Recognizing what grows on the roof
Three organisms settle on shingles, and people often mix them up.
- Algae. Black or dark brown streaks running down the slope. People mistake them for soot.
- Moss. Thick green cushions, lodged in the joints between shingles and along their edges.
- Lichen. Round, flat patches, grey-green or yellowish, stuck to the surface.
Algae, moss and lichen like shade and moisture. They show up first on the north slope, under trees and in areas that dry slowly after rain. A roof in full sun rarely has any.
Cosmetic or damaging
Growth on the roof shows from far away, but its effect varies a lot from one organism to another.
Algae changes the look of the roof. It forms a film on the surface that holds a little moisture, without piercing the shingle. Its effect on lifespan stays limited.
Moss is another matter. It acts like a sponge and keeps the shingles wet long after the rain. As it thickens, it slips under the edges and lifts them. Water then finds a path under the course, and winter frost pushes the shingles a little further apart every year.
Lichen clings to the granules. Pulling it off takes the granules with it. It is treated like moss, without scraping.
Cleaning without damage
Granules protect the asphalt from the sun. Any method that strips them shortens the life of the roof. Two methods are ruled out.
- Pressure washing. It blasts off the granules and drives water under the shingles.
- Scraping with a stiff brush, a scraper or a shovel.
The gentle method has two steps. A cleaning product made for roofs is applied at low pressure, following the shingle manufacturer's recommendations. Then it is left to work. The moss dies, dries, and comes off with the rain and wind over time.
A wet roof covered in moss is very slippery. Do not go up. This work is for a company equipped to work at heights.
Keeping it from coming back
- Have an arborist trim the branches hanging over the roof, to let in sun and wind.
- Have the debris removed. Built-up leaves and needles keep the roof damp.
- Ask us to install metal strips near the ridge. Rainwater running over the zinc or copper slows regrowth on the part of the roof below.
Frequently asked questions
Do black streaks mean my roof is finished?
No, black streaks are algae living on the surface. A stained roof can be in very good condition. Judge by the condition of the shingles instead, meaning flexibility, granules in place and edges lying flat.
Can moss cause a roof leak?
Yes, moss can cause a leak over time. By lifting the edges of the shingles and holding water, moss opens a path under the courses. The risk grows in the valleys and on low slopes.
How often should a shingle roof be cleaned?
A shingle roof should be cleaned only when needed. A roof without moss does not need regular cleaning, and each treatment wears the surface a little. Act when the moss thickens, and above all have the shade and the debris dealt with.