How to Get Mildew Out of Deck: 5 Proven 2026 Fixes

By David Park · January 9, 2026

Immediate Action (first 5 minutes)

When you spot black or green fuzz creeping across your outdoor wood, don't panic. Mildew spreads fast, especially after a humid summer weekend. I've dealt with this exact issue on my own cedar patio after hosting a dinner party. The spilled soda and damp shade created a perfect storm for fungal growth. Your first five minutes dictate how deep the stain sets into the grain.

Grab a stiff-bristled push broom. Sweep away loose debris, leaves, and standing water immediately. Don't use a leaf blower. It just scatters the microscopic spores into your nearby garden beds. Next, grab a garden hose with a spray nozzle. Rinse the affected area with cold water at around 60°F. Hot water actually opens the wood pores. This drives the fungal roots deeper into the timber. Keep the nozzle about 12 inches from the surface. You just want to wash away the surface slime. I'll be honest, this takes elbow grease if the wood is heavily textured. But doing this right away stops the mildew from eating into your expensive sealant.

If you have kids who track mud everywhere, those muddy footprints trap moisture and accelerate this whole process. Mud holds water against the wood fibers. Scrub those specific high-traffic zones with a nylon brush right away. When your landlord inspects next week, you need this done fast and thoroughly.

What You'll Need (tools/products)

I've tested dozens of products over my 12 years restoring outdoor spaces. You don't need a truckload of expensive chemicals to get the job done. Here is the exact gear I keep in my garage for this specific job.

Cleaning Solutions

Oxygen Bleach (e.g., OxiClean Outdoor): A 5-pound tub costs around $18-22. It's safe for plants and won't strip your wood's natural tannins.
Commercial Deck Wash (e.g., Simple Green Oxy Solve): Runs about $12-15 per gallon. Great for composite decking like Trex or TimberTech.
White Vinegar: Keep a gallon jug handy. It costs $4-6 and works wonders on light surface spots.

Tools and Equipment

Pump Sprayer: I use the Chapin 1-Gallon Poly Sprayer ($15). It gives you a consistent mist without killing your forearm.
Stiff Bristle Brush: Get the Quickie 8-inch Deck Scrub Brush ($12). The polypropylene bristles dig into grooves without splintering the wood.
Pressure Washer (Optional): If you own one, use a 40-degree white tip. The Sun Joe SPX3000 is my go-to model. Keep the PSI under 1500 for softwoods like pine or cedar.
Safety Gear: Nitrile gloves and basic safety goggles. Oxygen bleach isn't toxic, but it'll dry out your skin fast.

Cleaner Comparison Table

Product Type	Best For	Price Range	Plant Safe?
Oxygen Bleach	Natural Wood	$18 - $25	Yes
Chlorine Bleach	Concrete only	$5 - $8	No
Vinegar Mix	Light spots	$4 - $6	Yes
Commercial Wash	Composite	$12 - $18	Mostly

Step-by-Step Removal

This is the exact method I use when prepping a house for sale. It takes about 90 minutes for a standard 12x15 foot space. Follow these steps precisely to avoid damaging the timber.

Step 1: Prep and Protect Surroundings

Before you spray any chemicals, soak your nearby shrubs with plain water. This fills the plant's pores so they don't absorb the cleaning solution. Lay down a cheap plastic drop cloth over any delicate flowers. I always use 2-mil thick painter's plastic. It costs about $8 a roll and saves you from killing your wife's prized hydrangeas.

Step 2: Mix Your Cleaning Solution

Fill your 1-gallon pump sprayer with warm water. Aim for about 90°F. Add 2 cups of powdered oxygen bleach. Screw the lid on tight. Shake it vigorously for 30 seconds. Let it sit for five minutes so the powder fully dissolves. If you see clumps, shake it again. Undissolved powder will clog the spray nozzle and ruin your afternoon.

Step 3: Apply and Dwell

Pump the handle until you feel solid resistance. Spray the mildew-covered boards evenly. You want the wood to look wet, but not pooling. Work in 4x4 foot sections. Don't try to do the whole deck at once. The solution needs to dwell for exactly 15 minutes. This gives the oxygen ions time to break the fungal cell walls. If the sun is beating down and it dries fast, mist it lightly with plain water.

Step 4: Scrub the Grain

Grab your Quickie scrub brush. Scrub firmly along the length of the boards. Never scrub across the grain. You'll fray the wood fibers and create a fuzzy mess. Pay extra attention to the joints and corners. Mildew loves hiding in those damp crevices. I usually spend about 3 minutes per board. Your arms will burn a bit. That's normal. Wear knee pads if you have bad joints.

