Question: Our roof has black streaks on the shingles. Does this mean the roof is wearing out and the gravel has worn away?
Answer: Many of the roofs here in Sun City have streaking caused by blue-green algae known as Gloeocapsa Magma that is spread by airborne spores. Walk around your house and look at the roof from all sides. You will probably notice that the algae is worse on the north side. You may also notice that algae stains are absent directly below the metal flashing around chimneys or roof vents. This is due to fact that copper and the zinc coating on galvanized sheet metal are toxic to algae. Every time it rains, trace amounts of metals are washed down the roof, inhibiting algae growth.
Algae growth does little harm to roofing, but the stains are unsightly. In fact cleaning the algae often causes more damage to the shingles than the algae, since harsh chemicals are involved. Algae stains can be removed by cleaning, though they usually return. However, repeated use of harsh chemicals, or the erosive effects of pressure washing, can damage or shorten the life of asphalt shingles.
There are several products on the market specifically designed to remove algae stains from roofs, such as Wet & Forget and Moss Out. A mixture of trisodium phosphate (TSP), bleach, and water will also remove stains. Oxygen bleach lightens stains as well and is less harmful to the environment, but it doesn’t produce as immediate or dramatic an effect as chlorine bleach.
If you are over 55 years old, we strongly recommend against cleaning your roof yourself, as working on a roof can be dangerous, and it becomes even more hazardous when wet.
It’s best to have your roof cleaned on a cloudy day to prevent the cleaner from evaporating too quickly. If you know the manufacturer of your shingles, contact them or check their website for specific recommendations on cleaning. When using a product made for cleaning roofs or oxygen bleach, follow the directions on the container.
While the recipe for cleaning a roof with bleach can vary, the basic formula is:
1 quart bleach (6% sodium hypochlorite)
1 gallon water
1/4 cup TSP (trisodium phosphate) or other heavy duty cleaner (don’t use a cleaner that contains ammonia or is not recommended for mixing with bleach, as it can result in poisonous chlorine gas)
Pour the ingredients into a pump type garden sprayer, mix well, and spray. After wetting down the roof with the solution, allow it to remain on the roof for about 15 minutes before rinsing off with a hose.
Bleach can damage or kill other plants in addition to algae, so wet down any grass, shrubs, or other plants where runoff will occur before and after applying bleach. Covering bushes with plastic after wetting them down helps as well.
The bottom line is that our recommendation is to do nothing as any cleaning method will be more harmful to your roof than the algae.
If you have suggestions for future tips or have questions about maintenance around your home submit them to ask.the.woodchucks@gmail.com.
