Three Roof Cleaning Methods: Stained by algae or not, roofs are cleaned using any of three common methods, each of which has its own set of pros and cons. These methods are, pressure washing, and the application of solutions containing either, chlorine bleach or sodium hydroxide. When performed correctly and only once, these techniques clean roofs effectively without significant adverse effects. The problem is, they usually have to be repeated every six to 18 months to maintain cleanliness.

Chlorine Solution Washes: Chlorine bleach solution application requires a lot less rinsing than the two other methods, thereby allowing a cleaning procedure of shorter duration. One good consequence of a quicker process is less wear and tear for the roof. However, chlorine can be toxic to some people and plants, so care should be exercised to protect employees or residents, as well as the property itself, from spray runoffs. Asphalt shingle roofs are particularly well adapted to this type of cleaning.

Sodium Hydroxide-based Cleaning: Sodium hydroxide-based cleaners are very effective degreasers, but they require a lot more rinsing than chlorine solutions. As a result, using this type of cleaning agent can be time-consuming and carries with it greater risk for causing damage to the roof. And, while only 100 psi of force is needed to rinse away a sodium hydroxide-based compound on an asphalt shingle roof, some amount of asphalt granule loss is to be expected still. Even if advertisements would have us believe otherwise, sodium hydroxide can be toxic too, and should be kept from coming into contact with persons and property, and especially plants in the surrounding landscape. It is also caustic, so high levels of this compound may cause irreversible damage to roofs, especially those made of asphalt shingles.

Pressure Washing: Pressure washing is an option only for roofs made either of concrete, metal or barrel tiles. One of the distinct advantages of this method is that it does not require the use of chemicals, which does away with some costs, and which makes it more environment-friendly. This procedure, however, requires more time to perform and may exert more strain than some roofs can handle. Another downside is that it can wash away the thin layer of coloring with which some roof tiles are coated. The natural gray color of the concrete roof begins to show through, requiring the additional task of painting or staining.