A clean roof is about more than maintaining curb appeal. Proper cleaning increases its longevity. The damp weather and limited sunlight in the Pacific Northwest is perfectly suited to the growth of moss. Left alone, moss can lead to significant damage. 

How does moss lead to roof damage?

Moss spores become airborne, landing on a roof accumulating in the spaces between shingles. Moss does not contain roots, instead of drawing in water and nutrients through osmosis. They grow, as most plants do, through photosynthesis. Without roots, whose secondary purpose is to anchor a plant in place, mosses use rhizoids to secure themselves.

As moss grows, their size begins to loosen and lift shingles. This creates easy access for water to get to the roof decking. Spreading across the roof it begins to damage the granule coating on asphalt shingles – the coating designed to provide UV protection. 

Left alone, moss can grow into gutters and downspouts causing additional drainage problems. Added weight in the gutters can pull them free from fascia doors leading to additional water damage.

Removing the moss.

Moss can be removed through a variety of methods, but most ineffective or can cause additional damage. 

The traditional DIY approaches include:

• Store-bought chemicals and a garden hose
• Manually scraping the moss
• Moss control powders

Store-bought chemicals can be corrosive. They are usually not environmentally friendly and if they get into the soil or on plants, will kill them. 

Manually scraping, aside from being time-consuming and requiring getting on the roof (a safety risk) will almost certainly damage the granular coating on shingles. There is a significant risk of tearing shingles as well.

Neither of these methods will get to the rhizoids and scraping will leave parts of the moss behind. This will lead to new growth. Both are very ineffective.

Moss control powders seem like a less invasive option and are more effective at killing moss, but at a price. Most are not entirely environmentally friendly and can harm fish and aquatic invertebrates. With much of our watershed leading directly to the Puget Sound, they should not be used!

The traditional professional approach.

Most professionals use pressure washing to remove moss. Spraying water at high pressure, especially from below the roof is risky. High pressure can drive water below shingles, tear or loosen shingles and damage the surface of the shingles. While many businesses will state it is perfectly safe with the right pressure setting, it’s not completely accurate. Too little pressure will be ineffective at getting all of the moss and rhizoids.

The proper cleaning method – soft washing

” A soft wash incorporates special low-pressure nozzles on the end of a pressure washing gun or wand combined with a biodegradable chemical that removes the biological elements and other stains without damaging surfaces on your home or killing your plants.”– Angie’s List

Soft washing delivers the cleansers and water generally at a higher volume but lower pressure. This effectively kills and removes all of the moss and its rhizoids while having no impact on the roof surface.