This fall, when you were out raking your leaves, you may have noticed dark discolorations on your roof. A carefully raked yard, meticulously pruned hedges and flawless seasonal displays don’t do much for your home’s exterior when you have black streaks running down your shingles.
What Is It?
Sometimes streaks are caused by mold or mildew growth on shingles, but most of the time that’s not the issue. Roof experts say black streaks are caused by an algae known as gloeocapsa magma. Airborne spores deposit it on your roof, where it soaks up plenty of sunlight for photosynthesis and munches on the limestone filler in your shingles. The algae is blue-green when it’s alive and turns black after it dies and begins to decay.
Some manufacturers treat shingles with copper or zinc to prevent algae growth, but their effectiveness decreases over time. The streaks don’t just reduce your home’s curb appeal, they retain moisture and provide fertile ground for fungus and lichen. Lichen weakens shingles and is extremely difficult to remove.
Humid areas of the country are most susceptible to dark streaks from algae. The areas of your roof that receive less sunlight exposure are more likely to stay moist and encourage algae growth.
How to Get Rid of It
- Bleach, water and trisodium phosphate (TSP) or a commercial cleaner
- A pump sprayer
- Your garden hose
- A sturdy ladder
- Plastic sheeting
- Rubber soled shoes
- Safety goggles and rubber gloves
Cover the plants around the perimeter of your home to avoid dousing them with bleach or chemicals. If you can, clean your roof on a cloudy day so sunlight doesn’t cause your cleaning solution to evaporate too quickly.
Mix up a solution of one part bleach, three parts water and three tablespoons of TSP. Mix it well and pour it in your pump sprayer.
Place your ladder on firm, solid ground and carefully access your roof. It’s always best to leave roofs with a steep pitch to the professionals. Climb carefully to the top so you aren’t walking on wet shingles and solution doesn’t blow back in your face.
Soak the shingles with your cleaning solution and allow it to sit for at least 15 minutes, then rinse it off with your garden hose. If algae remain, repeat the treatment.
If You Hire Someone
Any time you hire someone to work on your home, be sure to check references online and check with family and friends. Roof work is dangerous, so confirm liability and worker’s compensation coverage.
Pressure washing removes the protective granules from your shingles, so make sure cleaners don’t use equipment that reduces the life of your roof. Find out what kind of cleaning solution they use and how they plan to protect your yard and property from runoff.