Mold and Mildew on Siding: How to Clean and Prevent It

Mold and mildew streaks on siding are almost always removable with a simple cleaning solution — typically a mix of oxygen bleach and water or a 70/30 water-to-white-vinegar ratio — applied with a soft brush and rinsed with a garden hose. The more important question is what's causing the growth, because if you don't address the underlying moisture or shade issue, the streaks will return within a season or two.

What Are Those Green or Black Streaks on Your Siding?

The dark streaks, green patches, or fuzzy growth you see on siding are usually one of three things:

• Mildew — a surface fungus that appears as flat, gray or white patches. It stays on the surface and is generally the easiest to remove.
• Mold — a fungus that can be green, black, or brown. It can penetrate porous surfaces like wood and fiber cement if left unchecked.
• Algae — the bright green film common on north-facing walls. It's not technically mold, but it looks bad and holds moisture against your siding.

All three thrive in the same conditions: moisture, shade, and organic material (pollen, dirt, tree sap) sitting on the surface. That's why you typically see growth on the north side of a house, under eaves, behind shrubs, or near downspout splash zones.

Is Mold on Siding Dangerous?

Exterior mold is usually a cosmetic issue rather than a health crisis, but it shouldn't be ignored. Left in place for months or years, mold can:

• Trap moisture against the siding surface, accelerating deterioration — especially on wood and fiber cement.
• Migrate to sheathing or insulation behind the siding if water is intruding through gaps, cracks, or failed caulk.
• Become a factor in allergies or respiratory issues for people spending time near the affected wall (decks, patios, entryways).

In most cases, surface mold on vinyl or aluminum siding is a maintenance task, not a structural emergency. But on wood siding or in areas where you also see peeling paint or warped boards, it can signal a deeper moisture problem worth investigating.

How to Clean Mold and Mildew Off Siding Safely

There are two reliable approaches: a DIY hand-scrub method and a low-pressure rinse. Both work well; the key is choosing the right cleaning solution for your siding material and avoiding methods that cause damage.

Step 1: Choose Your Cleaning Solution

Important: Never mix bleach with vinegar or ammonia-based products. The chemical reaction produces toxic fumes. Pick one approach and stick with it.

Step 2: Prep the Area

• Close all nearby windows and doors.
• Wet surrounding plants, grass, and landscaping with plain water before you start. This dilutes any runoff and reduces the chance of plant damage.
• Cover delicate plantings with plastic sheeting if you're using chlorine bleach.
• Wear rubber gloves, eye protection, and old clothes.

Step 3: Apply and Scrub

1. Wet the siding with a garden hose from bottom to top. This prevents dirty runoff from streaking dry siding below your work area.
2. Apply your cleaning solution to a manageable section — roughly 4 feet by 4 feet at a time.
3. Let the solution sit for 5–10 minutes, but don't let it dry on the surface.
4. Scrub with a soft-bristle brush (a long-handled car-wash brush works well). Use medium pressure — enough to loosen the growth, not enough to scratch vinyl or gouge wood grain.
5. Rinse from top to bottom with a garden hose.
6. Move to the next section and repeat.

What About Pressure Washing?

Pressure washing can work, but it's the most common source of DIY siding damage. The risks vary by material:

• Vinyl siding: High-pressure water can force moisture behind the panels and crack older or brittle vinyl. Keep the nozzle at least 12 inches from the surface and use a wide fan tip (25° or 40°). Never spray upward under the lap joints.
• Wood siding: Pressure washing etches wood grain and can splinter the surface. If you must use a pressure washer, keep it under 1,500 PSI with a 40° tip and maintain at least 18 inches of distance.
• Fiber cement (e.g., James Hardie): Moderate pressure is generally fine, but stay below 1,500 PSI. High pressure can chip the factory finish.
• Aluminum siding: Less risk of water intrusion, but high pressure can dent thin panels.

For most homeowners, a garden hose with a spray nozzle and a bucket of oxygen bleach solution does the job without any of the risk.

How Much Does Professional Siding Cleaning Cost?

