How to Remove Algae and Mildew from Siding (4 Safe Methods)

TL;DR: Algae and mildew appear as green, brown, or black streaks on north-facing or shaded siding. Clean them with diluted bleach, oxygen bleach, or a commercial algaecide using a soft brush or low-pressure sprayer—never a power washer above 1,500 PSI on vinyl. Regular cleaning every 12–18 months prevents the buildup from returning and keeps moisture from seeping behind the siding.

_Last reviewed: July 2026 · 6 min read_

Green or black streaks on your siding aren't just cosmetic—they hold moisture against the substrate, accelerate rot on wood trim, and lower resale value. Most growth is Gloeocapsa magma (a cyanobacterium) or common mildew, both of which thrive in shade and humidity. The good news: you can remove them with household ingredients and basic equipment.

Okoniq Property Hub lets you log cleaning dates, attach before-and-after photos, and set reminders so you stay ahead of buildup before it etches the finish or spreads to trim.

What causes algae and mildew to grow on siding?

Algae and mildew need three things: moisture, organic matter, and shade. North-facing walls, sections under roof eaves, and areas near sprinklers or downspouts stay damp longer. Pollen, dust, and bug residue provide food. Once spores land, they colonize in days.

Vinyl, fiber cement, and aluminum siding all support growth—texture and color don't matter. Wood siding is more vulnerable because the organisms can penetrate grain and accelerate rot. If you see black streaks near caulk joints or water stains inside the ceiling, check for gaps that let moisture behind the cladding.

Trim overhanging branches to increase sunlight. Redirect downspouts so water doesn't splash the wall. Both steps cut recurrence by 40–60 percent in typical suburban climates.

Which cleaning solution should you use?

Diluted bleach is the fastest killer. Mix one part household bleach (5.25–8.25% sodium hypochlorite) with three parts water. Wet plants and grass below, spray the solution bottom-to-top, wait 10 minutes, then rinse top-to-bottom with a garden hose. Bleach works in under 15 minutes but can discolor certain vinyl dyes and harm shrubs—test a hidden section first.

Oxygen bleach (sodium percarbonate) is gentler and won't bleach landscape plants. Mix per the package—usually 1 cup per gallon of warm water—apply with a pump sprayer, scrub with a soft brush, and rinse after 20 minutes. It takes longer than chlorine bleach but won't fade colored siding.

Commercial algaecides (brands like Wet & Forget or 30 Seconds) contain quaternary ammonium compounds. Spray, leave to dry, and let rain rinse over the next few weeks. They're convenient for tall sections but cost $20–40 per gallon versus $4 for a jug of bleach.

Vinegar and dish soap (1 cup white vinegar + 1 gallon water + a squirt of Dawn) work on light surface film but won't kill embedded roots. Use this method for monthly touch-ups between deeper cleans.

Never mix bleach and vinegar—the reaction produces chlorine gas. Store each solution in a labeled sprayer and rinse pumps thoroughly between uses.

How do you clean siding without damaging it?

Start with a garden hose to rinse loose dirt. Apply cleaner from bottom to top so drips don't streak over dry sections. Let the solution sit 10–20 minutes—check the product label—then scrub lightly with a soft-bristle brush on a pole. Rinse top to bottom so dirty runoff doesn't re-stain lower areas.

Pressure washing: Limit pressure to 1,200–1,500 PSI for vinyl, 1,800 PSI for fiber cement. Hold the nozzle 12–18 inches from the surface at a 45-degree downward angle—never spray upward under panel laps or you'll force water into wall cavities. A 25-degree green tip is safe; avoid the 0-degree red tip. If you've never used a power washer, rent an electric 1,500 PSI unit from a hardware store for $40/day instead of buying a gas model.

Ladder safety: Work in sections. Move the ladder rather than leaning. On two-story homes, use a stabilizer bar so the ladder doesn't crush gutters or dent siding. If you're uncomfortable above 12 feet, hire a crew—rates run $200–500 for a typical single-family exterior depending on height and square footage.

