Mildew on Siding: Why It Shows Up in Missouri Summers and How to Keep It Off

Mildew on siding is a fungus that feeds on dirt, pollen and moisture sitting on your exterior walls. In Missouri it spreads fastest in humid summers on north-facing and shaded walls, behind overgrown shrubs, and under leaky gutters. Clean it with a mildewcide or oxygen-bleach wash at low pressure, fix the moisture source, and recoat with a mildew-resistant finish.

Early June is when homeowners in St. Louis and Kirkwood start noticing it. The dew sits on the walls until midmorning, the trees have filled in, and that faint gray or green haze on the back of the house gets a little darker every week. Here's what's actually growing there and how to deal with it without damaging the siding underneath.

What is the black or green stuff on my siding?

Usually it's mildew, algae, or both. Mildew shows up as gray, black or brown speckling. Algae looks green and tends to coat a wall in a film, often thickest near the ground or under eaves where water drips.

A simple test tells mildew apart from plain dirt. Put a drop of household bleach diluted with water on a cotton swab and dab a dark spot. If it lightens within a minute or two, it's mildew. If it stays the same, you're looking at dirt, soot or pollen, and a regular wash will take care of it.

Neither one eats through siding on its own. The trouble is what they signal: a wall that stays damp for long stretches, which is where paint softens and wood starts to rot at the joints.

Why does mildew grow on the north side of the house?

North walls get little or no direct sun in Missouri, so they dry slowly after rain and morning dew. Mildew and algae need moisture and shade more than anything else. Give them both for most of the day, all summer, and they'll settle in.

The north side isn't the only problem area. Watch for growth in these spots:

• Walls shaded by mature oaks and maples, which is half the houses in older parts of the county
• Siding behind shrubs planted tight against the foundation, where air can't move
• Areas below clogged or overflowing gutters and leaky downspout joints
• The lowest courses of siding where lawn sprinklers hit the wall every morning

Our humid summers make all of it worse. Walls rarely get a full dry-out, and a house that looked clean in April can look tired by the Fourth of July.

How do you remove mildew from siding safely?

Use a cleaner made for mildew, apply it at low pressure, let it dwell, and rinse thoroughly. Soft washing does the work with chemistry instead of force. Blasting it off with a pressure washer turned all the way up is how siding gets damaged.

Bleach-only shortcuts are where a lot of DIY jobs go wrong. Straight chlorine bleach lightens the stain fast, so it looks like it worked. It doesn't remove the dirt the mildew was feeding on, it can burn your landscaping, and it can leave residue that interferes with new paint. Never mix bleach with ammonia-based cleaners. The fumes are dangerous.

A safer approach for most homes looks like this:

1. Wet down plants and cover anything delicate.
2. Apply a mildew house wash or oxygen-bleach solution at low pressure.
3. Let it sit as the label directs, but don't let it dry in full sun.
4. Scrub stubborn patches with a soft brush, then rinse top to bottom.

High pressure gouges wood, erodes stucco and older brick, and drives water behind lap siding if you spray upward. The editors at This Old House have good walkthroughs on house washing if you want to tackle a small area yourself. Two-story walls are another matter.

Can you paint over mildew?

No. Paint applied over live mildew traps it, and it'll grow right back through the new finish, sometimes within a season. Every bit of growth has to be killed and rinsed off, and the wall has to dry completely before primer goes on.

That's why cleaning comes first on any serious exterior job. Our coating application process starts with a thorough wash, then moves to repairs, caulking and sealing, and a primer matched to the surface before the topcoats go on.

What keeps mildew from coming back?

Two things: take away some of the moisture, and put a finish on the wall that gives mildew less to grab onto. You can't control Missouri humidity, but you can control a lot of what happens right around your house.

Start with the cheap fixes. Clean the gutters and point downspouts away from the foundation. Trim shrubs back so air can move between plants and the wall. Turn the sprinkler heads so they water the lawn, not the siding.

Then look at the finish itself. Old paint that has started to chalk holds dirt and moisture in a fuzzy surface layer, which is perfect habitat. Quality exterior paints include mildewcides, but those wear down, and a thin film that's baked in UV for years doesn't shed water like it used to.

A thicker film helps here. Rhino Shield is a ceramic elastomeric coating built with ceramic microspheres in a specialized resin, and it's formulated to resist mold and mildew. It goes on at 8 to 10 mils, compared with the 1 to 2 mils typical of conventional exterior paint. You can read more about how that compares on our exterior paint coatings page.

No coating makes a shaded wall under a big maple immune. Pollen still lands on it. What changes is how easily it comes clean and whether the finish is still sound years from now. One homeowner described the finished surface this way:

That thick look is the film build you're paying for, and on a north wall it's exactly what you want between the weather and your siding.

When should you call a professional about mildew?

Call someone if growth returns within a few months of cleaning, if you find soft wood under it, or if the walls are too high to reach safely. Fast regrowth usually means a moisture problem the wash didn't solve.

Rhino Shield of Missouri works on homes across the metro, from older wooded lots near our Kirkwood exterior coating customers to newer subdivisions in St. Charles County. On an estimate we'll look at what's feeding the growth and whether the siding needs repair first.

Related reading: Protect Your Brick Exterior From Moisture

Frequently Asked Questions

Is mildew on siding harmful?

Mildew on the outside of your siding usually isn't a health hazard for people inside the house. It's a sign the wall is staying wet, though, and long-term dampness shortens paint life and can lead to rot in wood siding and trim. Treat it as a maintenance warning.

What kills mildew on siding without bleach?

Oxygen-bleach (sodium percarbonate) solutions and commercial mildew house washes both work well and are gentler on plants than chlorine bleach. Apply at low pressure, let the cleaner sit as directed, scrub stubborn spots with a soft brush, and rinse thoroughly.

Can I pressure wash mildew off my siding?

You can use a pressure washer, but keep it on a low-pressure setting and let the cleaner do the work. High pressure can gouge wood, erode stucco and older brick, and drive water behind lap siding. Always spray downward, never up under the laps.

Does mildew-resistant paint really work?

It helps, especially when it's applied to a clean, dry wall. Mildewcides in paint slow growth, but they wear down over time and thin films lose their ability to shed water. Pair a quality finish with better drainage and airflow around the house for the best results.

Get a straight answer about your siding

If the same wall turns green every summer, it's worth having someone look at why. Request a free estimate or call us at 314-239-7947 and we'll walk the house with you.