How to Remove Algae and Mildew from House Siding

I Ripped Off My Own Paint So You Don’t Have To

The first time I tried cleaning mildew off my house, I turned my pressure washer up to “kill it.” I walked up to the siding, pulled the trigger, and watched a perfect six-inch strip of paint peel away like a banana. Water blasted under the lap siding. My neighbor came out and laughed. It took me a whole weekend to fix that mess.

I’ve cleaned dozens of houses since then—my own, my parents’, and some I wish I never agreed to. Green and black gunk on siding is ugly. But if you go at it like a car wash, you will cause real damage. Here’s how to do it right. No BS. Just what works.

What You Actually Need

Don’t use a tiny electric washer that spits at 1,200 PSI. It will take all day and do a bad job. But don’t grab a 4,000 PSI commercial rig either. That will cut wood like butter. I use a 2,800 PSI gas unit with a 1.2 GPM flow rate. That’s the sweet spot for house siding.

Your shopping list:

• Pressure washer: 2,500–3,000 PSI, 1.2–1.5 GPM. Anything above that is overkill.
• Downstream injector: A $25 kit that lets you spray bleach without wrecking the pump.
• 40-degree nozzle: The green one. Wide spray, low pressure. Never use a 0-degree nozzle on siding. I did that once. I now have a permanent stripe on my garage.
• Jug of bleach: Standard laundry bleach. 5–6% sodium hypochlorite. Don’t get the “splash-less” or concentrated stuff.
• Laundry detergent: Powdered Tide works. Yes, really. Dirt cheap and cuts grease better than “house wash” soap.
• Soft scrub brush: A 12-inch brush on a pole. You cannot blast mildew off with water alone. You have to touch the siding.
• Safety glasses and rubber gloves: Bleach mist in your eyes is not a fun afternoon.

Step One: Prep or You’ll Regret It

First, shut off your house’s outdoor water spigot. Wait—no. That’s wrong. Turn it on fully. The pressure washer needs full flow. My mistake was using a kinked hose once. The pump burned out in 10 minutes.

Walk around the house and look for trouble:

• Tape over electrical outlets and light fixtures with plastic bags and painter’s tape. Bleach drips into an outlet = short circuit or fire hazard. I learned this when my outdoor light started smoking.
• Cover any delicate plants with tarps. Hostas are expensive. Bleach kills them dead.
• Wet down all plants and grass around the foundation with a garden hose. This dilutes the bleach runoff and keeps your garden alive.

Don’t skip the wetting step. I once killed a rose bush because I thought “it’ll be fine.” It wasn’t fine.

Step Two: The Magic Mix (Don’t Screw This Up)

Here’s my exact mix for a 5-gallon bucket. This is for vinyl siding. If you have wood or stucco, scroll down to the next section—I cover that.

• 2 gallons of bleach
• 2 gallons of water
• Half a cup of powdered Tide

Stir it gently with a stick. Don’t splash. The soap helps the bleach stick to the wall instead of running off. Without it, the bleach slides down and you end up with cleaner streaks on a dirty wall. That’s a rookie look.

For wood siding (cedar, pine): Use only oxygen bleach (sodium percarbonate). Chlorine bleach eats wood fibers. I ruined a section of cedar siding once. It turned gray and fuzzy within a month. Oxygen bleach costs a bit more, but it’s $20 vs a $500 repair.

For stucco: Use the same bleach mix but never blast it with high pressure. Stucco is porous. High pressure drives water deep, and then you get mold inside your walls. Use a wide fan spray and keep the nozzle at least three feet away.

Step Three: Soak, Don’t Blast

Shut off the pressure washer. Connect the downstream injector. Drop the pickup hose into your bucket of cleaner. Start the machine. Pull the trigger—you’ll see the mix turn soapy.

Work from the bottom of the wall upward. Why bottom first? I’ll tell you. If you start at the top, the bleach runs down over dry, dirty siding and leaves a white streak mark that won’t rinse clean. Starting at the bottom prevents this. Trust me. I cleaned my entire north side top-down once and had to do it again.

