Keep the Pressure Off

While I was living in Florida, my cousin did a regular business in pressure-cleaning roofs.

June 11, 2000

While I was living in Florida, my cousin did a regular business in pressure-cleaning roofs. For residents of this hot and humid state, the sight of workers armed with pressure-sprayers is familiar--most homes require at least one pressure-cleaning a year--and it's a common complaint among homeowners.

Imagine the up-sell potential for anyone who installs roofing, if you had a system for preventing this problem.

Since strains of the Gloeocapsa Magma, the common form of algae that grows on roof shingles, have become hardier, more than 80 percent of the country has been afflicted. This form of blue-green algae is most commonly seen as a series of black streaks on the north sides of roofs, especially in the Middle Atlantic, Southeast and Pacific Northwest.

The algae spores are carried by the wind from roof to roof, so if one home in a neighborhood shows signs of algae infestation, it's a safe bet that more roofs will develop colonies. You can tell if streaking is algae related if chimneys, vents and roof flashing and the shingles beneath them are clean.

Algae are attracted to shingles as a food source. Gloeocapsa can feed on calcium carbonate, typically used as a filler material in asphalt shingles. The dark color develops over time, as the algae attempts to protect itself from sun damage. Although asphalt shingles are the biggest target, tile, wood shake, concrete and clay roofing are all susceptible to algae.

Regular pressure cleaning, such as that required in the most humid and warm areas, can shorten a roof's life span, and the algae itself can also cause roof damage, indirectly. The uneven spread of dark and light colored patches will attract sunlight unevenly, causing portions of a roof to wear more quickly.

According to Jeff Morris, building materials expert for Globe, "An algae-free roof can reduce the temperature in a home's attic by 15 to 20 degrees, while the black stains caused by algae absorb heat from the sun, resulting in premature shingle deterioration and increased energy costs to cool the home." He adds, "Often, the only permanent solution for containing the growth is to reshingle the entire roof."

Several manufacturers provide roofing shingles that are designed to combat this problem. Shingles containing copper granules repel roof algae growth, and certain copper-granule treatments also provide an effective solution. Copper strips can also be added to roof ridges, which inhibits algae growth, but sometimes does not cover the entire roof.

Zinc has also been found to be an effective preventative, although zinc pellets or strips can possibly leave white residue behind as a discoloration on the shingles themselves. Paint sealants are also available to prevent algae colonies from developing.

"Homeowners are becoming more aware of the problem of algae growth and how they can remedy it," says Morris. "Before [homeowners] embark on buying a new roof, [they] should walk through the neighborhood and see whether or not there are black streaks on the roofs of homes nearby. Then [they] should talk to their contractors about avoiding the risk."

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