Have you noticed green or yellowish spots on your roof, especially on the shaded side? This is a common problem on asphalt shingle roofs in Quebec, particularly in areas where moisture and shade persist. Although it may seem harmless, moss can actually shorten the lifespan of your shingles if left to grow. Here’s why it appears, what problems it can cause, and how to remove it without damaging your roof.
Key points to remember
- Moss grows in damp, shaded areas of your roof.
- It can lift the shingles and cause water to seep in.
- Pressure washing is strongly discouraged on asphalt shingles.
- Mild solutions and zinc strips can be used to remove and prevent foam.
- A professional inspection is recommended if the foam covers a large area.
Why Does Mold Grow on Asphalt Shingles?
Moss needs moisture, shade, and organic matter to take hold. Roofs in Quebec often meet all three of these conditions: snow, rain, and long periods of shade caused by surrounding trees. North-facing slopes, which receive less sunlight, are generally the first to be affected, as are areas under trees that block light and prevent the roof from drying quickly after rain.
Mold and moss grow in the presence of constant moisture, especially when air circulation is poor or when debris such as leaves and branches traps water on the roof. The same principle explains why mold forms inside a poorly ventilated home: without sustained moisture, neither mold nor moss can take hold.
What Are the Risks to Your Roof?
Moss isn’t just a cosmetic issue. When it absorbs water, it swells and can lift the shingles, creating small entry points for rainwater. Once moisture seeps in, it can reach the decking and, over time, spread into the interior of the house. This constant moisture also accelerates the deterioration of the shingles, shortening their useful life well before the manufacturer’s expected lifespan, and can promote the growth of mold indoors, an issue that directly affects indoor air quality and the respiratory health of occupants, as well as the roof’s longevity.
How to Safely Remove Moss
Here is the safest way to remove moss from an asphalt shingle roof:
- First, remove any leaves, twigs, and debris that have accumulated on the roof and in the gutters.
- Apply a mild solution (water and a non-abrasive detergent, or a cleaner specifically designed for roofs) using a spray bottle.
- Let the solution sit for about 15 minutes to loosen the foam without using force.
- Gently scrub with a soft-bristled brush, working from top to bottom.
- Rinse with low pressure using a garden hose fitted with a soft nozzle.
Important: Never climb onto the roof alone and without proper safety equipment. If the affected area is large or difficult to access, it is safer to have a roofer do the work .
Mistakes to Avoid
The most common—and most costly—mistake is to use a pressure washer. The force of the water jet strips the protective granules from the shingles, which accelerates wear and tear and may void certain manufacturer warranties. Also avoid stiff or metal brushes, which damage the surface, as well as cleaning in very hot or very windy weather.
How to Prevent Moss from Returning
| Situation | What it means | What to do |
| Moss found under trees | Lack of light and air circulation | Prune the branches above the roof |
| Moss That Comes Back Every Year | Persistent, unresolved moisture | Install zinc strips under the ridge |
| Thick, widespread rash | Advanced problem, risk to shingles | Have the roof inspected by a professional |
Zinc strips installed under the ridge shingles are a proven and durable solution. When exposed to rain, the zinc oxidizes and releases particles that prevent moss from growing back, without requiring repeated maintenance. An annual roof inspection also helps detect early signs of problems before they become more serious.
What to do now
Gentle cleaning and a visual inspection are often sufficient at first. But as soon as the moss returns year after year, covers a large area, or the shingles appear lifted or brittle, cleaning alone is no longer enough: the problem warrants a professional assessment.
An inspection allows you to check what isn’t visible from the ground—such as the condition of the sheathing under the shingles and the actual extent of the water intrusion—before the situation gets worse.
A well-maintained roof lasts longer
If addressed early and using the right methods, moss will have no lasting impact on your roof. The key is to use gentle products, avoid applying pressure, and consult a professional as soon as you have any doubts about the extent of the problem.
Our team can inspect your roof, identify the exact cause of the moss, and recommend the best course of action—whether it’s a simple cleaning or a more targeted treatment.
Contact us for personalized advice.
FAQ
Does moss really damage asphalt shingles?
Yes, if it is not treated. It traps moisture, can lift the shingles, and create conditions that lead to water infiltration over time.
Can I use a pressure washer to remove it?
No. Pressure washing strips the protective granules from the shingles and accelerates their wear. Always opt for a gentle method and rinse with low pressure.
Do zinc strips really work?
Yes. When zinc oxidizes upon contact with rain, it releases particles that prevent moss from growing back, although this effect develops gradually over several years.
How often should you check for moss?
A visual inspection once or twice a year—especially after winter—allows you to detect moss early and take action before it spreads.
When should I call a professional instead of cleaning it myself?
As soon as moss reappears each year, covers a large area, or shingles appear to be lifted or brittle, have a professional inspection performed to assess the roof’s actual condition before the problem worsens.
