Moss on Your Roof (and What You can do about It)

Moss can harm a roof, significantly shortening its lifespan. Ignoring moss on your roof can be an expensive proposition – when it becomes so invasive it resembles a green woolly blanket, it probably has gone beyond the scope of removal or DIY roof repairs and might mean a total roof replacement. As soon as it is detected, it should be thoroughly removed and measures taken to prevent future moss growth.

How Moss Impacts a Roof

There are many different varieties of moss. Moss is a living organism that thrives in damp, shaded places. It doesn’t have a root system; instead it absorbs water from the air through its spores. Because it doesn’t have any roots, moss grows low and readily spreads on flat surfaces. Moss impacts a roof in a number of ways including:

  • Adds weight
  • Eats into roofing tiles
  • Covers roofing materials such as asphalt shingles designed to block out UV rays, allowing tiles to retain heat
  • Promotes wood rot on cedar shingle roofs
  • Creates holes in the underlayment, letting even more moisture in
  • Detracts from curb appeal

It Should be Removed

Moss on a roof can’t just be cleaned off; it must be removed or it will keep coming back. Whether you DIY moss removal or hire a roofing contractor will depend on the following factors:

  • The extent of the moss growth
  • Accessibility of the roof
  • Low or steep slope
  • If you can work safely on the roof

DIY Moss Removal

If the moss growth is contained in one area or is limited to a few patches here and there, it might be possible to kill it by one of several methods. Moss killing products, either dry powder or liquid, can be applied at any time of the year, but they seem to perform better in early fall. Check to see if the moss is entirely brown (dead) before attempting to remove it.

Bleach: Apply a solution of bleach, water, and a small amount of mild liquid detergent to the affected area. Let sit for at least 20 minutes; then rinse with a garden hose. Using a garden hose is preferable, but if you do use a power washer select a low setting to minimize water damage to older roofing shingles.

Dry powder: Dry powder moss killers are applied directly on the moss in parallel lines. When the rain comes the powder will be mixed with the rain and washed across the roof and down into the gutters. The process will kill the moss anywhere from four to seven days. Drawbacks to using a dry powder moss killer are: it’s difficult to apply evenly; it can leave white streaks behind that don’t disappear until several hard rains; and if it’s a windy day between the time the moss killer was applied and it rains, it could be blown away.

Liquid: A liquid moss killer typically comes in a container that is attached to a garden hose. Liquid moss killers are more straightforward to handle and spread more easily than their powder counterparts. You don’t want to be on the roof while applying the moss killer – spray it on to the roof from the ground or from a ladder. The roof should be dry before the moss killer is applied.

Hiring a Roofing Contractor

You want to remove the moss is such a way that it doesn’t harm the roof. Homemade solutions that use household cleaning chemicals can damage roofing shingles or have adverse reactions with other roofing materials such as flashings. A homeowner might do structural damage to a roof by walking on it, depending on what the roofing shingles are made of and how old the roofing system is. Hire a pro if one or more of the following conditions apply to you or your home:

  • The house has a steep slope roof
  • The house is more than two storeys
  • The moss growth is extensive
  • You have mobility issues, a health condition, or fear of heights
  • You don’t feel safe working on the roof
  • Sand cast, clay, and concrete roofing tiles can be cracked if directly walked on

A professional roofing contractor will know the best methods and techniques for moss removal based on their extensive experience with all types of roofs