Deck mold, mildew, or slippery surface

Symptoms

• Green, black, or gray discoloration on deck boards
• Surface feels slick or slippery, especially when wet
• Growth concentrated in shaded or low-airflow areas

Likely causes

• CommonAlgae or mildew growth from moisture and shadeDamp, shaded wood or composite surfaces are prime conditions for growth.
• CommonPoor drainage or standing water on the deck surfaceWater that doesn't run off quickly gives growth more time to establish.
• PossibleFailed or worn sealant no longer shedding waterOnce sealant wears off wood decking, the surface holds moisture longer.

What to do

1. Sweep away leaves and debris that trap moisture against the deck surface.
2. Scrub the affected area with a deck-safe cleaner made for mold and mildew, following label directions.
3. Rinse thoroughly with a garden hose; avoid high-pressure washing that can gouge wood or composite surfaces.
4. Let the deck dry fully, then assess if wood needs resealing to help shed water going forward.
5. Trim back overhanging branches or plants that keep the area shaded and damp.
6. Improve airflow under the deck if possible, since trapped humidity underneath contributes to surface growth.
7. Repeat cleaning seasonally in shaded, damp-prone areas rather than waiting for heavy buildup.

Tools and parts

• Stiff outdoor broom
• Garden hose
• Scrub brush

When to call a pro

Call a deck cleaning or pressure-washing service for larger decks or persistent growth.

Safety