Moss, algae and lichen on shingles

Three occupants to tell apart

From the ground, with binoculars, they are fairly easy to tell apart.

• Algae form long black or dark brown streaks running down the slope. They look like soot.
• Moss forms thick green tufts, settled in the gaps between two shingles and along their edges.
• Lichen draws flat circles, greenish grey or leaning toward yellow, fused to the shingle.

All three seek shade and moisture. The north slope, the area under trees and corners slow to dry are their first ground.

Of all the shingle problems, vegetation is the one most noticeable from the street, without being the most serious.

Which one does harm

Algae, little. They stay on the surface, as a film that holds a bit of moisture without going through the shingle. They change the roof's appearance far more than its lifespan.

Moss, yes. It soaks up water like a sponge, and the shingles stay wet long after the shower. As it grows, it works its way under the edges and lifts them. Water can then get under the row. Each winter, frost widens the opening a little.

Lichen, when touched. It clings to the granules. If it is forced off, the granules go with it. Lichen is treated like moss, without tearing anything off.

What a cleaning must never do

Granules protect the asphalt from the sun, and a roof that loses them ages faster. Two methods are to be refused, even when a company offers them.

• High-pressure spraying carries off the granules and sends water under the shingles.
• Scraping, with a stiff brush, a scraper or a shovel, also tears off the granules.

A careful company applies a cleaner made for roofs at low pressure, following the shingle manufacturer's recommendations, then lets the product work. The dead moss dries and comes off gradually with rain and wind.

A wet, moss-covered roof is very slippery. You do not go up on it, and the cleaning goes to a company equipped for work at height.

Limiting regrowth

Three measures, all handled by tradespeople, tackle shade and moisture.

• A tree trimmer clears the branches reaching over the roof, so the sun and wind dry the shingles.
• Leaves and needles piled up on the roof, which keep it damp, are removed.
• We can fasten zinc or copper strips near the ridge. Rain running over this metal slows regrowth further down the slope.

The City of Laval's Plan de foresterie urbaine (urban forestry plan) notes a higher proportion of trees at the end of their life in Vimont and Chomedey. This finding is about trees, not roofs. For the tree hanging over your roof, a tree trimmer judges its condition.

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