Moss is a simple plant organism that can grow on trees, concrete and even roof shingles. If your roof (or any section thereof) receives limited sunlight and periodic exposure to water or damp conditions, it can become an ideal breeding ground for moss. If you notice this grassy organism growing on your roof, take action to remove it immediately.
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Moss itself is harmless. It does not produce any dangerous spores or fumes, it contains no poisons or irritants and it lacks the mass to physically damage any structures, including roof shingles. The danger of moss lies in its ability to retain moisture. When moisture like rainwater or humidity settles on a roof, it typically air dries before it can linger for too long. When moss covers the roof, however, it can trap the moisture, preventing it from properly evaporating.
Dangers of Moisture
When excess moss growth causes moisture to remain trapped on a roof, the shingles themselves can weaken and rot over time. If you ignore the moss on your roof for years at a time, you may significantly shorten the lifespan of your roof. The moisture actually softens the porous material, leaving it with a much higher vulnerability to elements like rain and wind.
Persistent moisture can contribute to another, more serious problem: mold. Mold is much more hazardous than moss, and it grows on persistently moist surfaces. If a mold colony grows on your roof, the fungus can cause permanent damage. You can only ensure a mold-free roof by keeping the shingles free from excess moisture, and you can only free your roof of excess moisture by eliminating moisture-trapping organisms like moss.
If you find moss growing on your roof, This Old House recommends killing it with equal parts bleach and water. Spray the bleach over the moss and scrub it with a scrubbing brush. If the moss proves really stubborn, spray it with the bleach and wait about 15 minutes before scrubbing. The lapse in time will allow the bleach to break down the organisms. Use extreme caution when working on the roof, or call a professional roofing service to remove the moss on your behalf.