Green streaks on vinyl siding, black stains running down the north slope of a roof, and pink film building up in the bathtub all share the same root cause and respond to the same chemistry. The right algae cleaner kills the organism on contact, lifts the staining, and leaves behind enough residual that re-colonization stays delayed for months. Picking the wrong product, or the right product at the wrong dilution, either fails to clean or strips the finish off the surface you are trying to save. The chemistry is simpler than the marketing makes it look once you know which active ingredient solves which problem.
What You Are Actually Killing
The green streaks on north-facing siding are usually Trentepohlia or Klebsormidium, two algae species that thrive in shaded, damp microclimates. The black stains on asphalt shingles are Gloeocapsa magma, a cyanobacterium often miscalled algae. The pink slime in showers and around hose bibs is Serratia marcescens, a bacterium. Moss and lichen are separate organisms that often grow alongside algae and need different treatment.
Each responds to one of three active ingredients. Sodium hypochlorite, also known as bleach. Sodium percarbonate, which is a powdered oxygenated cleaner. Or quaternary ammonium compounds, the same family used in hospital disinfectants. Match the right active to the right surface and the cleaning is straightforward.
The Best Off-the-Shelf Products
For exterior siding and concrete, Wet & Forget is the most-recommended option. It uses alkyl dimethyl benzyl ammonium chloride at a 5 percent concentration and works without scrubbing or rinsing. A 64-ounce concentrate at $18 covers 750 square feet of siding when diluted 1:5 with water in a pump sprayer.
- Wet & Forget Outdoor: $18 per 64 oz, no rinse, gentle on plants
- 30 Seconds Outdoor Cleaner: $22 per gallon, sodium hypochlorite based, faster acting
- Mold Armor House Wash: $14 per gallon, garden hose end sprayer, bleach concentrate
- Spray & Forget Roof Cleaner: $48 per gallon concentrate, specifically labeled for shingles
- OxiClean Concentrated: $15 per 7 lb tub, sodium percarbonate, safe for plants and pets
For interior algae on grout, tile, and shower walls, Tilex Mold and Mildew Spray or CLR Bath and Kitchen Cleaner handle most jobs at $5 to $9 per bottle. Both contain sodium hypochlorite at a higher concentration than household bleach so they work faster on porous grout.
The DIY Bleach Solution
For a fraction of the cost of name-brand products, a homemade algae cleaner mixes one part household bleach (sodium hypochlorite 5.25 percent) with three parts water and adds one tablespoon of liquid dish soap per gallon as a surfactant. The soap helps the solution cling to vertical surfaces long enough to penetrate the algae cell walls.
This mix costs about $1.50 per gallon to make. Apply with a pump sprayer, wait 15 to 20 minutes, then rinse with a garden hose at 40 to 60 psi. Pre-wet surrounding plants before application and rinse them thoroughly after to dilute any overspray. Never mix bleach with ammonia, vinegar, or any acid. The chlorine gas released is toxic and has killed homeowners attempting DIY combinations.
Cleaning Roof Algae the Right Way
Roof algae need a soft wash, not a pressure wash. Pressure washing strips the granules off asphalt shingles and shortens roof life by 10 to 15 years. The Asphalt Roofing Manufacturers Association explicitly recommends a chemical-only soft wash using a 1:1 mix of household bleach and water with a surfactant.
Apply from the bottom of the roof working upward to prevent stripes, soak for 15 to 20 minutes, then rinse from the top down using a garden hose. Most stains disappear immediately. Stubborn stains may need a second application a week later as remaining algae cells die and release their pigment.
Zinc or copper strips installed along the ridge prevent re-growth for 15 to 20 years. Each rainfall washes a thin film of zinc oxide down the slope and inhibits algae colonies before they establish. Strips cost $25 per 50 linear feet at Home Depot and install with the same nails used for ridge cap shingles.
Deck and Concrete Algae
Pressure-treated decks accumulate algae fastest on north-facing or shaded sections. A 50:50 mix of oxygen bleach (sodium percarbonate) and water applied with a stiff-bristle brush kills algae and lightens grayed wood at the same time. Scrub with the grain, wait 10 minutes, then rinse. The deck will look noticeably brighter and the algae will be gone.
Concrete patios, walkways, and driveways tolerate a stronger mix. A 1:2 bleach-to-water dilution applied with a pump-up sprayer and allowed to dwell 20 minutes lifts algae and the dark staining underneath. Follow with a pressure wash at 2,500 to 3,000 psi using a 25-degree tip.
What Surfaces Need a Gentle Approach
Three surfaces require special care. Painted wood siding can be damaged by sodium hypochlorite if left to dwell more than 5 minutes. Always rinse painted surfaces within that window. Composite decking like Trex and TimberTech tolerates oxygen bleach but should never see chlorine bleach, which can lighten the manufactured color. And stone surfaces, especially limestone and travertine, etch under acidic cleaners and lose color under bleach. For natural stone, stick to a dedicated stone cleaner like StoneTech Mold and Mildew Stain Remover at $30 per quart.
Preventing Re-Growth
Even the most thorough cleaning fails if the underlying conditions stay the same. Algae need three things: moisture, shade, and a food source like organic dust or pollen. Trim back overhanging tree branches to let more sun reach affected walls, redirect downspouts away from siding, and rinse the wash-prone elevations twice a year with a garden hose.
For roof and siding, an annual or biennial application of Wet & Forget keeps growth from re-establishing. The product is engineered to leave a residual film that disrupts new algae colonies before they become visible. Combined with the right cleaning chemistry up front, that residual approach delivers years between visible regrowth events.
When to Hire a Pro
For multi-story siding, steep roofs, and large commercial walkways, a soft-wash contractor charges $200 to $600 for a typical house and brings two advantages. A 12-volt soft-wash pump that delivers the cleaning solution at low pressure, and proper containment of runoff to protect storm drains. Most insurance policies cover the cost as part of routine maintenance, so a written quote is worth obtaining before tackling steep or high work yourself.