Step 5: The Rinse Off

Switch your garden hose to the flat or shower setting. Rinse the deck thoroughly from the highest point down to the lowest. Push all the dirty water off the edge. If you're using a pressure washer, switch to the 40-degree white tip. Hold the wand 18 inches away. Sweep in long, even strokes. Keep the wand moving. Lingering in one spot for even two seconds will gouge the wood permanently.

Step 6: Inspect and Repeat

Let the wood dry for at least 4 hours. The sun needs to hit it. Wet wood looks darker, which hides remaining stains. Once it's bone dry, check for stubborn black spots. If you see any, mix a paste of baking soda and water. Apply it directly to the spot. Let it sit for 20 minutes, scrub, and rinse again.

Alternative Methods

Sometimes the standard oxygen bleach route doesn't cut it. I've had to pivot when dealing with older, deeply stained pressure-treated pine. Here are two backup strategies I rely on when the first attempt fails.

The Vinegar and Baking Soda Fizz

This works best for small, localized patches near your garden beds. Pour undiluted white vinegar straight onto the mildew. Let it soak for 10 minutes. The acetic acid kills about 82% of mold species. Next, sprinkle a heavy layer of Arm & Hammer baking soda over the wet vinegar. It will fizz aggressively. This mechanical action lifts the spores out of the wood pores. Scrub with a stiff nylon brush. Rinse with cold water. This isn't the cheapest option for a massive deck, but it's incredibly safe for your landscaping.

Commercial Sodium Percarbonate Strippers

If your deck hasn't been sealed in five years, mildew gets trapped under peeling stain. I use DEFY Wood Cleaner for this. It costs around $35 a gallon. You mix it at a 1:4 ratio with water. Apply it with a paint roller. Let it sit for 20 minutes. The sodium percarbonate actually strips the old, failing sealant while killing the mildew. You'll need to rinse it off and then apply a fresh coat of penetrating oil stain afterward. It's a bigger project. But it saves you from renting a $75 floor sander from the local hardware store.

Prevention Tips

Killing the mildew is only half the battle. If you don't change the environment, it will return by next spring. I learned this the hard way after cleaning my parents' shaded backyard oasis. Follow these rules to keep the wood pristine.

Trim the Canopy: Cut back tree branches so they are at least 3 feet above the deck. Sunlight is the ultimate mildew killer. UV rays dry the wood and destroy spores naturally.
Clear the Gaps: Use a 5-in-1 painter's tool to scrape out debris between the boards. Leaves and pine needles trap moisture. I do this every November before the snow flies.
Apply a Water Repellent: Once the wood is completely dry, roll on a water-repellent sealer. I prefer Thompson's WaterSeal Advanced Natural Wood Protector ($28 per gallon). It contains mildewcides that prevent future growth.
Rearrange Furniture: Move your planters and outdoor rugs every two weeks. Stagnant air under a heavy ceramic pot breeds fungus. Give the wood a chance to breathe.
Clean Your Gutters: Clogged gutters overflow right onto your deck edges. That constant dripping creates a permanent damp zone. Clear them out twice a year.

FAQ

Can I use regular household chlorine bleach on my wood deck?

I strongly advise against using chlorine bleach on natural wood. It breaks down the lignin, which is the glue holding the wood fibers together. Your deck will turn a sickly gray and become structurally brittle over time. Stick to oxygen bleach or specialized deck washes.

Why does my composite decking still get mildew?

Composite boards like Trex or TimberTech are made of wood flour and plastic. The wood flour inside the plastic matrix can still feed mildew if the surface gets scratched. Dirt and pollen build up in those scratches, creating a food source. Wash your composite deck with soap and water every few months to prevent this.

How long should I wait to stain my deck after washing it?

You must wait at least 48 to 72 hours before applying any stain or sealer. The wood needs to reach a moisture content below 15%. If you seal damp wood, you'll trap the moisture inside and cause the new stain to peel within six months. Buy a cheap $12 wood moisture meter to be absolutely sure.

Will a pressure washer alone remove the mildew?

A pressure washer will blast away the surface fuzz, but it won't kill the fungal roots. The spores will simply grow back in a few weeks. You always need a chemical or natural cleaner to break down the cell walls. Use the pressure washer only as a rinsing tool on a low setting.