Protect windows and doors with painter's tape and plastic sheeting. Rinse foundation plantings before and after to dilute any overspray. If you're also maintaining bathroom exhaust fans and other moisture sources inside, schedule both tasks in the same weekend to maximize efficiency.

When should you call a professional instead?

Call a pro if you see:

• Mold or mildew inside wall cavities behind siding panels—visible when you remove an outlet cover or look through weep holes
• Rotted sheathing or studs where the siding flexes inward when you press it
• Asbestos siding (common on homes built before 1980)—disturbing it releases fibers; certified abatement costs $1,500–3,000 but is legally required in most states
• Three-story sections or areas above a steep roof pitch

Professional soft-wash services use lower-pressure pumps (under 500 PSI) with surfactants that cling longer and penetrate deeper. They charge $0.15–0.35 per square foot, so a 2,000 sq ft exterior runs $300–700. That's worth it if the buildup is severe or you lack safe ladder access.

If algae returns within six months despite cleaning, check for a leaking gutter or an attic ventilation imbalance that's pushing humid air through soffits and condensing on the exterior. Fixing the root moisture source stops the cycle.

How often should you clean siding to prevent regrowth?

Clean once every 12–18 months in humid climates (Southeast, Pacific Northwest), once every 24 months in arid regions. Schedule in spring or fall when temperatures are 50–80°F—cleaner evaporates too fast in summer heat and freezes in winter. Mark the date in Okoniq and set a 15-month reminder.

After cleaning, apply a siding protectant (siloxane or acrylic sealer) to smooth surfaces like vinyl or aluminum. These products form a hydrophobic film that sheds water and slows algae colonization by 30–40 percent. One gallon covers 200–400 square feet and lasts 2–3 years. Skip sealers on porous fiber cement—they trap moisture and cause spalling.

Trim shrubs 12 inches from the wall to improve airflow. Redirect sprinklers so they don't mist the siding daily. Both changes cut cleaning frequency in half over a five-year span.

FAQ

Can algae or mildew damage siding permanently?

Surface growth won't crack vinyl or aluminum but can etch the UV-protective top coat, causing chalking and fading within 3–5 years if left untreated. On wood siding, mildew penetrates grain and accelerates rot, especially around joints and nail holes. Clean within a few months of noticing streaks to prevent substrate damage.

Is it safe to use a pressure washer on vinyl siding?

Yes, if you stay below 1,500 PSI and angle the nozzle downward. Spraying upward under panel laps forces water into wall cavities, leading to mold behind the drywall or soaked insulation. Hold the wand 12 inches away and move steadily—hovering in one spot can crack brittle vinyl in cold weather.

Will bleach kill plants near the foundation?

Undiluted bleach will burn foliage within hours. Dilute to 1:3 (bleach to water), wet plants before spraying, and rinse them immediately after the siding rinse. Alternatively, use oxygen bleach or a commercial algaecide labeled "plant-safe." Cover prized shrubs with a tarp during application.

How do you remove black streaks that won't scrub off?

Persistent black streaks are often artillery fungus (Sphaerobolus stellatus), which ejects spores that bond to surfaces like tar. Scrub with a Magic Eraser or plastic scraper dipped in mineral spirits, then clean the area with bleach solution. On textured vinyl, you may need to repaint or replace affected panels—there's no chemical that reliably dissolves the spore adhesive without damaging siding.

Does cleaning siding increase home value?

A clean exterior can add 3–5 percent to perceived value during showings, according to real estate agents. Buyers interpret streaked siding as deferred maintenance and often lowball offers to account for future repair costs. Spending $300 on a professional clean before listing can return $3,000–8,000 in final sale price on a median-priced home.

This is educational information, not professional advice. If you discover structural damage, leaks, or suspect asbestos, consult a licensed contractor or environmental specialist before proceeding.