A clean roof contributes significantly to a home’s curb appeal and extends the lifespan of asphalt shingles. Organic growth and debris can compromise the integrity of roofing materials, leading to premature wear and costly repairs. This guide provides a safe and effective method for cleaning your roof, focusing on low-pressure techniques that protect the shingle surface. Following these steps ensures the removal of contaminants while preserving the protective qualities of your roof.
Understanding Common Roof Contaminants
The dark streaks commonly seen on roofs are caused by a cyanobacterium called Gloeocapsa magma. This organism feeds on the limestone filler material embedded in asphalt shingles, a common component in modern roofing. As the bacteria multiply, they form a dark, protective coating that absorbs heat, which can increase attic temperatures and contribute to premature aging of the shingles.
Moss and lichen represent two other threats to roof health. Moss is a non-vascular plant that holds water, trapping moisture against the shingle surface. This constant dampness can lift the edges of shingles, breaking their seal and allowing water to penetrate underneath. Lichen is a composite organism that attaches securely to the shingle surface, often embedding its root-like structures into the protective granules. Attempting to forcibly remove moss or lichen without a chemical treatment can strip away the protective granules, causing irreversible damage.
Essential Safety and Preparation Steps
Working on a roof presents fall hazards, making safety the highest priority. Use a sturdy ladder placed on level ground and ensure it extends at least three feet above the roofline for safe access. Wearing non-slip, soft-soled shoes is necessary to maintain traction and avoid dislodging the protective shingle granules. For roofs with a steep pitch, using a safety harness secured to a reliable anchor point is a necessary precaution.
Before applying any cleaning solution, protect the surrounding environment. The cleaning solution contains sodium hypochlorite, which can damage vegetation. Thoroughly wet all nearby plants, shrubs, and grass with plain water before starting the application. This pre-soaking allows the plants to absorb clean water, making them less susceptible to the chemical solution. Cover sensitive plants with plastic sheeting or tarps and turn off power to any exterior electrical outlets near the work area.
The Recommended Low-Pressure Cleaning Method
The most effective way to clean asphalt shingles is through a low-pressure application method, often called soft washing. High-pressure washing, which can exceed 1,200 pounds per square inch (PSI), will blast away the protective granules, drastically shortening the roof’s lifespan and potentially voiding the manufacturer’s warranty. The soft wash method relies on a chemical solution to kill the organic growth, using water pressure no greater than that of a standard garden hose for rinsing.
A common and effective DIY cleaning solution uses a mixture of water and sodium hypochlorite, which is the active ingredient in household bleach. A final solution strength of 3% to 6% sodium hypochlorite is recommended to neutralize the organic growth. This can be achieved by mixing one part of standard 5% to 6% laundry bleach with one part water, or by using three parts water to one part 12.5% commercial-grade sodium hypochlorite. Adding liquid dish soap acts as a surfactant, helping the solution adhere to the roof surface for a longer dwell time.
The application process begins by applying the solution using a garden pump sprayer or a low-pressure system, working from the roof’s ridge down to the gutters. The solution must be allowed to dwell on the roof for 15 to 20 minutes to allow the chemical to break down and kill the organisms. During this dwell time, periodically rinse the surrounding landscaping with plain water to prevent chemical burn. Once the dwell time is complete, the roof must be thoroughly rinsed with a low-pressure stream of water to wash away the dead organisms and residual cleaning solution.
Long-Term Prevention and Maintenance
After the roof is clean, implementing preventative measures slows the recurrence of organic growth. One effective method involves installing strips of zinc or copper near the roof’s ridge line. As rainwater flows over these metal strips, it leaches out metal ions that are toxic to algae, moss, and lichen. These ions are then carried down the roof slope, creating an inhospitable environment for new growth.
Copper strips are more potent and can protect a wider area of the roof compared to zinc, though both are effective at inhibiting growth. Trim back any tree branches that overhang the roof surface. Overhanging branches deposit organic debris that feeds the growth and create shaded areas that retain moisture, which is the primary condition for moss and algae to thrive.