There has been significant interest in roof washing lately, including a lot of questions about what to use to clean off those black streaks and stains (mold algae also known as 'gloeocapsa magma'). These stains will significantly affect the look of any home. In the past, the only way to get rid of them was to replace the roof. It has been estimated that as much as 50% of all roof replacements have been done because of the appearance of the roof and not actually because of its functionality.
These stains are impossible to remove without the aid of chemicals. Caustics are inexpensive and very effective for killing mold.
When Sun Brite Services washed roofs for customers a decade ago, we used the following method: First, using an X-Jet, we wet the roof slightly to cool it down. Second, we apply QwicClean?s Super Roof Kleen to the shingles, from the ground, using an X-Jet. We allow the cleaner to work for about 15 minutes before we begin power washing. (Note: Roof Kleen costs as little as 70 cents per gallon, which is less expensive than bleach!)
Walking gently on the roof, we washed using very light pressure ? about 300 PSI. We are very careful to avoid knocking off gravel from the surface of the shingle. When we meet up with a tough stain area, we used a very soft scrub brush with a 5? handle and gently scrub this area. Finally, we rinse the surface well. Rinsing can be done from the ground using your X-Jet again.
Walking most roofs requires special gear to prevent falls. After all, a good cleaner can make any surface pretty slippery. We used body harnesses tied off to solid objects like tress. A chicken ladder was another tool we used to make it safer to move around the roof.
An alternate roof cleaning method is to use a surface cleaner designed for roofs. Using that, the operator perches safely on the peak of the roof and lowers and raises the surface cleaner over the surface with a rope. This is an excellent method for cleaning barrel tiles, for example, and is commonly used in Europe.
Today, two more advanced techniques have made asphalt shingle roof cleaning much simpler, safer, and more profitable. Both are referred to as "soft wash" or "low pressure cleaning".
The first of these techniques is to clean the roof by shooting a bleach-based cleaner on to the roof from the ground. For this method, you need a pump designed to handle bleach like the Roofster. It shoots a stream as far as 40', which delivers the cleaner all the way to the peak.
The bleach mixes used are exceptionally strong (often a concentration as high as 6%) so much care must be taken to avoid the cleaning mix hitting other surfaces (like the house siding). With a little practice and effort, good technique comes quickly. The job is frequently complete within an hour or less.
The second technique still involves walking the roof, but unique roof anchors make the job very safe. A super-strong bleach mix is applied in a light covering. Since you are on the roof, little chance of damage to other surfaces is possible even though you are using a super-strong bleach cleaner.
Both of these techniques mean pressure is virtually zero, which makes them superior to the method we used.