If you'd rather hire it out, professional soft-wash or low-pressure cleaning typically runs $0.15–$0.50 per square foot of siding area, or roughly $300–$800 for an average two-story home (approximately 1,500–2,000 square feet of siding). These are approximate figures and vary by region, the severity of the growth, and whether the company includes gutter or window cleaning in the package.

Professional soft-wash services use pump-sprayed cleaning solutions (usually sodium hypochlorite or surfactant blends) followed by a low-pressure rinse. This method is effective and avoids the high-pressure damage risks homeowners run into with rented power washers.

How to Stop Mold and Mildew from Coming Back

Cleaning solves the immediate problem, but regrowth is almost guaranteed if you don't address the conditions that caused it. Here's what actually makes a long-term difference:

Reduce Moisture on the Siding Surface

• Fix gutter issues. Overflowing or misaligned gutters dump water directly onto siding. Make sure downspouts extend at least 4 feet from the foundation and that gutters are draining properly.
• Redirect sprinklers. Irrigation heads that spray the house are a top cause of recurring mold on lower siding sections. Walk your sprinkler zones while they run and adjust any heads that hit the house.
• Improve grading. The soil next to your foundation should slope away from the house at roughly 1 inch per foot for the first 6 feet. Standing water near the foundation keeps the lower courses of siding constantly damp.

Increase Airflow and Sunlight

• Trim vegetation. Shrubs, hedges, and tree limbs that touch or shade the siding create a microclimate perfect for mold. Keep all plantings at least 12 inches from the siding surface, and trim overhead branches to let sunlight reach north-facing walls.
• Check dryer vents. A dryer vent that exhausts warm, moist air against the siding — or worse, a dryer vent that's disconnected inside the wall — feeds mold growth on the exterior and can cause hidden moisture damage.

Keep the Surface Clean

Pollen, tree sap, and airborne dirt act as food for mold and algae. An annual rinse with a garden hose — even without soap — goes a long way. Many homeowners do this in late spring after pollen season or in early fall before winter moisture sets in. Twice a year is enough for most climates.

Consider Mold-Resistant Siding Materials

If you're due for a siding replacement and recurring mold has been a headache, some materials resist growth better than others:

• Vinyl siding is non-porous, so mold sits on the surface and cleans off easily. It doesn't rot. Typical cost: $4–$9 per square foot installed.
• Fiber cement siding (like James Hardie) resists moisture well and doesn't support mold growth the way wood does. Typical cost: $8–$14 per square foot installed.
• Engineered wood (like LP SmartSide) uses zinc borate additives to resist fungal decay, but it still needs proper painting and caulking. Typical cost: $6–$11 per square foot installed.
• Natural wood siding is the most vulnerable to mold and mildew. It requires regular staining or painting and careful moisture management. Typical cost: $7–$15 per square foot installed.

When Mold on Siding Means a Bigger Problem

Sometimes surface mold is a symptom of something more serious behind the siding. Call a contractor for an inspection if you notice any of these:

• Mold that returns within weeks of cleaning in the same spot.
• Soft, spongy, or warped boards behind the mold growth.
• Paint blistering or peeling from the inside out (this suggests moisture is coming from behind the siding, not from the exterior surface).
• A musty smell inside the house near the affected wall.
• Visible gaps, cracked caulk, or missing flashing around windows, doors, or trim near the mold.

These signs can point to failed weather-resistant barriers (the material behind your siding that keeps water out of the wall cavity), inadequate flashing, or even ice-dam damage on walls below the roofline.

Bottom Line: Clean It, Then Fix What Feeds It

Mold and mildew on siding look alarming but are usually a straightforward cleaning job. The real fix is eliminating the moisture, shade, and dirt that let it come back. Budget an afternoon for cleaning and a walk around your house to check gutters, sprinklers, and vegetation clearance — that combination handles the problem for most homes.

If the mold keeps returning or you're seeing signs of damage underneath, it may be time to talk with a siding professional. Get matched with a local contractor using the form on our home page.