Moss, Algae and Black Streaks on Roofs: What Is Harmful and How to Clean It

Algae and moss need moisture and shade. In West Michigan, the north and east slopes, the slopes under big trees and the sections next to tall walls dry out slowest after rain and keep snow longest in spring. That is where growth starts, and often where it stays.

Humid summers, long wet springs and falls, and roofs that never fully dry between rains give it plenty of time. Leaves and seeds caught in the shingle gaps give moss something to root in. Once one roof in a neighborhood has heavy streaking, the airborne algae spores have an easy trip to the next one.

ARMA's guidance is that algae can be lightened, not erased, with a solution of household chlorine bleach and water, mixed to the shingle manufacturer's directions, since those vary. Other chemicals or methods should not be used without the manufacturer's approval. ARMA also encourages using professionals, because a wet roof covered in bleach is about as slippery as a roof gets.

A moss killer is a pesticide under Michigan law, even though it is sold for roofs rather than crops. Part 83 of the Natural Resources and Environmental Protection Act defines a pest to include fungi, weeds and other forms of plant life, and defines a pesticide as a substance intended for preventing, destroying, repelling or mitigating pests. A product sold to kill moss fits that description, and so do many products sold to kill roof algae.

That matters when you pay someone else to apply it. MCL 324.8314 says a person shall not apply a pesticide for a commercial purpose or in the course of employment unless that person is a certified applicator or a registered applicator. MCL 324.8313 adds that commercial applicators who hold themselves out to the public as being in the business of applying pesticides must obtain a commercial applicator license for each place of business. The Michigan Department of Agriculture and Rural Development (MDARD), which runs the program, says commercial and registered applicators who are hired to apply pesticides must meet its business licensing requirements to legally provide that service.

A homeowner treating their own roof with a ready-to-use product is in a different position from a crew doing it for pay, and the label is the law either way: MCL 324.8311 requires everyone to use a pesticide in a manner consistent with its labeling. That is the legal side of the warnings above about rain, runoff, plants and copper.

In practice, if a roof cleaning company, a handyman or a roofer offers to spray a moss or algae killer, it is fair to ask for their MDARD credential. MDARD's licensing pages include a public lookup for applicator certifications and licensed businesses. The rules do not stop anyone from brushing moss off or rinsing a roof with plain water; they apply to the chemical part of the job.

Algae streaks on a sound roof are a cosmetic choice. Heavy moss on a roof that is already 15 or 20 years old is different: it often marks the slope where the roof is failing first. If shingles have lifted, cracked or lost their granules under the moss, cleaning will make the roof look worse, not better, and may open it up to leaks.

That is when a roofer should look before anyone cleans. Sometimes the answer is a section repair along the eaves where the deck has softened. Sometimes the moss has simply been hiding a roof that is near the end of its life. Either way, it is better to know before the first freeze.