Asphalt Shingle Cleaning 101

Reasons for cleaning include aesthetics, property values, energy efficiency, and increased longevity of the roof. Whatever the main reason, if it is going to be done, it is in your best interest to know what choices are available to you. A quality roof should last upwards of 30 years if cared for and maintained properly. Unfortunately, many roofs here in Florida (and increasingly throughout the US) are subjected to unnecessary stress while cleaning that greatly reduces the life expectancy of the roof. With repeated roof cleaning comes certain pitfalls that every homeowner, property manager, and professional cleaner should be aware of in order to avoid damage to property, unnecessary repair costs, and the premature replacement of the roof.

WHY HAS THE ROOF TURNED BLACK? The black streaking typically found on homes in areas where moisture settles on shingles is not caused by mildew or fungus; it's roof algae. 

3M scientists have identified the most common form as Gloeocapsa Magma. Algae spores are carried by the wind, which is why so many houses in the same neighborhood can end up with this problem. During the last 20 years, this particular algae strain has become hardier and so has been able to migrate to less humid environments than it has in the past. And in areas where it traditionally has been found, the staining is showing up earlier, is more severe and settles on a greater number of roofs.

HOW CAN IT BE CLEANED? Typical roof cleaning methods involve either pressure cleaning or chemicals solutions containing Chlorine Bleach or Sodium Hydroxide. If used properly, these methods will clean the roof with minimal adverse impact - but the effects can be very temporary and will generally have to be repeated every 6-18 months in order to maintain the appearance of the roof.

WHAT IS THE BEST CLEANING METHOD? Each method has its advantages and disadvantages. A chlorine and water solution can be used followed by a thorough rinse. Of course, since chlorine can be toxic to people and plants, proper care should be taken to protect employees and the surrounding property from overspray and runoff. The benefits to using chlorine include a much faster cleaning process and minimal rinsing as compared to the other methods. This translates into much less wear & tear on the roof - particularly asphalt shingle roofs. Sodium hydroxide based cleaners are generally advertised as safer for the landscaping, but sodium hydroxide is caustic, very toxic, and care should also be used to protect employees and surrounding property. Sodium hydroxide is also a very effective degreaser. So high levels of sodium hydroxide in a cleaning product can cause damage to an asphalt shingle roof that can be irreversible. These products generally require much more rinsing than the chlorine method. The rinsing process is not only time consuming, but it can also be potentially damaging to the roof if not done correctly. A quality sodium hydroxide based product should not require any more than 100 psi to effectively rinse an asphalt shingle roof and even then some granule loss should be expected. Pressure cleaning is an option that should only be used on a concrete, barrel tile, or metal roofs. Pressure cleaning has the advantage of not requiring chemicals, which eliminates some of the cost - in addition to landscaping and chemical exposure concerns. The down side to pressure cleaning is that it is very time consuming and it exerts tremendous force on a surface that is not really designed to handle it. In addition, repeated pressure cleaning can wear away the surface of some tiles that are only covered by a thin layer of coloring. When this happens, the grey concrete color starts to show through and the roof must be stained or painted.

THESE METHODS ARE NOT A LONG-TERM SOLUTION. While these methods are relatively safe and effective when done properly, repeated use will lead to premature aging of the roof. The natural erosion that occurs over time from the basic forces of heat, cold, wind, and rain alone are enough to weaken most roofs to the point where annual inspections and minor repairs are necessary to prevent significant repair costs. But when the roof is repeatedly subjected to high pressures or harsh chemicals, the aging of the roof structure is significantly accelerated. With repeated cleaning, tiles are more likely to shift, slip, or break and asphalt shingles are more likely to become brittle and crack. When this happens, there is a greater likelihood of tearing the roof membrane simply by walking on the roof. Once gaps are created in the roof and subsequent pressure cleaning or chemical cleaning takes place, these gaps provide the water or caustic solutions a path to the roof membrane and increase the chance of costly roof leaks.

WHAT IS THE ALTERNATIVE? A proper program of preventative maintenance will eliminate the need for future cleaning and the potential for damage that comes along with it. Preventing the recurrence of the algae growth will require a light spray of chlorine and water or an algae prevention product. Most quality algaecides will last for approximately 6-12 months before reapplication of the product is necessary. Obviously, the longer the time frame, the better your chances are of minimizing any unnecessary damage to the roof. Prevention programs should be done in conjunction with neighbors whenever possible and many can be applied without a need for walking on the roof. One important note about algaecides: Algaecide products are regulated at both the state and federal level. Any algaecide product used on a roof requires EPA registration as a pesticide or biocide and approval for use on a roof. Contractors found violating the law in this regard could find themselves facing fines up to $10,000. per occurrence.