Remove Mildew from Deck Boards

Mildew on deck boards isn't just ugly — it's the visible edge of a deeper problem. Those black or green streaks signal moisture sitting too long, organic matter breaking down, and wood fibers starting to compromise. Left alone, mildew becomes mold, mold becomes rot, and rot becomes replacement. The good news is that catching it early means a Saturday afternoon with the right cleaner brings the wood back. Done right, this isn't about bleaching the surface white — it's about killing the organism at root level and restoring the wood's ability to shed water. The work itself is straightforward but specific. Oxygen bleach does the heavy lifting without the toxicity or wood damage of chlorine bleach. You're not power-washing at full blast or scraping with wire brushes. You're applying chemistry, giving it time to work, then removing what's dead. The difference between a clean that lasts six months and one that lasts three years comes down to how thoroughly you rinse and whether you follow up with a sealer once the wood dries.

1. Strip the deck bare first. Remove all furniture, planters, and grills. Sweep the entire surface with a stiff push broom, working debris out from between boards. Pay attention to corners and edges where organic matter collects. You want bare wood with no loose dirt — the cleaner needs direct contact with the mildew.
2. Mix the chemistry fresh. Fill a 5-gallon bucket with warm water and add oxygen bleach powder according to package directions — typically 1 cup per gallon for heavy mildew. Stir until fully dissolved. The solution works best when fresh and warm, so mix only what you'll use in the next hour.
3. Spread and wait it out. Pour or pump-spray the solution onto the deck boards, working in sections you can complete before the solution dries. Use a deck brush or push broom with synthetic bristles to spread it evenly and work it into the wood grain. Let it sit for 15 minutes. You'll see the mildew start to lift and foam slightly as the oxygen does its work.
4. Let chemistry win the fight. After the dwell time, scrub any remaining dark spots with the deck brush using circular motions. Most mildew will have already broken down, but older stains need mechanical help. Don't scrub so hard you're fuzzing up the wood fibers — let the chemistry do most of the work.
5. Flush it all away. Rinse the entire deck with a garden hose, using a fan nozzle to push water across the boards. If using a pressure washer, keep it under 1200 PSI and hold the wand at least 12 inches from the surface. Work systematically to avoid missing spots. Rinse until no suds or residue remain — leftover cleaner will interfere with sealers later.
6. Assess in full daylight. Let the deck dry for 24 hours, then inspect under direct sunlight. Any remaining dark spots are either deep stains or actual wood discoloration, not active mildew. If needed, spot-treat with a second application of oxygen bleach solution.
7. Restore wood's true color. If the wood looks clean but dull or grayish, apply a deck brightener solution to restore the natural color. This is optional but recommended before sealing. Follow product directions for mixing and application, then rinse thoroughly and let dry for 48 hours.
8. Lock protection in place. Once the wood is completely dry, apply a water-repellent sealer or stain to protect against future mildew. Clean wood is vulnerable — sealing it while it's open and receptive prevents moisture from re-establishing. Choose a product with mildewcide if your deck stays damp or shaded.