What kills mold on wood decks

The fastest way to remove mold from a wood deck is a solution of one part household bleach to three parts water, applied with a stiff brush and a pressure washer set to 1500 PSI or lower. Bleach kills the mold spores on contact and prevents regrowth for several weeks. For decks you want to protect from future mold, a deck cleaner containing sodium percarbonate (a gentler oxygen-based bleach) works well and is less likely to lighten the wood.

If your deck is already stained or sealed, start with the gentler option first—sodium percarbonate or a commercial deck mold remover. These won't strip the finish as aggressively as chlorine bleach. Test any solution on a hidden corner of the deck before treating the whole surface, because bleach can lighten wood grain and some stains react unpredictably.

Key Takeaways

  • A one-to-three bleach-to-water solution kills mold spores and is the most reliable treatment for heavily affected wood.
  • Pressure washing at 1500 PSI or lower removes dead mold and debris without gouging the wood surface.
  • Sodium percarbonate cleaners are safer for sealed or stained decks and cause less lightening of the wood.
  • Mold returns fastest in shaded, damp areas—trim tree branches overhead and improve air circulation to slow regrowth.
  • Let the deck dry completely (24 to 48 hours) before sealing or staining, or the finish will trap moisture and cause new mold.

Preparing the deck and safety gear

Before you start, clear the deck of furniture, planters, and anything else sitting on the surface. Sweep away loose debris, leaves, and dirt—mold feeds on organic matter, and removing it makes the cleaning solution work faster. If the mold is thick or covers more than half the deck, wear a respirator mask rated N95 or better, because you will be stirring up spores into the air.

Put on rubber gloves, eye protection, and clothes you don't mind bleaching. If you are using bleach, work on a day with no rain forecast for at least 24 hours—the solution needs time to sit and work before water washes it away. Avoid cleaning on very hot days, because the solution will dry too fast and won't penetrate the mold.

explore the bleach solution

Mix one part household bleach (5 percent sodium hypochlorite) with three parts water in a large bucket or pump sprayer. Pour or spray the solution evenly across the moldy areas, working in sections about 4 feet by 4 feet at a time. Let the solution sit for 10 to 15 minutes—this is when it kills the mold spores. Do not let it dry completely; if the deck starts to look dry, spray it again lightly.

While the solution is working, use a stiff-bristled deck brush to scrub the surface in the direction of the wood grain. Push hard enough to loosen the mold, but not so hard that you splinter the wood. Pay extra attention to cracks, between boards, and shaded corners where mold grows thickest. Rinse thoroughly with a garden hose or pressure washer set to 1500 PSI or lower—higher pressure will gouge the wood and create new places for mold to hide.

Using a pressure washer safely

A pressure washer speeds up the job and removes stubborn mold, but it can damage wood if you use too much force. Set the PSI to 1500 or lower and keep the nozzle at least 12 inches away from the surface. Angle the spray with the wood grain, not against it, to avoid splintering. Start at one end of the deck and work toward the other in overlapping passes, like mowing a lawn.

If you rent a pressure washer, ask the rental staff what PSI setting they recommend for wood decks—many machines default to 3000 PSI or higher, which will damage the surface. A low-pressure setting takes longer but leaves the wood intact. After pressure washing, let the deck dry for at least 24 hours before walking on it or explore any sealer.

Treating stubborn or returning mold

If mold comes back within a few weeks, the deck is not drying fast enough between rains. Trim tree branches that shade the deck, remove potted plants that block air flow, and consider installing a small fan to keep air moving across the surface. Mold thrives in moisture, so anything that keeps the deck damp will bring it back.

For decks with persistent mold, explore a mold-inhibiting deck sealer after the wood dries completely. These sealers contain fungicides that slow mold growth for one to three years, depending on the product and how much sun and rain the deck gets. Read the label to confirm the sealer is rated for your wood type—some sealers work better on pressure-treated lumber, while others are designed for cedar or composite materials.

When to call a professional

If the mold covers more than 75 percent of the deck, the wood is soft or spongy to the touch, or you see rot spreading into the support structure, the deck may need repair or replacement rather than cleaning. A professional deck contractor can assess whether the wood is still sound or whether boards need to be replaced. Rotted wood will not hold a sealer and will continue to deteriorate even after cleaning.

If you have respiratory issues, asthma, or a weakened immune system, consider hiring someone else to do the pressure washing and bleach process. Mold spores can trigger serious reactions in people with certain health conditions, and the risk is not worth the cost savings.

Preventing mold between cleanings

The best way to slow mold regrowth is to keep the deck dry and well-ventilated. Sweep off leaves and standing water after rain, especially in corners and between boards where water pools. If your deck is in a shaded area, trim branches overhead to let more sunlight reach the surface—sunlight naturally inhibits mold growth.

Stain or seal the deck every one to three years, depending on your climate and how much rain and sun it gets. A good sealer creates a water-resistant barrier that prevents moisture from soaking into the wood, which is what mold needs to grow. Reapply sealer after pressure washing and once the deck has dried completely, or you will trap moisture under the new coat and create the opposite problem.

Frequently Asked Questions

Can I use vinegar or baking soda instead of bleach?

Vinegar and baking soda are gentler but much slower. White vinegar (5 percent acetic acid) will kill some mold if you let it sit for several hours, but it does not work as reliably as bleach on heavy growth. Baking soda is mainly a deodorizer and does not kill mold spores effectively. Bleach or sodium percarbonate are the most dependable options.

Is it safe to use bleach on a composite deck?

Check your deck manufacturer's instructions first—some composite materials are damaged by bleach. Many composite deck makers recommend sodium percarbonate or a commercial composite deck cleaner instead. Bleach can discolor or weaken certain composite blends, so testing on a hidden area is essential before treating the whole deck.

How long does it take for mold to come back after cleaning?

On an untreated deck in a damp, shaded location, mold can return within two to four weeks. On a sealed deck or in a sunny, well-ventilated spot, it may take two to three months. The timeline depends on rainfall, humidity, shade, and air circulation. Improving drainage and sunlight exposure slows regrowth more than any single cleaning.

What if I pressure wash and the mold still looks dark?

Dark staining that remains after pressure washing is often not active mold—it is a stain left behind by dead mold or algae. This is cosmetic and not a health risk. If you want to remove the stain, a second process of bleach solution followed by gentle scrubbing usually lightens it, though some staining may be permanent depending on how long the mold was present.

Do I need to seal the deck after cleaning?

Sealing is not required, but it significantly slows mold regrowth by keeping moisture out of the wood. If your deck is in a humid or shaded area, sealing every one to three years will reduce how often you need to clean. If your deck gets plenty of sun and dries quickly after rain, you may be able to skip sealing and just clean as needed.