Roof Algae Facts

1. Bleach mixtures can injure sensitive plants. Because it is difficult to totally protect them from the mix, spray them down with clean water before applying the mix. And do immediately after, whether you cover them or not. Roof algae stains

2. Using a plastic pump-type garden sprayer, thoroughly soak the affected area with your choice of chemical. Allow it to sit for at least 5 minutes.

3. Wash off all dirt and fungus residue.  NOTE:  Though power washing was recommended for years, the industry has moved towards low-pressure cleaning due to evidence of permanent asphalt shingle damage caused by high-pressure power washing.

4. Allow the roof to dry thoroughly. Inspect the roof for any remaining fungus.  If it is slimy, or any sort of residue comes off on your hand, repeat the above steps.  You must let the roof dry before a second application of mix.  A second application of cleaner will not have its full effect on a damp surface!
If you think it wise to leave the bleach-brew on the roof longer than the recommended time, think again!  As mentioned earlier, long-term exposure to bleach can damage most any surface, and your roof is no exception.  However, this piecemeal approach is the likely to cause the least damage.

Problems with using bleach on asphalt roofs

There is some concern regarding the use of chlorine-based products on asphalt roofs.  Household bleach can damage asphalt due to its high sodium content.  Apparently, the sodium causes an electrochemical reaction that reduces the elasticity of the asphalt leading to stiffness, brittleness and curling of the shingles.  (I suffer from two of those symptoms myself!)

Thankfully, there are some “killer” alternatives to bleach.

1) Sodium hydroxide products are not recommended!  A chemical alternative to bleach uses the antifungal agent sodium hydroxide… also known as lye.  Lye-based products appeared on the market a few years ago, but turned out to be even more dangerous to use than bleach!  Even at low concentrations, permanent damage could be done to the roofing, possibly even dissolving the roofing nails!
Fortunately, most lye-based products have been removed from the market.

2) Here are a few bleach-free chemical treatments that can help keep fungus under control:

Spray and Forget is a new product that offers a long-term solution to fungus problems.  It is sprayed on and allowed to dry without rinsing.  Though it is slow-acting when compared to bleach solutions, it can prevent mildew growth for years!  If you want to see quick results, you can do a one-shot bleach cleaning.  Rinse thoroughly, allow to dry and then apply Spray and Forget according to the instructions.

Moss Out! from Lilly Miller Brands, comes in a number of formulations, one specifically for roofs and others designed to remove and/or inhibit moss and fungus growth on siding, masonry surfaces and even lawns.  Their formulae use tried and true zinc compounds that linger on surfaces to provide continuing protection from fungus regrowing.

If you wish to prevent reoccurrence of fungus, there are a few options. Zinc strips have been used for many years as an algae preventative. Savetime Corporation offers sets of 3′ zinc strips, which are attached near the peak of the roof. When it rains, a slight amounts of zinc dissolves from the strips and coats the roof, inhibiting algae and fungus growth.

Anti-Growth from GSI/Camden Products offers long-term protection against the return of mildew once the cleaning is done. Unlike the other products mentioned on this page, Anti-Growth can be used for interior applications.
Would you believe… fungus-resistant shingles?

3M Company has developed an additive for asphalt roofing shingles using copper granules.  This unique advance in shingle technology gives long-term fungus protection for the entire roof. They call it the AlgaeBlock system. You can find more information on these special roofing shingles at

3M is not a manufacturer of asphalt shingles, just the granules. However, they do list a number of companies that are producing shingles meeting their specifications.

The less expensive solution is to spray wash the roof with a 50 percent mix of water and bleach to get rid of the algae. (No pressure washers please. They’re likely to damage the shingles.) Just be sure to wet your foundation plantings first, and rinse everything in clean water when you’re done. Plants don’t like bleach, and wetting them with plain water first protects them.

To keep the algae from coming back, insert 6-inch-wide strips of zinc or copper under the row of shingling closest to the roof peak, leaving an inch or two of the lower edge exposed to the weather. That way whenever it rains, some of the metal molecules will wash down the roof and kill any algae trying to regain a foothold on your shingles.