If your drive for spring cleaning demands a clean house inside and out, you may wonder what to do with those dirty shingles.

Before you pay high costs to replace your roof, try cleaning it.

In high-humidity areas, roofs often turn dark brown or black within five to seven years because fungi and algae feed on dirt in shingles.

The fungus can start growing on a new roof right after the first shingle is laid down, says Dale Dorman, a housing specialist with the University of Georgia Extension Service.

"The growth first appears as black streaks or wedge- shaped areas that spread across the roof,"> Dorman says. "After a few years, the discolored areas merge, and uniform discoloration eventually results.

"Fungus and algae growth is usually heaviest on west- or north-facing roofs or on those shaded by trees," she says. "Dew dries more slowly in these areas."

The good news is that these stains don't mar the roof's strength or service life. Research at Mississippi State University found several chemicals can remove shingle discoloration caused by fungi, algae and lichen.

"One of the best cleaners is liquid household bleach," Dorman says. "And it doesn't damage the shingles."

Apply a 75 percent solution of household bleach (three parts bleach to one part water) to asphalt shingles. Use one gallon of solution per 30 to 50 square feet of roof surface. About 15 gallons of bleach will treat 1,000 square feet of roofing.

"Roofs will remain clean for at least five years if sprayed with the 75 percent bleach solution," Dorman says.

Cleaning power decreases with less bleach. A 10 percent bleach solution will kill the fungus, but it won't clean the roof immediately. The dead organisms will eventually wash away with rain. But the roof will remain clean for only about a year.

Clean your roof in strips starting at the peak and working toward the eaves, Dorman says.

"Treated roofs are slippery when wet, so work from a ladder," she says. "Use a clean garden sprayer to apply the mixture.

"Avoid skin contact with the solution," she says, "and cover any shrubs or plants below the eaves with plastic. Dilute any solution reaching the ground by spraying it with water.

If you have rain gutters at the eaves, remove all leaf screening and place a garden hose in the gutter. This will dilute the solution as it drips from the roof. You don't need to scrub the roof or rinse off the solution.

"While cleaning the shingles, look for damage on your roof," Dorman says. "Note cracks in the shingle surface, curling corners, buckling front edges or loss of granules."

"If you have to reroof, use a fungus-resistant shingle that carries a 20-year limited warranty against fungus growth," she says. "The shingles release zinc granules when it rains, destroying fungus and algae."

The fungus-resistant shingles offer an inexpensive way to maintain beauty, Dorman says.