Wet & Forget Outdoor Cleaner Concentrate eliminates black and green stains caused by mold, mildew and algae stains without scrubbing, power washing or using bleach. Just mix 5 parts water with 1 part Wet & Forget in a pump up garden sprayer and apply to the surface - that's it! Wet & Forget goes to work with the wind and rain to gently clean the surface over time. Green stains clean up within a few days or weeks. Black stains generally clean up in 4-6 weeks. Black roof stains or stains that have been on the surface for many years can take up to 6 months to come clean. Be Patient, Good Things Take Time! Surfaces are properly cleaned so you get an extended clean. Surfaces stay clean up to 1 year or longer. Re-apply at the first sign of reoccurrence or annually. Use Wet & Forget on decks, siding, roofs, walkways, patios, sealed brick, sealed cement, or any hard, non-porous outdoor surface. Half-Gallon Concentrate makes 3-Gallons and covers 375-1, 125 sq. ft. depending upon severity of the stain and porosity of the surface.