Can zinc strips stop roof algae?

The Asphalt Roofing Manufacturers Association does mention zinc strips, and the place it puts them in its advice is the first thing to know.

ARMA's page on algae and moss prevention for asphalt roofing names other preventative measures, “such as adding zinc or copper strips to prevent both algae and moss,” or applying asphalt shingles that incorporate algae resistant copper granules, and then gives the timing: they “may be employed when it is time to replace a roof.” The same passage adds that asphalt shingles are currently available that use algae resistance technology.

Two things in that sentence are easy to miss. The first is the verb: strips may be employed. ARMA lists them as an option; it does not describe results, a coverage area or a number of years. The second is the moment: a replacement, when the roof is being built again anyway. Nothing in the passage places strips on a roof already finished and sealed.

What the strips are aimed at is the growth ARMA describes elsewhere on the same page as the most common type, “gloeocapsa magma or blue green algae,” which protects itself from ultraviolet rays with a dark pigmented sheath. ARMA's algae bulletin calls Gloeocapsa magma probably the most prevalent of several species behind roof discoloration. Copper shows up in two forms in these roofing documents, a strip and a granule, and that difference is set out on the page asking whether copper strips keep roofs clean. The shingle side of the replacement choice, including GAF's copper-releasing shingles, belongs to the page on whether algae-resistant shingles are worth considering in Florida.

For a roof that is not being replaced, ARMA's answer is short: “Adding zinc or copper strips to an existing roof is not recommended.”

ARMA gives the reason as a pair of bad options. A strip has to be held on. Fastened on top, it needs exposed nails, which ARMA says “can cause leaks over time.” Slid up under the shingles instead, it means breaking the sealant bond, which ARMA says “may eventually result in wind damage.” There is no third way in the passage. The trade-off is not between a strip that works well and one that works less well; it is between two ways of disturbing a roof that is otherwise intact. How those two sit among the other leak paths the documents name is on the page about whether roof cleaning can make a roof leak.

Tile is outside ARMA's passage. TRI Alliance, writing about concrete and clay tile, does not discuss strips, and it knows of no job-site coating with a long-term preventative effect on the growth either. Whether anything should be fastened to a tile or shingle roof is a question for a roofing contractor, not for a cleaner.

With no measured strip to lean on, the published advice for a roof where algae keeps returning comes down to three plainer lines.

• Light and less debris. ARMA says trimming tree branches back to let more sunlight reach the roof and minimize debris accumulation “will provide an environment less inviting for such growth.” It also says air flow used to clear debris should be directed down the slope, so debris is not driven under the shingle edges.
• A cleaning, honestly described. ARMA's algae bulletin says discoloration may be lightened with a solution of household bleach and water, and that “The effectiveness of such cleaning is only temporary, and the discoloration may recur.” TRI Alliance says the same for tile in its own words: if the growth returns, treatment can be repeated as necessary. Why it comes back is on the page about a roof that is dirty again after a year, and what the sources say about the interval is on the page about how long a roof stays clean.
• A choice at reroof time. The replacement moment is where ARMA puts both strips and algae-resistant shingles, so both of ARMA's longer-term options belong to the day a roof is rebuilt, not to a roof already finished.

How each roof maker wants its own material cleaned, tile, shingle or metal, is laid out on the roof cleaning page's section on what each maker says about cleaning a roof. Moss follows the same routing ARMA gives for algae methods: its effects and removal are questions for the shingle manufacturer.