How Moss Grows on Roofs and How to Prevent It

A moss-covered roof can present the look of a countryside cottage or just be unsightly but, either way, the growth will be incurring long term damage on the roofing materials upon which it sits. Moss, which decomposes organic materials including wood and shake shingles, will eventually weaken the structure of these shingle types to the point where they become vulnerable to cracking and breaking, while also retaining moisture on the roof’s surface leading to a breakdown of adhesives and dry rot.

Moss, of which there are over 12,000 species, has three general requirements to develop:

1) Constant shade – In the northern hemisphere, sloped roofs that are north facing see little or no sunlight, making for one environment that supports moss growth. The north facing roof slopes, however, aren’t the only surfaces that can be in shade during the entirety of the day as structures that are surrounded by trees or taller buildings may remain in shade throughout the day as well.

2) Moderate temperatures – The moss species that thrive on rooftops require moderate temperatures that stay above freezing and don’t exceed 70 degrees Fahrenheit. Temperatures can slide outside of these parameters temporarily, but areas that are typically hotter or colder than this range are considered as being inhospitable for moss growth.

3) Humidity and or moisture – Moss thrives on moisture, which can take the form of precipitation, fog, or humidity. Most moss species trap moisture, leading to roof surfaces that never dry out completely, which speeds the breakdown of wood and shake shingles as well as other roofing components.

Moss growth can be managed and/or prevented using the following methods:

• Being diligent about removing leaves and other debris – Accumulations of leaves on a roof surface can provide shade, moisture, and insulation, which are the three requirements for moss growth. Removing leaves and twigs from the roof can hinder moss growth and reduce the amount of debris that ends up in the drainage system.

• Trimming back surrounding trees – Trimming shade trees to allow direct sunlight on roof surfaces can kill existing moss growths and prevent them from recurring.

• Do a treatment routine on your roof – Untreated wood and shake shingles are more porous and hold moisture longer than a treated roof surface, which presents a tougher environment for moss growth by shedding moisture faster.

If the area in which you live and the specifics of your roofing system are conducive to moss growth, prevention is the best cure. By following these steps, you can create an inhospitable environment for moss growth and extend the life of your roof.

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KTM Roofing was founded in Atlanta in 1984 by Kelly and Tim McLoughlin, and incorporated as KTM Roofing Company, Inc, in 1995.