How to Clean a Composite Deck

Composite decking looks like wood and ages gracefully, but it's actually a blend of wood fibers and plastic binders that require different care than pressure-treated lumber. The appeal of composite is low maintenance, but that doesn't mean no maintenance. Dirt, algae, and mildew accumulate in the texture just like on wood, and if you ignore it for seasons, staining and discoloration set in harder than they should. A proper cleaning every spring and fall keeps the deck looking rich and prevents the graying that makes composite look neglected. The work is simple—no special skills, no renting equipment—and takes an afternoon.

1. Clear Everything First. Remove all furniture, planters, grills, and loose items. Sweep out leaves, dirt, and debris with a broom. Pay attention to corners, beneath railings, and the spaces where boards meet the rim joists—this is where organic matter collects and feeds algae growth.
2. Mix the Right Solution. Fill a bucket with one gallon of warm water and add one cup of mild deck cleaner or a biodegradable all-purpose soap. For stubborn algae or mildew, use a composite-specific cleaner (not wood stripper or oxygen bleach, which can damage the binders). Stir the mix well. Do not use chlorine bleach, which breaks down the resin matrix in composite.
3. Soak Before You Scrub. Use a standard garden hose to wet the deck thoroughly. This opens the pores of the composite and helps the cleaning solution penetrate. Spray from one end and work toward a drain point so water flows freely rather than pooling.
4. Brush the Entire Surface. Dip a soft-bristle deck brush into the cleaning solution and scrub the deck boards in the direction of the wood grain pattern (if your composite has one). Use firm, circular motions for heavy soiling, longer strokes for general cleaning. Work in sections from one end toward the other, keeping the surface wet. Don't let the solution dry on the boards.
5. Hit the Problem Spots. If algae, mildew, or rust stains persist after the initial scrub, dip the brush in full-strength composite cleaner and work the spot for 30 seconds. Do not let the concentrated solution sit; keep scrubbing and working toward rinsing. For tannin stains (reddish discoloration), a second application of solution and brush usually lifts them. For grease or oil, apply a few drops of dish soap directly to the stain, scrub, and rinse immediately.
6. Rinse Until Clean Water Flows. Use the garden hose to rinse the entire deck with clean water. Start at the high end and move toward the drainage direction. Make two passes: the first to remove soap and loosened dirt, the second to ensure no residue remains. Pay special attention to seams, under railings, and board edges where soap can hide.
7. Dry Out Hidden Seams. Walk the deck and look for water pooled in board seams or in the gaps between boards and railings. Use a dry cloth or old towel to wipe out these spaces. If your deck has a significant slope or drainage issue, use a leaf blower to force air through the gaps and accelerate drying.
8. Wait for Complete Dryness. Wait for the deck to dry completely before using it or returning furniture. In warm, sunny conditions, this takes 2–4 hours. In overcast or humid conditions, plan for 6–8 hours or overnight. Don't walk on the deck or replace items until it's bone dry to the touch.
9. Spot-Check in Daylight. Once dry, take a walk around the entire deck in daylight. Look for streaks, soap residue, or spots you missed. If you find any, spray those areas with water and wipe with a damp cloth. Check the underside of railings and any built-in benches—these catch dripping water and develop algae that spreads to the main surface.
10. Restore and Protect. Once the deck is fully dry, replace your furniture and outdoor items. If your composite deck is older or the manufacturer recommends it, apply a composite deck sealer in the fall (typically once every 3–5 years). Follow the sealer's drying time before using the deck again.