Roof Algae -- Not a DIY Job...
It's generally not necessary to power-wash shingle roofs when it comes to simple tasks like removing ordinary dirt and leaves. In most cases, broom sweeping or a rinse with a garden hose usually does the trick. However; the exception to the rule are roofs that have grown algae, mold or moss. These fungal growths can become so entrenched that they damage the shingles. Furthermore, they are not easily addressed by "conventional" methods. It is not uncommon to see well-meaning home owners turn to power washing to rid their homes of the unsightly mess - especially when selling their property.
Roof algae is a living organism that thrives in moist conditions and sunlight. Pressure washing your roof is not going to kill algae. It will only remove the black streaks that ooze down the roof or the roof moss. Power washing, in the hands of a novice, will break down the protective layers of shingle stone and cause spores to spread across the roof covering and possibly get worse.
Most roof manufacturers will not honor warranty items such as the amateur use of a pressure washer on a roof. So what is a homeowner to do? The safest way is to hire a professional who either applies herbicidal chemicals or employs mechanical force, such as pressure-washing. Ideally both.
The video below is a classic example of what can happen when a home owner, wanting to make their property more presentable for a sales transaction, uses a pressure washer.
Roof Mold from Michael Thornton on Vimeo.
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