Glossary
Algae
Algae are photosynthetic organisms EPA describes as primary producers in water; on a roof the word covers the airborne growth ARMA and TRI Alliance document.
Read itGlossary
Algaecide
EPA files this idea under a different spelling, and a Naples homeowner hears the trade's version when talk turns to the black streaks on a roof.
Definition
Algaecide is a substance that kills algae. EPA spells the class "algicides" and describes them as killing algae in lakes, canals, swimming pools, water tanks and other sites; the trade's spelling adds a letter and changes nothing about the meaning.
The spelling
EPA's "algicides" against the trade's "algaecide"
US EPA's own pesticide vocabulary spells this word without the extra e: "Algicides kill algae in lakes, canals, swimming pools, water tanks and other sites." The trade's common spelling, algaecide, adds a letter that changes nothing about the meaning; both point at the same class of substance.
That single-letter difference is worth naming because a search for one spelling can miss a source written in the other. Neither spelling changes what the word describes: a substance meant to kill algae.
EPA's own scope
The sites EPA names, and the catch-all after them
EPA describes pesticide active ingredients by the pests they control or how they work, and its algicide entry names water sites first: lakes, canals, swimming pools and water tanks. It ends with "and other sites," so a roof is neither named nor ruled out. EPA's antimicrobial page, written for inanimate objects and surfaces, also places products that control algae of economic and aesthetic significance in its non-public-health category.
Algicides sit on EPA's list beside biocides, antimicrobials and fungicides; that wider frame, and what an EPA registration on a label means, is the subject of the biocide entry.
What the roofing bodies say instead
ARMA lightens, TRI treats, neither uses the word
ARMA's shingle bulletin says algae discoloration "may be lightened by applying a solution of liquid household bleach (sodium hypochlorite) and water," and never calls that solution an algaecide.
TRI Alliance's wording for tile is different again: mildew, algae and moss "can be removed or treated." Neither association uses algicide or algaecide in the documents quoted in this entry; both describe a solution or a result instead of naming a product class. What both say about growth that comes back is quoted in the black streaks section of the roof cleaning page.
Not a registration claim
Whether a product is a registered algaecide is the label's question
No roofing bulletin settles whether a given product is a registered algaecide. That is a question for the specific product's own label, and a registration printed there is not read on this page as a statement about any roof.
Whether a soft wash kills the algae outright or clears it from the surface is argued at does soft washing kill the algae or just remove it.
Does EPA's algicide entry name a roof?
No, but it does not rule one out either. EPA's entry names lakes, canals, swimming pools and water tanks and then adds "other sites," and EPA's antimicrobial page places products that control algae of economic and aesthetic significance in its non-public-health category. Neither page mentions a roof by name.
Is an algaecide the same thing as a biocide?
Not in EPA's wording. EPA's list gives each its own entry, named for what it kills: algicides for algae, biocides for microorganisms. Neither entry is written as a subset of the other, and the biocide entry in this glossary covers that word and EPA's registration frame.
Sources
Where this page's facts come from.
Every technical statement above is drawn from one of these documents. Nothing on this page about 239 Exteriors goes beyond what the company itself states.
- U.S. Environmental Protection Agency (EPA). "Types of Pesticide Ingredients" (updated May 5, 2026)
- U.S. Environmental Protection Agency (EPA). "What are Antimicrobial Pesticides?" (updated January 29, 2026)
- Asphalt Roofing Manufacturers Association (ARMA). Technical Bulletin, "Algae Discoloration of Roofs" (revised June 2024)
- Tile Roofing Industry Alliance (TRI Alliance). Roof Maintenance Guide
Page reviewed September 14, 2026. Florida statutes are quoted from the 2026 Florida Statutes; every image on this page is a labeled concept illustration, not a photograph of a completed job.
Keep reading
Where to go next.
Answers
Does soft washing kill the algae or just remove it?
Two stages, not one. ARMA's bleach solution may lighten the stain, then algae washes away with later rains and moss loosens over time.
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