Dealing with Moss

by: The Great Canadian Roof Doctor                            Nov 12, 2017

Moss is associated with a sharp drop in roof life (often to 15 years on the Island).  Let’s start with some of the moss removal basics. Once you remove your moss, it will come back … moss will grow on a rock, although a little better footing will help it. Your chance of having moss is really all about your local climate (moss likes moisture, shade, and high humidity; but dislikes a drying wind). New moss moves from place to place as airborne spores, and normally shows first growth in already accumulated dirt (usually found at the bottom edge of each shingle). Once the new moss starts growing, it will shade any moisture in the soil beneath it, and will help keep that soil moist. This new growth will also help to catch additional dust and dirt.

Interestingly, the moss doesn’t cause any direct roof damage on its own, but the moisture stored beneath it does … by breaking the chemical bond between the shingles’ underlying black asphalt layer, and the shingles’ attached protective granules. It’s this “glue” that breaks down. Where the granules come loose, and leave bare asphalt patches behind, the asphalt will quickly disintegrate with exposure to the sun’s ultraviolet (UV) rays.

To remove the moss, someone will have to get on the roof (note: walking on the roof will void most shingle warranties) and then (optionally) use soap and/or bleach (which can both destroy the asphalt layer directly … each in a different way), and then water plus scrubbing (which will break away the protective granules). If you must remove the moss, at least don’t use soap or bleach. You shouldn’t be scrubbing either, and the walking should be done very gently if you choose to go there at all. A cleaning using any of these methods can keep moss from view for up to about 2 years.

An alternate approach is to kill the moss with a chemical spray, then leave it to die. Signs of new growth at the sprayed locations will normally reappear in the year after the spraying. Unfortunately, the chemical killing doesn’t remove either the moist dirt layer or the shade providing dead moss, so the existing water water-trapping and sheltering structure remains … still catching new moss spores from the air, and encouraging continued shingle degradation.

The bottom line is that each time you remove the moss, more of the protective granules will be lost (decreasing the roof’s life expectancy). It may seem counterintuitive; but your roof will last longer if you don’t take any of the above de-mossing approaches. If the moss is simply left in place, the granules in the moist soil under the moss will still break free from the asphalt, but they won’t roll down the roof. Instead, the mat of moss will hold the free granules in place, where they can still protect the asphalt from the sun’s UV rays. A rain storm with a strong wind could of course still take the mat of moss (and the granules held in the moss) off the roof (and all at one time), once the granules are loose.

There is a different approach available. We remove all moss, dirt, and algae (using only water) as the first step in our “Weatherproofing” process (a deep cleaning, followed by a coating of penetrating clear acrylic sealant). This extends your roof’s lifespan significantly, and saves you thousands. Moss can grow on a sealed roof too, but it’s less likely to get a good foothold on the coated surface. Even if it does return … removal becomes optional after sealing, as moss (or its removal) can no longer do any damage, and walking on the roof won’t either. If you do plan to Weatherproof your roof, we recommend that you don’t do any simple de-mossing before Weatherproofing. Our “Tech-Stuff” document contains detailed information on our Weatherproofing process, shingle warranties, shingle lifespan, environmental issues, technical notes on moss and algae, some pricing information, and other related topics too.


Bob Cassady, P.Eng.
District Sales Manager
778.992.1922 – direct Comox Valley line
1.888.677.6310 – toll free

Click here to download the PDF.