Removing Moss, Mildew, Fungus and Associated Stains on Roofs

Roof cleaning is a topic that draws yawns in New England but causes shudders and screams of agony from Florida and points south, where vegetable life seeks world domination!  How do we keep that "fungus among us" under control.  (Graphic left courtesy Spray and Forget mildew and moss remover.)

Since starting on a quest for the ideal method of cleaning fungus and algae from roofs, I have contacted dozens of individuals and companies. Some of my readers came forward with suggestions and a few companies also supplied information helpful to this research.

For the purposes of this article, the terms "mildew", "fungus", "algae" and "moss" are interchangeable... even though their mothers' might object!

Chlorine bleach cleaning mixtures are one option...

If you've had experience with mold and mildew in the home, you know the best mildew remover is chlorine bleach. But outdoor, heavy duty cleaning usually calls for more heavy-duty mixtures such as TSP and chlorine bleach (see the TSP page for more details), or specialty additives such as Jomax from Rustoleum, which combines a cleaner with a bleach activator.

How to use bleach mixtures on your roof...

The following procedure is for bleach-cleaning only using JOMAX.  (Other cleaning options are mentioned later in this article.)  If JOMAX is not available in your area, use a TSP mixture with bleach added.

  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 immediately after, whether you cover them or not!
  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 powerwashing was recommended for years, the industry has moved towards low-pressure
    cleaning due to evidence of permanent asphalt shingle damage caused by high-pressure powerwashing.
  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

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

Moss Out! 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 regrowth.

Preventing reinfestation... zinc strips are one option

If you wish to prevent reoccurrence of fungus, think zinc! Zinc strips (left) have been used for many years as an algae preventative. offers sets of 3' zinc strips, which are attached near the peak of the roof. When it rains, a slight amount of zinc dissolves from the strips and coats the roof, inhibiting algae and fungus growth.

Would you believe... fungus-resistant shingles??

Yessiree... the 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 HERE.

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