Do copper strips keep roofs clean?

People say copper on a roof and mean one of two things: a metal strip attached to the roof, or copper built into the shingle itself.

The Asphalt Roofing Manufacturers Association names both in one sentence and keeps them apart. Its algae and moss prevention page lists “adding zinc or copper strips to prevent both algae and moss” and, as a separate item, applying asphalt shingles that incorporate algae resistant copper granules, and says either may be employed when it is time to replace a roof. One is hardware added to a roof. The other is a property of the shingle as it leaves the factory.

When a shingle maker talks about copper and algae, it is talking about something in its product, not something added on top.

GAF's technical bulletin on blue-green algae staining puts algae-resistant shingles first among its solutions and says its Time-Release Algae-Fighting Technology is “engineered with thousands of microsites that release algae-fighting copper over time.” CertainTeed says its StreakFighter technology “employs numerous copper-containing granules,” that copper possesses natural anti-algae properties, and that “the copper in the granules is gradually released” over time. Both makers are describing the same idea: copper sits in the surface and is let out slowly.

The organism in view is the one GAF says often causes the dark stains, the cyanobacteria Gloeocapsa magma, commonly called blue-green algae. What the black streaks on a roof are, and why they form, is set out on the roof cleaning page's section on the black streaks on a roof.

Where that copper lives also bears on ARMA's cleaning cautions. CertainTeed describes granules as crushed rock particles pressed into the asphalt that block ultraviolet light from reaching it. ARMA warns that scrubbing may loosen granules and that a pressure washer will cause granule loss. On a shingle with copper-containing granules, as CertainTeed describes its own, granule loss means losing the layer that carries the copper as well as the layer that shades the asphalt.

Neither maker prints a performance figure for the copper itself on the pages read. GAF attaches StainGuard algae protection limited warranties of stated lengths to listed products and says to see its Shingle & Accessory Limited Warranty “for complete coverage, restrictions, and qualifying products.” CertainTeed attaches a limited warranty against blue-green algae to its algae-resistant shingles, for a period that depends on the product. Whether those shingles make sense for a reroof is the subject of the page on whether algae-resistant shingles are worth considering in Florida.

A copper strip put on a roof that is already built has no maker or association figure behind it, and ARMA advises against it.

The reason ARMA gives is a fork with two bad branches: nails left exposed, which ARMA says can let leaks develop over time, or a sealant bond broken open, which it ties to eventual wind damage. That warning, and what ARMA offers instead, is argued in full on the page about whether zinc strips can stop roof algae. For tile, TRI Alliance does not discuss strips at all, and says no manufacturing procedure or job-site coating is currently known to prevent the growth over the long term.

That leaves the cleaning, described honestly. ARMA says a bleach and water solution may lighten discoloration and that the effect is only temporary. A house down the street that seems to stay clean has explanations of its own, taken up on the page about why a neighbor's roof stays clean. A clean look, from any cause, is also only a look: whether cleaning a roof changes what an inspector finds is a separate question, on the page about whether cleaning a roof before an inspection changes the result.