Green stains on a deck usually indicate that the wood has fallen prey to algae. Algae will grow almost anywhere if the conditions are right, and your deck is no exception. An algae stain on wood will be greenish brown in appearance and slippery to the touch. Because algae is caused by excessive moisture, you are more likely to find it in shady areas of the deck that do not have the benefit of being rapidly dried by the sun after it rains. Rather than hire a professional, you can clean these pesky green stains yourself with a little bit of hard work.
Things You'll Need
Acid- or bleach-based deck cleaner
Remove all furniture and other items from the deck. Not only will this make cleaning easier, but you are likely to find additional spots of algae lurking in areas such as behind the grill or beneath patio furniture.
Sweep the deck well. Be sure to remove all leaves and debris from the cracks and corners. Piled-up leaves lock in moisture and algae commonly thrives beneath them.
Dilute an oxygen-based bleach with water. Clean the entire deck with the bleach mixture. This will kill the algae you can see along with the developing algae you cannot.
Allow the bleach mixture to sit on the deck for 10 to 20 minutes. After the time is up, rinse the deck thoroughly with the garden hose on the highest pressure setting.
Allow the deck to dry, and examine the algae stains. Stains that are still visible when the deck is dry may require further treatment.
Examine your acid- or bleach-based deck cleaner. Make sure any acid-based cleaners that you use are compatible with your current deck sealant. If you do not wish to use harsh chemicals, you may treat the algae with a potassium-based algae-removal product.
Saturate the plants surrounding your deck with the garden hose. It is likely they will be exposed to the cleaner you will be using, and this will help protect them from damage.
Pour the cleaner directly onto the algae stains. Allow it to sit for 10 minutes. If you have particularly stubborn algae stains, you may opt to use a stiff-bristled brush to scrub the stain with the cleaner.
Rinse the deck thoroughly with the garden hose. The algae should be completely gone when the deck dries.
Water the surrounding plants again to help disperse any of the chemicals that they have come into contact with.
Consider sealing your deck with a clear epoxy to prevent algae growth.
Oxygen bleach is nontoxic to plants, and thus it is not necessary to spray plants before and after using it.
Acid-based cleaners are very caustic. Always wear gloves and protect your eyes when working with them.