mold and mildew from reappearing? It’s aggravating. What about some sort of paint? I’m open to any and all ideas. – Robert C., Sylvania, Ohio

A. I have the same issue at my own home, although its siding is fiber cement. You may have black mold and mildew or green algae growing on the north side of your home if you live north of the equator. The north exposure provides the perfect conditions for this organic growth.

Here’s the bad news. There’s no magic paint or coating I’m aware of that will keep your siding free of mildew, mold and algae. Here’s why: These three living organisms are just about everywhere. They just need two things to grow: water and food. The northern exposure of houses provides ample amounts of water.

Because direct sunlight doesn’t wash north-facing walls, water there is often slower to evaporate. Morning dew can and does form on all the surfaces of the house when the surface temperature drops below the dew point of the air.

Those surfaces on the north side of the house lose this dew last, allowing the mold, mildew and algae to get a drink before the next morning. The soil just below the siding stays far moister than the soil on the other sides of the house, all things being equal. Water evaporates from this soil, and some of this water vapor might also quench the thirst of the organisms.

Food is readily available in almost all areas of the USA for mold, mildew and algae growth. In Ohio, the plentiful deciduous trees, especially maples, broadcast a fine aerosol. These microscopic drops of elixir contain sugars that are delicious food for mildew, mold and algae.

Dust and other pollutants also can be food for some of these organisms. Realize that food is everywhere, even though it’s invisible. Old oil-based paints were made from alkyd oil resins. These oils are scrumptious food for mildew and mold.

This is why some modern deck sealers made using natural oils turn black in months after you go to all the trouble of making your deck look nice. You’re slathering on food for the mold and mildew to devour. Some natural-oil sealers contain chemicals to slow organic growth.

Three years ago, I wrote a book, “Roofing Ripoff,” about why asphalt shingles are falling apart faster than they should. The inspiration for the book came when I discovered that copper ions will extend the life of asphalt shingles for decades.

Many have known for hundreds of years that copper also is an amazing natural biocide. That’s why the hulls of clipper ships and “Old Ironsides,” the historic U.S. Navy frigate, have copper sheathing. I’ve used liquid copper solution to keep algae off patios. The only issue is that the copper washes away with each rainfall, and it can create a bluish cast. Some people are very much against the use of aqueous copper like this, thinking it harms the environment.

If you have problems with mold and mildew on your vinyl siding, you might want to experiment with spraying a copper sulfate on the siding. However, if you’ve got valuable landscaping or trees nearby, the copper is not going to help them live. The issue is the copper on the vinyl siding is temporary, and it will wash off with repeated rainfalls.

I think the easiest thing to do is to just wash the siding once a year around Flag Day. It’s warm by then, and the chore won’t be too hard. If you wash the house when the siding doesn’t yet have mold or mildew growth, you might be able to accomplish it using a garden spray attachment to your garden hose.

Instead of putting dangerous chemicals in the sprayer, just use powdered oxygen bleach as well as liquid dish soap. Spray this soapy solution on the house and allow it to sit for 15 or 20 minutes. This allows the oxygen bleach to attack and break down any invisible food particles.

The liquid dish soap lubricates the surface so food comes off with minimal effort when you rinse the siding. This simple cleaning procedure ensures the siding looks great all year.