We manufacture all products to be highly resistant to mold and other damage from moisture. In rare cases and depending on environmental conditions (temperature, moisture in the air, etc.), mold and mildew could appear on any outdoor surface. If mold occurs on the surface of TimberTech decking, it does not penetrate through the deck board. Surface mold and mildew on TimberTech decking can be removed using TimberTech DeckCleaner or a 1-to-4 mixture of bleach and water.