How to Remove Algae and Mildew from House Siding

Why You Need to Kill the Green Stuff

Last spring, I nearly pressure-washed the paint right off my own house. I was blasting away at what I thought was just dirt, and suddenly I was looking at bare wood in a patch the size of a pizza box. That was the day I learned a hard lesson: algae and mildew aren't just ugly. They're slippery, moisture-holding, siding-destroying little monsters. And you can't just power-wash them off like a dirty truck.

The good news? Removing that green-black slime is dead simple if you do it right. I've done this on vinyl, wood, and fiber cement siding. This guide is everything I wish I'd known before I stripped half my neighbor's fence (true story).

What You Need: Tools and Chemicals

Don't buy the "specialty" house siding cleaner at the big box store for $30. It's just sodium hypochlorite with a fancy label. Here's what I actually use:

• Pressure washer — anything over 2,000 PSI works fine. I use a 2,300 PSI gas model.
• Turbo nozzle or 40-degree nozzle — never use a 0-degree or 15-degree nozzle on siding. You'll carve grooves into the material. I learned this the hard way on my shed.
• Bleach-based cleaner (1 part bleach, 3 parts water + a squirt of dish soap) — OR a commercial product like "Wet & Forget" if you're too lazy to mix. I'm not lazy, so I mix my own.
• Garden sprayer — a cheap 1-gallon pump sprayer, about $15.
• Brush with a long handle — a soft-bristle brush on a pole (I use a 12-foot extension) to scrub the stubborn spots.
• Bucket and sponge — for windows and trim. You'll thank me later.
• Eye protection and rubber gloves — bleach burns. Trust me, I've had splashback.

Prep Step: Don't Skip This

First, wet down your garden plants, bushes, and grass near the house. Bleach runoff will scorch leaves. I killed a hydrangea bush last year because I thought "a little spray won't hurt." It hurt. Cover delicate plants with a tarp or plastic sheeting if you can.

Next, close all windows and doors. Seal outlets and vents with plastic wrap if they're exposed. Water inside the walls = mold and rot. I once soaked my living room carpet because I got overzealous near a first-floor window. My wife still brings that one up.

Wait for a cloudy day. Direct sun dries the bleach cleaner too fast, and it won't kill the mildew. Plus, the pressure washer water will leave streaks if it evaporates instantly. Overcast and 60-70°F is perfect. I always check the forecast for no wind too — wind blows bleach spray onto my face and car.

Step 1: Spray the Killer

Mix your cleaner in the garden sprayer. I do one gallon of water, one cup of bleach, and a squirt of Dawn dish soap. That's enough for about 200 square feet of siding. Adjust for your house size.

Spray the algae and mildew directly. Start from the bottom and work up. Why bottom first? Because if you start at the top, the cleaner runs down over untreated areas and dries before it hits the ground. You want it to sit wet on the siding for 10-15 minutes. I let it dwell for a full 15 minutes on heavy black mildew patches.

Don't let it dry. If the sun comes out, mist the siding with a garden hose to keep it wet. Dried bleach cleaner turns into a white powder that does nothing.

Step 2: Scrub the Bad Patches

After the dwell time, hit the really nasty spots with the long-handled brush. I use a circular motion and light pressure. You don't need to grind — just lift the dead gunk. The bleach has already loosened it.

Focus on north-facing walls and shaded areas. That's where mildew thrives. My porch overhang is a nightmare every year.

Step 3: Pressure Wash — Correctly

Now the fun part. Attach the turbo nozzle or the 40-degree nozzle. Set your pressure to no more than 1,200–1,500 PSI at the gun. Most washers let you adjust the unloader valve or use a wider tip to reduce pressure. If you don't have a gauge, test on an inconspicuous spot. You want it to clean, not etch.

Hold the nozzle at least 12 inches from the siding. Work from the top down — start at a gable or the top row of siding, then work lower. Why top down? Gravity pulls dirt and water down, so if you start at the bottom, you're washing over grime that's already been loosened. It's cleaner this way.

Keep the stream moving. Do not stop on one spot. I paused for a second to answer my phone once, and I left a perfect ring-shaped dent in the siding. Never again.

Use a sweeping motion that follows the direction of the siding boards. On lap siding, spray horizontally, not vertically. Spraying vertically forces water up under the overlapping boards and into the wall cavity. I did that once and had water dripping from inside my laundry room for two days.

Step 4: Rinse and Inspect

Once the whole wall is done, give it a final rinse with plain water, keeping the nozzle at least 18 inches away. Look for any remaining green or black streaks. If you see them, spray more cleaner, scrub, and blast again. One pass rarely gets the stubborn stuff on north-facing walls.

Check your ground. If you see white or green runoff pooling, you probably didn't dilute the bleach enough. That's okay. Hose it down with plain water.

What Not to Do: Mistakes I Made

• Using a metal nozzle on a fiber cement house. It scratched the surface. Use only plastic or brass nozzles.
• Bleaching my driveway too. The runoff from the siding turned my concrete white in patches. Now I use a driveway-specific cleaner for that, not house wash.
• Thinking one treatment lasts forever. It doesn't. You'll need to do this every 12-18 months, especially in humid climates. Don't let it build up for three years like I did.
• Not checking the weather. I started washing at 3 PM on a sunny day, and the siding was dry before I could rinse. The cleaner left a white film that took me two days to scrub off. Cloudy morning only from now on.

FAQ

Should I use a pressure washer or a garden hose?

Garden hose is fine for maintenance washing of light dust. But for algae and mildew that's been growing for months, you need pressure. I'd say 2,000 PSI is the sweet spot. Anything under 1,500 PSI won't cut through the slime without a lot of scrubbing.

Can I use vinegar instead of bleach?

I've tried it. Vinegar kills some mildew on contact, but it doesn't penetrate the spores the way bleach does. You'll end up scrubbing three times as hard. Bleach is faster and cheaper. Just rinse your plants afterward.

What if my siding is painted wood?

Be extra careful. Painted wood is soft. I'd drop pressure to 1,000 PSI max, use a 40-degree nozzle, and never hold it closer than 16 inches. Test in an inconspicuous corner first. If paint comes off, you're too aggressive. Consider hand scrubbing instead.

How long does the whole job take?

For a single-story 1,200 sq ft house with average algae, I spend about 2 hours prepping and spraying, 30 minutes letting it dwell, and 1.5 hours washing and rinsing. Total: about 4 hours. A two-story house with heavy mildew can take 6 hours. Budget a full morning.

Do I need to treat the siding afterward?

I apply a clear mildew-resistant sealer every two years on wood siding. Vinyl doesn't need it. For fiber cement, a coat of paint every 5-7 years is enough. The bleach wash removes protection, so you're smart to re-seal afterward if your siding is bare wood.

That's the whole deal. Grab your gear, pick a cloudy day, and show that green stuff who's boss. Your house will look brand new, and you won't have to fix any pressure washer mistakes like I did. Happy washing.