How to Remove Algae and Mildew from House Siding

Algae and mildew on your home's siding isn't just ugly — it can damage the exterior over time. This guide shows you how to remove green streaks using a pressure washer with the right attachments, cleaning solutions, and technique — no ladder required for most one-story homes.

Algae and mildew on your home's siding isn't just ugly — it can damage the exterior over time. This guide shows you how to remove green streaks using a pressure washer with the right attachments, cleaning solutions, and technique — no ladder required for most one-story homes.

Algae and mildew on your home's siding isn't just ugly — it can damage the exterior over time. This guide shows you how to remove green streaks using a pressure washer with the right attachments, cleaning solutions, and technique — no ladder required for most one-story homes.

Algae and mildew on your home's siding isn't just ugly — it can damage the exterior over time. This guide shows you how to remove green streaks using a pressure washer with the right attachments, cleaning solutions, and technique — no ladder required for most one-story homes.

The Truth About Green Walls

I bought my first pressure washer because I was tired of my house looking like it belonged in a swamp. The north side of my siding was green. Not a nice shade of green either. It looked like moss was plotting a takeover. I fired up that machine, grabbed the zero-degree nozzle (big mistake), and blasted away. I blew a groove straight through my lap siding. Water got behind the boards. I had to replace three sections of siding and deal with a small leak inside my utility room for a month. Don't do that.

Algae and mildew are different, but they both wreck your paint and make your house look abandoned. Algae shows up as dark green or black streaks, usually on shady sides. Mildew is lighter, almost gray or white, and it’s that powdery stuff that smears when you touch it. Both need moisture and shade. You can beat them without destroying your house. I’ve done it about twenty times now. Here’s exactly how.

What You Actually Need

You don’t need a $800 professional machine. I use a 2,300 PSI electric unit with 1.2 GPM. That’s enough. Anything over 2,800 PSI on wood siding is asking for trouble. For vinyl, you can go a little higher, but don’t push past 3,000 PSI.

Here’s your shopping list:

• Pressure washer (2,000–2,800 PSI, 1.1+ GPM)
• 40-degree nozzle (the white one – widest spray, lowest risk)
• Garden pump sprayer ($15 at the hardware store)
• Laundry detergent with bleach (Tide powder works great – I’ll explain below)
• Sodium hypochlorite pool shock (10% or 12.5% concentration – plain bleach is too weak at 5% or 6%)
• Eye protection and rubber gloves (bleach burns. Trust me.)
• Plastic sheeting (for covering plants and electrical outlets)

Step 1: Mix the Killer Chemicals

Don’t just spray bleach straight from the bottle. That’s wasteful and it can stain your siding. I mix a strong solution in my garden sprayer. Here’s the recipe I’ve settled on after ruining a few patches of lawn:

• 1 gallon of 10% pool shock (sodium hypochlorite)
• 1/2 cup of Tide laundry detergent powder (the bleach kind, not the stain-lifting stuff)
• 2 gallons of water

Why Tide? It acts as a surfactant. It makes the solution stick to vertical siding instead of running straight off. The bleach kills the algae and mildew. The detergent helps lift the grime. Mix it in the sprayer, shake gently, and let it sit for five minutes. Do not use dish soap. Dish soap creates too much foam and you’ll be rinsing for an hour.

Step 2: Prep the Zone

I learned this the hard way. Bleach runoff kills plants. I killed a row of hostas that my wife planted. She was not happy. Cover every plant near the siding with plastic sheeting. Wet them down first, then drape the plastic. Wetting helps protect them from bleach fumes. Also cover any outdoor outlets. Tape the sheeting down so it doesn’t blow away when you spray.

Wet the siding itself with plain water before you apply the chemical. This slows down the bleach reaction so it doesn’t dry out and leave residue. Work in sections. I go 10 feet wide at a time. Don’t try to do the whole house at once. The chemical dries and your results look blotchy.

