What are those stains on my roof and how do I treat them?

Cleaning stains from your roof shingles

Published by Pawel Matonog

I frequently get calls from clients who are worried about the state of their roofs because of unsightly discolorations that can be found on roofs such as: rust, cement or paint staining. Generally, cleaning shingles that have been discolored by these stains is rather a difficult task. Rust stains may be reduced by trained staff using a chemical solution, but it is usually best to replace the damaged shingles.

Another concern for homeowners is the appearance of algae, moss or lichen on their roofs. Due to Toronto’s hot, humid weather in the summer, these problems can creep up on many homeowners’ roofs without proper maintenance.Algae moss lichens

Algae staining consists of those dark streaks creeping down that shingles that can continue to get larger and larger over time. Sometimes wrongly called ‘fungus’, the dark stains on roofs are usually caused by the blue-green colour of the algae. If it takes hold, within five years, the algae can cause the entire roof surface to look ‘muddy’. Algae stains can be prevented for a time through a zinc oxide coating over the shingles that prevents the growth of the algae.  They can also be lightened with an algicidal treatment. Using algae-resistant shingles in the first place, like the ones we install by CertainTeed, is the most important line of defence.


Moss and lichens growing on this Toronto roof replaced by Archer Roofing


It’s a regular garden party up here!

Moss and lichens are more difficult to keep in check than algae, as cleaning the surface of the roof will only do so much. Preventing the moss and lichens from taking hold is the best method—such as keeping your roof clear of debris like leaves and pine needles and trimming your tree limbs back from the roof. Since trees produce sap, they will naturally drop some of the sap onto a roof if kept in close proximity to a home. Sap contains nutrients and sugars that moss and lichens love to feed off of, especially if the roof is in a shaded area. Trees offer shade to a roof, which also allows moisture to be retained on the surface and may inhibit air flow across the roof. All of this, combined with dropped leaves and debris make a perfect environment for algae, moss and lichen growth.


This Toronto roof needs a total replacement due to damage by time and lack of yearly maintenance


Trees close to the roof have contributed sap, debris, and leaves that have aided the growth of organisms

Don’t let these micro-organisms take hold of your roof! Regular maintenance and inspection by Archer Roofing is key to keeping your roof free from damage by these organisms, preserving the life of your roof. Call Toronto’s best roofer to discuss a treatment plan for your roof today 416-427-2437.


Regular maintenance is in order for a home with lots of trees and greenery, like in this Toronto home


A closer look at the new roof after replacement of shingles free from moss and lichens

As a final note, even new roof shingles can be temporarily stained or discolored through storage, or picking up asphalt oils from adjacent shingles. Natural weathering eliminates this temporary discoloration within a few months.