How to Clean a Deck: Wood vs Composite, Mildew and When to Seal

For a wood deck, the safest effective cleaner is an oxygen bleach (sodium percarbonate) deck cleaner mixed per the label, scrubbed in with a stiff synthetic brush and rinsed with a garden hose. For a composite deck, start with warm water, dish soap and a soft brush, and follow your decking maker’s care guide for anything stronger. Skip the pressure washer on wood unless you know how to use it at low pressure, because it can gouge and splinter the boards.

Most decks need one thorough cleaning a year, usually in spring, plus regular sweeping. Here is how to do it for each decking type.

Wood vs composite at a glance

What you need

• A broom and a putty knife or thin scraper for the gaps between boards
• Oxygen bleach deck cleaner (for wood) or dish soap (for composite)
• A stiff synthetic-bristle brush on a long handle (soft-bristle for composite)
• A garden hose with a spray nozzle and a pump sprayer or watering can
• Rubber gloves and safety glasses
• Plastic sheeting to protect plants, siding and furniture

How to clean a wood deck

1. Clear and sweep. Move furniture, grills and planters. Sweep thoroughly and push debris out of the gaps with a putty knife so water can drain.
2. Protect the surroundings. Wet nearby plants and cover them, and cover the lower edge of the siding if the cleaner label advises it.
3. Wet the deck. Rinse the boards with a hose. Work on a cool, overcast day or in the morning so the cleaner does not dry too fast.
4. Mix and apply the cleaner. Mix the oxygen bleach product with water exactly as the label says and use it soon after mixing, since it loses strength. Apply with a pump sprayer or watering can, section by section.
5. Let it work for the dwell time on the label, keeping it wet.
6. Scrub with the grain using the stiff synthetic brush.
7. Rinse thoroughly with the hose until no residue remains, and rinse the plants again.
8. Let it dry completely before putting furniture back or applying any finish.

Some deck-care systems follow the cleaner with a wood brightener, often based on oxalic acid, to restore color. If you use one, apply it only after the cleaner is fully rinsed off, as its label directs. Never mix a brightener or any acid with chlorine bleach.

Why not chlorine bleach on wood?

Household chlorine bleach can lighten mildew stains quickly, but deck-care guides commonly warn that it can strip the wood’s natural color, weaken surface fibers, corrode metal fasteners and harm plants. Oxygen bleach cleaners are the usual recommendation for wood decks. If you do use any bleach product, follow its label exactly, rinse well and never combine it with other cleaners; see our guide on cleaning products you should never mix.

Pressure washing a wood deck

A pressure washer can erode the softer wood between the growth rings, leaving a fuzzy surface, raised splinters and visible lines. That damage is hard to undo without sanding. A brush and cleaner usually give a better result.

If you choose to use one, keep the pressure low, use a wide 25- or 40-degree tip, hold the wand well back from the surface and keep it moving along the length of the boards. Never use the 0-degree tip, and never point the wand at skin; high-pressure spray can cause serious injection injuries that need emergency care. Our guide to pressure washing a house explains the tips and safety rules in more detail.

How to clean a composite deck

1. Sweep and clear the gaps. Debris trapped between boards holds moisture.
2. Wash with soap and water. Use warm water, a little dish soap and a soft-bristle brush, scrubbing along the board’s embossed grain.
3. Rinse thoroughly. Composite makers note that dirty water left to dry can leave a film on the surface.
4. Use only approved cleaners for mildew, grease or rust spots. Each manufacturer lists what its boards can tolerate.

Pressure washer rules differ by brand and even by product line. For example, Trex’s care and cleaning page says certain current lines can be cleaned with a pressure washer up to 3,100 psi using a fan tip held at least 8 inches from the surface, while earlier generations should be cleaned with a hose, soap and a soft brush instead. Going beyond the maker’s limits can damage boards and affect the warranty, so look up the rules for your exact decking.

Mildew, algae and stains

• Mildew and green algae grow in shade and on boards that stay damp. Clean with the appropriate cleaner for your decking type, trim back plants for airflow and keep gaps clear so the deck dries faster. For a primer on what you are looking at, see black mold vs. mildew.
• Tree sap on boards and railings can be loosened with the methods in our guide to getting tree sap off; test any solvent on a hidden area first, especially on composite.
• Rust marks from planters and furniture: wood brighteners often lift them on wood; on composite, use the maker’s recommended product.

Wear gloves and eye protection when using deck cleaners, keep products in their original containers and keep children and pets off the deck until it is rinsed and dry.

If someone swallows, inhales or gets a cleaning product in the eyes, call Poison Control at 1-800-222-1222 (US) or emergency services.

When to seal or stain a wood deck

A simple check: sprinkle a little water on the boards. If it beads up, the finish is still working. If it soaks in and darkens the wood quickly, it is time for a fresh coat.

• Clean first. Stain and sealer need a clean, dry surface to bond.
• Wait for the wood to dry. Follow the product label for dry time and the weather window, which typically means a stretch of dry days without rain in the forecast.
• New pressure-treated wood often needs to dry out before it can be finished; check the lumber and stain makers’ guidance.
• Composite does not need sealing or staining unless the maker says otherwise.

Before you start, check for loose or protruding nails and screws, soft or rotting boards and wobbly railings. Structural problems are a job for a qualified deck contractor.

Bottom line

Sweep regularly and give the deck one proper cleaning a year. Use oxygen bleach deck cleaner and a stiff brush on wood, soap and water plus the maker’s rules on composite, and keep the pressure washer gentle or leave it in the garage. Reseal wood when water stops beading on the surface.

FAQ

Can I use vinegar to clean my deck?

Diluted white vinegar can help with light mildew on some decks, but it is not a disinfectant and is weaker than a dedicated deck cleaner. Check your composite maker’s guide before using it, and never combine it with bleach.

How often should I clean my deck?

Sweep as often as leaves and dirt collect, and do a full cleaning at least once a year. Shady or damp decks may need it more often.

Does oxygen bleach harm plants?

It is generally considered gentler than chlorine bleach, but concentrated solution can still affect plants. Wet them before and after, cover them, and follow the label.

Should I clean my deck before winter?

Yes, a sweep and rinse in fall clears leaves and debris that would sit wet all winter and encourage mildew and stains.