Step 3: Apply the Chemical

Put on your gloves and goggles. No shortcuts. I got bleach in my eye once and it felt like a hot coal. Spray the chemical onto the siding from bottom to top. Why bottom first? If you start at the top, the chemical runs down over dry spots and you miss stuff. Working from the bottom up forces you to cover every inch. Coat the siding generously. You want it dripping wet for a solid five minutes.

Let it sit for 10 to 15 minutes. Don’t let it dry out completely. If it starts drying, spray a little more on. This is where the surfactant pays off. On a hot day, you might only get 8 minutes before it dries. On a cool cloudy day, you can push to 15. Watch your watch. Set a timer.

Step 4: Rinse Properly

Here’s where most people mess up. They grab the zero-degree or 15-degree nozzle and blast the siding from close range. That’s how you gouge wood or bend vinyl. I use the 40-degree nozzle and hold it about 18 inches from the siding. Rinse from top to bottom now. Your goal is to wash the dead algae off, not to strip paint.

Keep the wand at a slight angle downward. If you spray upward into the siding seams, you force water behind the boards. That leads to rot. I’ve pulled apart rotted trim that was perfectly fine five years ago because somebody aimed the spray wrong. Light overlapping passes. Let the water do the work.

For stubborn spots that didn’t come clean, don’t press the nozzle closer. Walk away, mix a fresh batch of chemical, reapply to that spot, wait 10 minutes, and rinse again. Scrubbing with a brush on a pole works too, but I only do that for heavy mildew stains on wood. Use a plastic-bristle brush, never wire.

Step 5: The Gutter and Trim Trap

Algae loves inside corners and under the eaves. Mildew builds up on window sills and door frames. Don’t ignore these. They’ll regrow and spread back to your siding within a month. I use a 15-degree nozzle (the yellow one) for gutters only, but I hold it at a flat angle and keep it 24 inches away. I’d rather spray twice than risk blowing a gutter seam loose. I’ve blown gutters off before. It’s loud and expensive.

For window trim, I use the pump sprayer with chemical and a soft scrub brush. Rinse with the garden hose, not the pressure washer. Glass cracks easily. I’ve broken one window in my life. That was enough.

Step 6: When to Stop

Your siding will never look brand new if it’s old and weathered. Accept that. I’ve had people complain that their cedar siding still had a slight green tint after cleaning. That’s called “patina.” Bleach kills the surface algae, but deep stains in the wood grain take several cleanings over two years to fade. If you keep blasting with high pressure, you’re just sanding your siding away. Stop while the paint is still intact.

If you have vinyl siding, check for gaps or loose panels after rinsing. High pressure can pop seams. I once flooded my crawl space because a vinyl seam popped and water ran down inside the wall. Don’t assume your house is waterproof. It isn’t.

FAQ

Can I just use bleach without a pressure washer?

Yes, but it’s slower. A garden hose with a spray nozzle and a brush works for small areas. But you’ll use way more chemical and risk streaking. For anything over 200 square feet, use the pressure washer for rinsing at least. The force helps lift dead spores off the surface.

How often should I clean house siding?

I clean mine every two years. North-facing sides might need it yearly if you have lots of shade trees. If you wait four or five years, the algae roots into the paint and you’ll have to repaint. I repainted after year four once. Not doing that again.

What if I have cedar shake siding?

Be very careful. Cedar soaks up water like a sponge. I use half-strength chemical (less bleach) and I rinse with the garden hose only. No pressure washer over 1,500 PSI on cedar shakes. I’ve turned shake siding into fuzzy mess by going 2,000 PSI. Learn from my error.

Do I need to seal or paint after cleaning?

If your paint is still good, just let it dry for three dry days. If you see bare wood, prime and paint within two weeks. Mildew loves bare wood. I once left a section unpainted for a month and it was green again before I got back to it.

The chemical killed my grass. What now?

Water the grass heavily right after you rinse the siding. Flush the bleach out of the soil. If it’s already brown, reseed in the spring. Move your plant covers further out next time. I use tarps that extend five feet from the house now. That fixed the problem.