Apply the cleaner in horizontal passes. Get a good layer of foam on there. Don’t be shy. Let it sit for 10 minutes exactly. Not 5 minutes—the algae won’t die. Not 20 minutes—the bleach dries out and leaves a white residue that’s a pain to scrub.

Step Four: The Scrub (This Is Where Most People Fail)

Pressure alone will not remove mildew. You need physical scrubbing. Grab that soft brush on a pole. Dip it in your leftover bleach mix. Scrub in a circular motion, hitting each section of siding. Focus on north-facing walls and shaded spots—those hold moisture longest and grow the most gunk.

Here’s a mistake I made: scrubbing with a wire brush. The bristles scratched the hell out of my vinyl. Now I use a soft nylon brush. A dollar-store dish brush works in a pinch for small areas.

If you have a two-story house, use a brush extension. I clamp mine to a fiberglass pole. It bounces less than an aluminum extension. Falling off a ladder while scrubbing is not a fun story to tell.

Step Five: Rinse the Right Way

Take the cleaner bucket off your injector. Run clean water through the system for 30 seconds. Switch to the 40-degree nozzle. Start at the top of the wall and work downward. Keep the nozzle 12-18 inches from the siding. Move in smooth, overlapping passes.

Angle the wand downward at a 45-degree angle. If you spray upward, water gets behind the siding. Water behind siding = mold inside your walls. I did this on my own house in year one. Now I have a fun rot spot I still haven’t fixed.

Rinse twice. The first rinse gets the big stuff. Walk away for 10 minutes. Come back and rinse again. You’ll still see soap residue dripping down. That’s what leaves white streaks when it dries. A thorough second rinse saves you from standing there later with a rag.

The One Product That Actually Works for Tough Stains

I’ve tried every chemical on the shelf. Bleach and Tide is best for vinyl. But if you have persistent black mildew on wood, grab a bottle of Wet & Forget Outdoor. It’s not a pressure washer chemical—you spray it on with a garden sprayer and leave it. It takes two weeks to work. But when bleach fails, Wet & Forget always wins. I used it on my deck railings last year. They went from black to new wood color without any scrubbing. Costs about $25 a gallon.

Avoid anything that says “mildew stain remover” with hydrogen peroxide. It foams up and evaporates too fast. You’ll be scrubbing a dry foam mess in 5 minutes. Not worth it.

Mistakes I’ve Made So You Don’t Have To

• Bleach on cold day: Bleach doesn’t work well below 50°F. I had to redo a whole side because I was impatient. Wait for a warm day above 60°F.
• Not taping windows: Bleach splatter etches glass. I have tiny white spots on my kitchen window that are permanent. Tape or soak the windows down first.
• Using a pressure washer on old peeling paint: If your paint is already cracking, a pressure washer will rip it off. Consider repainting first or use a soft-wash method (just bleach spray and a hose, no pressure).
• Walking backward: You will trip over the hose. I have a scar on my elbow. Walk forward. Pull the hose to your side.

Quick Answers to Questions You Probably Have

Can I use a garden hose instead of a pressure washer?

For non-mildew dirt, yes. For algae and mildew? No. The bleach needs to dwell and the garden hose won’t get enough water volume to rinse properly. You’ll just spread the mess around. Rent a washer for $40 if you don’t own one.

What about bleach on painted brick?

Fine. Brick loves bleach. But pressure wash brick with caution—the mortar is soft. Use the 40-degree nozzle at 2 feet and keep the wand moving. Mortar pits easily.

My siding is vinyl. Can I ever use a turbo nozzle?

No. Turbo nozzles have a rotating jet of water. On vinyl, that rotating jet hits the seams and blows them apart. I popped a whole row of siding off once. Not joking. Stick to the green 40-degree.

How often do I need to do this?

Every 1-2 years for north-facing walls. Every 3 years for the rest. If you live in a humid climate, you’re looking at yearly cleaning. The bleach kills the spores, but they float back in eventually.

Can I pressure wash my siding in direct sun?

If you have to, do it early morning. Direct sun dries the bleach too fast. You get white residue and half-dead mildew. I did a section at 2 PM once and it looked like a zebra. Misted the wall down first and it helped, but not much.

That’s the whole deal. Go kill that green stuff. And don’t forget to tape the outlets.